Article

Topsoil foraging - An architectural adaptation of plants to low phosphorus availability

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Abstract

Low phosphorus availability is a primary constraint to plant productivity in many natural and agricultural ecosystems. Plants display a wide array of adaptive responses to low phosphorus availability that generally serve to enhance phosphorus mobility in the soil and increase its uptake. One set of adaptive responses is the alteration of root architecture to increase phosphorus acquisition from the soil at minimum metabolic cost. In a series of studies with the common bean, work in our laboratory has shown that architectural traits that enhance topsoil foraging appear to be particularly important for genotypic adaptation to low phosphorus soils (`phosphorus efficiency'). In particular, the gravitropic trajectory of basal roots, adventitious rooting, the dispersion of lateral roots, and the plasticity of these processes in response to phosphorus availability contribute to phosphorus efficiency in this species. These traits enhance the exploration and exploitation of shallow soil horizons, where phosphorus availability is greatest in many soils. Studies with computer models of root architecture show that root systems with enhanced topsoil foraging acquire phosphorus more efficiently than others of equivalent size. Comparisons of contrasting genotypes in controlled environments and in the field show that plants with better topsoil foraging have superior phosphorus acquisition and growth in low phosphorus soils. It appears that many architectural responses to phosphorus stress may be mediated by the plant hormone ethylene. Genetic mapping of these traits shows that they are quantitatively inherited but can be tagged with QTLs that can be used in plant breeding programs. New crop genotypes incorporating these traits have substantially improved yield in low phosphorus soils, and are being deployed in Africa and Latin America.

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... These responses become important because plant Pi-uptake capacity is greatly increased by an enhancement in the root's ability to explore the soil which is accompanied by higher expression of Pi-transporter and phosphatase genes and an increase in exudation of organic acids from root tips (Sánchez-Calderón et al. 2005;Ruiz Herrera et al. 2015). Earliest evidence of these type of responses was reported in common bean and subsequently in Arabidopsis (Lynch and Brown 2001;López-Bucio et al. 2002). ...
... Root architecture modifications are oriented to the development of a shallow root architecture in order to more efficiently explore the top layers of the soil where Pi tends to accumulate. This adaptation is known as topsoil Pi foraging (Lynch and Brown 2001) and is common to multiple plant species, including cereals that do not form a main root growth axis (Lynch and Brown 2001;Zhu et al. 2005; see Péret et al. 2014 for review). Depending on the species, modifications of root architecture and morphology also include an increase in the density and length of root hairs (Bates and Lynch 1996), an increase in the emergence of lateral roots with a shallower root angle (Lynch and Brown 2001;López-Bucio et al. 2002), the formation of adventitious roots (Ochoa et al. 2006), and the inhibition of primary root growth (Gutiérrez-Alanís et al. 2018). ...
... Root architecture modifications are oriented to the development of a shallow root architecture in order to more efficiently explore the top layers of the soil where Pi tends to accumulate. This adaptation is known as topsoil Pi foraging (Lynch and Brown 2001) and is common to multiple plant species, including cereals that do not form a main root growth axis (Lynch and Brown 2001;Zhu et al. 2005; see Péret et al. 2014 for review). Depending on the species, modifications of root architecture and morphology also include an increase in the density and length of root hairs (Bates and Lynch 1996), an increase in the emergence of lateral roots with a shallower root angle (Lynch and Brown 2001;López-Bucio et al. 2002), the formation of adventitious roots (Ochoa et al. 2006), and the inhibition of primary root growth (Gutiérrez-Alanís et al. 2018). ...
Article
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Improving phosphorus (P) crop nutrition has emerged as a key factor toward achieving a more resilient and sustainable agriculture. P is an essential nutrient for plant development and reproduction, and phosphate (Pi)-based fertilizers represent one of the pillars that sustain food production systems. To meet the global food demand, the challenge for modern agriculture is to increase food production and improve food quality in a sustainable way by significantly optimizing Pi fertilizer use efficiency. The development of genetically improved crops with higher Pi uptake and Pi-use efficiency and higher adaptability to environments with low-Pi availability will play a crucial role toward this end. In this review, we summarize the current understanding of Pi nutrition and the regulation of Pi-starvation responses in plants, and provide new perspectives on how to harness the ample repertoire of genetic mechanisms behind these adaptive responses for crop improvement. We discuss on the potential of implementing more integrative, versatile, and effective strategies by incorporating systems biology approaches and tools such as genome editing and synthetic biology. These strategies will be invaluable for producing high-yielding crops that require reduced Pi fertilizer inputs and to develop a more sustainable global agriculture.
... They can increase their P-use efficiency by reducing their P requirement, optimizing P allocation and increasing the residence time of P, for instance by remobilizing P from senescing organs, including leaves, roots and the sapwood (Chapin III, 1980;Veneklaas et al., 2012;Heineman et al., 2016). They can enhance P i acquisition by investing in absorptive roots and acclimating their root architecture and morphology to improve their P i -foraging capacity and efficiency (Lynch & Brown, 2001;Lambers et al., 2006). They can release phosphatase enzymes that mineralize P o forms into available P i (Nannipieri et al., 2011) and release low-molecular-weight organic acids that can mobilize P i and P o from the soil matrix (Lambers et al., 2006). ...
... Fine roots are commonly defined as below 2 mm in diameter (but see McCormack et al., 2015). In highly weathered soils, where P input from parent material is negligible, P typically accumulates in the topsoil; therefore, root traits and adaptations that improve topsoil exploration are advantageous (Lynch & Brown, 2001, 2008. In Amazonia, fine roots in the upper 30 cm of soil can account for more than 50% of the total fine-root biomass in the soil profile (Nepstad et al., 1994;Trumbore et al., 2006;Cordeiro et al., 2020). ...
... Fine roots can acquire P i through strategies that can be classified as foraging and mining. Here, we define fine-root P i foraging as related to the capacity of the roots to find and explore soil P i -rich patches, which has been associated mainly to root architectural traits, such as branching intensity and angles, and morphological traits, such as root hairs, length, diameter, specific root length, specific root area and root tissue density (Lynch & Brown, 2001, 2008Hodge, 2004). By contrast, fine-root P i mining is related to the capacity of the roots to acquire P i from less available forms, for instance through the release of phosphatases or organic acids. ...
Article
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In the tropical rainforest of Amazonia, phosphorus (P) is one of the main nutrients controlling forest dynamics, but its effects on the future of the forest biomass carbon (C) storage under elevated atmospheric CO2 concentrations remain uncertain. Soils in vast areas of Amazonia are P-impoverished, and little is known about the variation or plasticity in plant P-use and -acquisition strategies across space and time, hampering the accuracy of projections in vegetation models. Here, we synthesize current knowledge of leaf P resorption, fine-root P foraging, arbuscular mycorrhizal symbioses, and root acid phosphatase and organic acid exudation and discuss how these strategies vary with soil P concentrations and in response to elevated atmospheric CO2. We identify knowledge gaps and suggest ways forward to fill those gaps. Additionally, we propose a conceptual framework for the variations in plant P-use and -acquisition strategies along soil P gradients of Amazonia. We suggest that in soils with intermediate to high P concentrations, at the plant community level, investments are primarily directed to P foraging strategies via roots and arbuscular mycorrhizas, whereas in soils with intermediate to low P concentrations, investments shift to prioritize leaf P resorption and mining strategies via phosphatases and organic acids.
... For example, in dicotyledonous plants (e.g., Arabidopsis, soybean (Glycine max L.), tomato (Lycopersicon esculentum L.)), cotyledon abscission occurs faster when seedlings grow in shade than when plants grow in normal light (Clouse, 2001;Devlin et al., 2003;Li et al., 2017b;Zhou et al., 2019). Shade enhances the metabolism of carbohydrates, nutrients in the cotyledons, as the seed storage reserves (rather than nutrients taken up by roots and leaf-captured carbon) are the main energy sources for plant growth and development at the early seedling stage (Lynch and Brown, 2001;Obendorf et al., 2009;Nadeem et al., 2013;Wingler and Wingler, 2018). Furthermore, plant preferred storing carbohydrate in the stems and roots except cotyledons enhances long-term survival in shade by enabling seedlings to cope with periods of biotic and abiotic stress ( Myers and Kitajima, 2007). ...
... The decreased leaf length and weight under high light intensity might be induced by the increasing translocation of IAA from shoots to roots (Halliday et al., 2009;Vanstraelen et al., 2012). Generally, apocarotenoid biosynthesis in flowers increases with increasing light intensity (Llorente et al., 2017), but the response of crocin I concentration in the stigma to increasing light intensity shows the tendency opposite to most of the apocarotenoids in flowers. One possible explanation might be that genes such as CsβCH1, CsZDS, and CsCCD2 involved in crocin biosynthesis in saffron were highly expressed under dark conditions (Zhou et al., 2020a). ...
Article
Saffron (Crocus sativus L.) two-segment (TS) cropping system, with mother corms flowering in a controlled environment first and then growing daughter corms in the field, can greatly increase stigma yield and quality, but may decrease daughter corm yield (mainly due to depletion of nutrient reserves in mother corms). Light plays crucial roles in regulation of nutrient reserve metabolism in tubers, corms, and other storage organs. However, how the metabolism of mother corm reserves, which determines the daughter corm yield, responds to light intensity and quality during the reproductive stage remains unclear. Plants grew under five light intensities (56, 200, 400, 600, and 800 μmol m⁻² s⁻¹) and three combinations of red (R) to blue (B) light (4R1B, 3R1B, and 3R2B) at 200 μmol m⁻² s⁻¹ during the reproductive stage. After flowering, the plants were transplanted to the field. Leaf growth, net photosynthesis, carbohydrate and nutrient content, stigma yield and quality, and daughter corm yield were assessed. The stigma yield and quality decreased with increasing light intensity, whereas the carbohydrate and total N, P, and K content in the mother corm showed the opposite trend. The light treatment of 200(3R2B) not only maintained the stigma yield (up to 482.4 mg (10 corms)⁻¹) at a highest level, but also improved the apocarotenoid content (up to 33.6%) in stigma, and carbohydrate and nutrient content in mother corm. Compared with 56 μmol m⁻² s⁻¹, light 200 (3R2B) decreased the length and weight of leaves, ratio of leaf to corm weight, and starch and nutrient decomposition in corms, but increased the rate of photomorphogenesis, stomatal opening, chlorophyll concentration (up to 72.9%), net photosynthesis (up to 448.6%), and leaf sucrose concentration (up to 156.7%). A decreased demand for leaf development and increased carbon capture inhibited the decomposition of carbohydrates and nutrients in the mother corm. The high sucrose concentration in leaves served not only as a carbon substrate for leaf growth, but also as a signal for inhibiting the translocation of carbon from corm to leaves and then the decomposition of starch in corm. The increased reserves in mother corms and improved stomatal opening combined to result in large leaf area, well-developed vascular tissue, and then high rate of leaf photosynthesis in the field, which provided assimilates for achieving yield potential and a high proportion of large-size corms. This is the first report on optimizing light intensity and quality during the reproductive stage to partially eliminate daughter corm yield decreases in the TS cropping system. Furthermore, this work also contributed to understanding the metabolism of nutrient reserves and leaf growth in saffron as regulated by light intensity and quality.
... Production of greater root biomass could increase nutrient uptake from the soil. As P bioavailability is generally greatest in the topsoil layer, topsoil foraging is known to be an efficient strategy to increase P uptake, which could attain through increased production of axial roots, greater lateral root density, and greater number of root hair and its length (Lynch and Brown, 2001;Lynch, 2019). For example, common bean (Phaseolus vulgaris L.) cultivars with greater biomass of basal roots were capable of acquiring more P and producing greater yield under P-deficient stress than cultivars with fewer number of basal roots (Walk et al., 2006;Rangarajan et al., 2018). ...
... Diffusion of water-soluble nutrients such as nitrate, sulfate, calcium, magnesium (for short distances), and mass movement (for longer distances) decreases due to lack of soil moisture (Mackay and Barber 1985). Roots tend to increase their length and surface area so that they can take up more or less mobile nutrients such as phosphorus (Lynch and Brown 2001). Under arid conditions, root growth is reduced and root function impaired, resulting in reduced nutrient uptake capacity of the root systems (Marschner 1995). ...
... Among the developmental changes, the most important ones are the increase of the root-to-shoot growth ratio and the modification of the root system architecture (RSA) by a reduction of the primary root growth and increase in the number of lateral roots with smaller basal angle. These changes lead to more shallow roots that more efficiently forage the topsoil, with higher Pi levels due to its low mobility (Lynch and Brown, 2001;Ló pez-Bucio et al., 2003;Lynch, 2011). An additional PS-triggered modification in the root is the increase in root hair number and length, which enlarges the root-soil surface and helps to overcome the problems of Pi acquisition caused by its low diffusion in soil (Bates and Lynch, 2001;Kochian et al., 2015). ...
Article
Phosphorus is an essential nutrient for plant growth and reproduction. Plants preferentially absorb P as ortophosphate (Pi), an ion that displays low solubility that is readily fixed in the soil, making P limitation a condition common to many soils and Pi fertilization an inefficient practice. To cope with Pi limitation, plants have evolved a series of developmental and physiological responses, collectively known as the Pi starvation rescue system (PSR), aimed to improve Pi acquisition and use efficiency (PUE), and protect from Pi starvation-induced stress. Intensive research has been carried out during the last 20 years to unravel the mechanisms underlying the control of the PSR in plants. Here we review the results of this research effort that have led to the identification and characterization of several core Pi starvation signaling components, including sensors, transcription factors, miRNAs and miRNA inhibitors, kinases, phosphatases, and components of the proteostasis machinery. We also refer to recent results revealing the existence of intricate signaling interplays between Pi and other nutrients and antagonists, N, Fe, Zn and As, that have changed the initial single-nutrient centric view to a more integrated view of nutrient homeostasis. Finally, we discuss advances towards improving PUE and future research priorities.
... Steeper root angles increase the depth of the roots, thereby increasing the root distribution in the deep soil, which in turn allows roots to access water and nutrients that have leached into the deep-soil profile [7,11]. Under conditions of adequate water and nutrients, the root growth angle is relatively flat because the moisture and nutrients of the shallow soil profile meet the needs of aboveground growth [12]. In addition to typical RSA traits, such as angle and depth, lateral root length and density play important roles in plant nutrient uptake [7,13]. ...
Article
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Roots are important plant organs for the absorption of water and nutrients. To date, there have been few genome-wide association studies of maize root system architecture (RSA) in the field. The genetic basis of maize RSA is poorly understood, and the maize RSA-related genes that have been cloned are very limited. Here, 421 maize inbred lines of an association panel were planted to measure the root systems at the maturity stage, and a genome-wide association study was performed. There was a strong correlation among eight RSA traits, and the RSA traits were highly correlated with the aboveground plant architecture traits (e.g., plant height and ear leaf length, r = 0.13–0.25, p < 0.05). The RSA traits of the stiff stalk subgroup (SS) showed lower values than those of the non-stiff stalk subgroup (NSS) and tropical/subtropical subgroup (TST). Using the RSA traits, the genome-wide association study identified 63 SNPs and 189 candidate genes. Among them, nine candidate genes co-localized between RSA and aboveground architecture traits. A further co-expression analysis identified 88 candidate genes having high confidence levels. Furthermore, we identified four highly reliable RSA candidate genes, GRMZM2G099797, GRMZM2G354338, GRMZM2G085042, and GRMZM5G812926. This research provides theoretical support for the genetic improvement of maize root systems, and it identified candidate genes that may act as genetic resources for breeding.
... The desired root phenotypes by plant breeders will be ones that enhances plant adaptation to the edaphic stress while maintaining or increasing yields, for example, deeper and proliferating roots are desired during water-deficient stresses in the changing climate (Gaur et al., 2008;Aski et al., 2021). Lynch and Brown (2001) coined the term "topsoil foraging ideotype, " which is characterized by proliferation of lateral roots, long root hairs, association with mycorrhizal fungi, and suited to uptake of the immobile phosphorus mineral from the topsoil stratum (White et al., 2013). The "steep, cheap, and deep" ideotype (Lynch, 2013) optimizes on the uptake of water and the soluble nitrogen in the soil minimizing leaching. ...
Article
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Mung bean [ Vigna radiata (L.) Wilczek] is a drought-tolerant, short-duration crop, and a rich source of protein and other valuable minerals, vitamins, and antioxidants. The main objectives of this research were (1) to study the root traits related with the phenotypic and genetic diversity of 375 mung bean genotypes of the Iowa (IA) diversity panel and (2) to conduct genome-wide association studies of root-related traits using the Automated Root Image Analysis (ARIA) software. We collected over 9,000 digital images at three-time points (days 12, 15, and 18 after germination). A broad sense heritability for days 15 (0.22–0.73) and 18 (0.23–0.87) was higher than that for day 12 (0.24–0.51). We also reported root ideotype classification, i.e., PI425425 (India), PI425045 (Philippines), PI425551 (Korea), PI264686 (Philippines), and PI425085 (Sri Lanka) that emerged as the top five in the topsoil foraging category, while PI425594 (unknown origin), PI425599 (Thailand), PI425610 (Afghanistan), PI425485 (India), and AVMU0201 (Taiwan) were top five in the drought-tolerant and nutrient uptake “steep, cheap, and deep” ideotype. We identified promising genotypes that can help diversify the gene pool of mung bean breeding stocks and will be useful for further field testing. Using association studies, we identified markers showing significant associations with the lateral root angle (LRA) on chromosomes 2, 6, 7, and 11, length distribution (LED) on chromosome 8, and total root length-growth rate (TRL_GR), volume (VOL), and total dry weight (TDW) on chromosomes 3 and 5. We discussed genes that are potential candidates from these regions. We reported beta-galactosidase 3 associated with the LRA, which has previously been implicated in the adventitious root development via transcriptomic studies in mung bean. Results from this work on the phenotypic characterization, root-based ideotype categories, and significant molecular markers associated with important traits will be useful for the marker-assisted selection and mung bean improvement through breeding.
... Soil integrity by erosion Every mineral nutrient [117] Transpiration driven mass flow Calcium, magnesium, silicon, nitrates, and sulfates [117] Root growth P and K [118] Biological nitrogen fixation Loss of N [119] Soil microbial activity Loss of N [120] ...
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Each year, the global population and agriculture suffer critical agricultural output losses as a result of severe drought devastation. Physiological drought occurs when plants are unable to extract water from the soil, even though it is available in the root zone. Apart from having a significant effect on plant physiology, drought stress has the effect of reducing crop yield. Drought stress influences plant metabolism both directly and indirectly. Drought stress alters the morpho-anatomical, physiological, and biochemical composition of plants, thereby decreasing transpiration water loss and increasing the efficiency with which plants use their water. Constant water loss through transpiration, combined with previously lost water, results in leaf water deficits. Nonetheless, drought stress has a wide variety of effects, ranging from lesions to confusion. Plant health is harmed when their ability to absorb water and nutrients, interact with their environment, and breathe is harmed. Apart from oxidative damage to plants, it may also result in cell death, which can occur under certain conditions when cells are exposed to their environment. Drought induces a plethora of physiological and molecular changes in plants, the majority of which assist them in adapting to the harsh environment. To mitigate drought's adverse effects, we must first gain a better understanding of how drought affects plant physiology. The purpose of this research is to better understand how drought affects plant development by examining the causes and effects of drought stress.
... Both nutrient and hormonal signals act locally to regulate GSA (Bai et al., 2013;Rosquete et al., 2013Rosquete et al., , 2018Roychoudhry et al., 2013Roychoudhry et al., , 2017Roychoudhry et al., , 2019Trachsel et al., 2013). The net effect of this adaptive response is an increase in the surface area of the plant root system for resource capture (e.g., horizontal LRs for phosphorus uptake), or the securing of anchorage (Lynch & Brown, 2001;Trachsel et al., 2013). Similarly, shoots with vertical lateral branches show enhanced efficiency of light capture, allowing higher density planting and higher yields (Sakamoto et al., 2006;Vriet et al., 2012). ...
Article
The role of jasmonates (JAs) in primary root growth and development and in plant response to external stimuli is already known. However, its role in lateral root (LR) development remains to be explored. Our work identified methyl jasmonate (MeJA) as a key phytohormone in determining the branching angle of Arabidopsis LRs. MeJA inclines the LRs to a more vertical orientation, which was dependent on the canonical JAR1‐COI1‐MYC2,3,4 signalling. Our work also highlights the dual roles of light in governing LR angle. Light signalling enhances JA biosynthesis, leading to erect root architecture; whereas, glucose (Glc) induces wider branching angles. Combining physiological and molecular assays, we revealed that Glc antagonizes the MeJA response via TARGET OF RAPAMYCIN (TOR) signalling. Moreover, physiological assays using auxin mutants, MYC2‐mediated transcriptional activation of LAZY2, LAZY4 and auxin biosynthetic gene CYP79B2,and asymmetric distribution of DR5::GFP and PIN2::GFP pinpointed the role of an intact auxin mechanism required by MeJA for vertical growth of LRs. We also demonstrated that light perception and signalling are indispensable for inducing vertical angles by MeJA. Thus, our investigation highlights antagonism between light and Glc signalling and how they interact with JA‐auxin signals to optimize the branching angle of LRs. This article is protected by copyright. All rights reserved.
... In groundwater sources, P may be distributed more homogenously due to turbulent flowing water, so RGA will not assist within the water column. Nevertheless, even with homogenous P distribution, plants with shallower root systems have been shown to encounter less inter-root competition with roots on the same plant so RGA could provide an adaptive value in this sense [83]. Cumbus & Robinson (1977) studied P absorption by the adventitious and basal roots of watercress and found that the adventitious roots absorbed a higher proportion of P at low P concentrations, despite having a lower biomass compared to the basal root tissue [32]. ...
Article
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Watercress is a nutrient-dense leafy green crop, traditionally grown in aquatic outdoor systems and increasingly seen as well-suited for indoor hydroponic systems. However, there is concern that this crop has a detrimental impact on the environment through direct phosphate additions causing environmental pollution. Phosphate-based fertilisers are supplied to enhanced crop yield, but their use may contribute to eutrophication of waterways downstream of traditional watercress farms. One option is to develop a more phosphate use efficient (PUE) crop. This review identifies the key traits for this aquatic crop (the ideotype), for future selection, marker development and breeding. Traits identified as important for PUE are (i) increased root surface area through prolific root branching and adventitious root formation, (ii) aerenchyma formation and root hair growth. Functional genomic traits for improved PUE are (iii) efficacious phosphate remobilisation and scavenging strategies and (iv) the use of alternative metabolic pathways. Key genomic targets for this aquatic crop are identified as: PHT phosphate transporter genes, global transcriptional regulators such as those of the SPX family and genes involved in galactolipid and sulfolipid biosynthesis such as MGD2/3, PECP1, PSR2, PLDζ1/2 and SQD2. Breeding for enhanced PUE in watercress will be accelerated by improved molecular genetic resources such as a full reference genome sequence that is currently in development.
... Differences in root architecture and geometry have been found to play an important role in nutrient acquisition. For example, shallow basal root growth enhances topsoil foraging for phosphorus (P) because in most soils P is concentrated in the topsoil (Lynch and Brown 2001). In addition, root hairs are implicated to increase the absorption surface of the root and therefore the volume of soil that can be scavenged for nutrients (Gahoonia and Nielsen 2004). ...
Article
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Increasing food demand coupled with climate change pose a great challenge to agricultural systems. In this review we summarize recent advances in our knowledge of how plants, together with their associated microbiota, shape rhizosphere processes. We address (molecular) mechanisms operating at the plant–microbe-soil interface and aim to link this knowledge with actual and potential avenues for intensifying agricultural systems, while at the same time reducing irrigation water, fertilizer inputs and pesticide use. Combining in-depth knowledge about above and belowground plant traits will not only significantly advance our mechanistic understanding of involved processes but also allow for more informed decisions regarding agricultural practices and plant breeding. Including belowground plant-soil-microbe interactions in our breeding efforts will help to select crops resilient to abiotic and biotic environmental stresses and ultimately enable us to produce sufficient food in a more sustainable agriculture in the upcoming decades.
... Low mobility of P compounds in soils contributes to a basic heterogeneity of P placement in soils [19]. Plants can modify root architecture to place more fine roots in P-enriched soil patches at the lowest cost of root morphology [20][21][22][23]. In calcareous soils, the precipitation of P further increases the heterogeneity of P patches; hence, root foraging behavior in karst plants has uncertainties for P acquisition. ...
Article
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Ecosystem is vulnerable due to large areas of rocky desertification, which results in patchy soils and inlaid stone soils. Karst landform is typically characterized by heterogeneous phosphorus (P) distributions in soils at high calcium (Ca), but root foraging behavior has not been fully docu-mented in agronomical plants. In this study, Bidens pilosa and Plantago asiatica were raised in pots in a simulated soil environment with sands at high Ca (2.00 g kg-1) and low Ca levels (0.63 g kg-1). Inner spaces were divided into four sections to receive P in homogeneous (Homo.) (four quarters: 2.00 mg P kg-1) or heterogenous (Hete.) (one quarter: 8.00 mg P kg-1; three quarters: no-P input) patterns. Both species had longer roots in high P sections compared to no P sections. Foraging scale (highest length or SA) was higher in P. asiatica plants subjected to the Hete. pattern than to the Homo. pattern in low Ca pots. Foraging precision (length or SA differences between P patches as a proportion of the total) was also higher for P. asiatica subjected to the Hete. pattern but did not change in response to Ca level or P placement pattern. Overall, P. asiatica has a higher foraging ability than B. pilosa due to higher levels of foraging scale and precision from high-P (8.00 mg kg-1) patches in soils subjected to low Ca (0.63 g kg-1).
... The spatial configuration of the root system in the rhizosphere is often referred to as root architecture. It relates root distribution patterns and root topology in response to soil physiochemical properties including any nutrient deficiency (Lynch, 1995;Lynch and Brown, 2001). Therefore, root architecture determines the extent of root exploration capacities throughout the soil (Péret et al., 2014). ...
Article
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Being a macronutrient, phosphorus (P) is the backbone to complete the growth cycle of plants. However, because of low mobility and high fixation, P becomes the least available nutrient in podzolic soils; hence, enhancing phosphorus use efficiency (PUE) can play an important role in different cropping systems/crop production practices to meet ever-increasing demands in food, fiber, and fuel. Additionally, the rapidly decreasing mineral phosphate rocks/stocks forced to explore alternative resources and methods to enhance PUE either through improved seed P reserves and their remobilization, P acquisition efficiency (PAE), or plant’s internal P utilization efficiency (IPUE) or both for sustainable P management strategies. The objective of this review article is to explore and document important domains to enhance PUE in crop plants grown on Podzol in a boreal agroecosystem. We have discussed P availabilities in podzolic soils, root architecture and morphology, root exudates, phosphate transporters and their role in P uptake, different contributors to enhance PAE and IPUE, and strategies to improve plant PUE in crops grown on podzolic soils deficient in P and acidic in nature.
... Due to the immobile nature of phosphorus in the soil, phosphorus uptake is linked strongly to soil exploration, especially in the topsoil where phosphorus availability is greatest [116]. For the same reason, competition between roots is small, which explains why the phenotypes with high lateral branching densities are the best performers in the low phosphorus, high nitrogen soil, again confirming previous in silico [155,154] and field [221,92] studies. ...
Article
The green revolution led to a drastic increase in crop yields through chemical fertilisers, dwarf varieties and introduction of new methods of cultivation. We now face challenges such as climate change and soil degradation that require the development of crops resilient to adverse conditions in order to maintain an adequate food supply for the still growing world population. In order to maintain crop yields in soils with low nutrient availability or drought conditions we need to get a better understanding of the interactions between root system traits, soil environment and plant development. Compared to shoots, roots are hard to study because they are hidden from view by the soil. This makes mathematical models of roots a powerful tool to help us study root systems. We use OpenSimRoot, a functional-structural plant model to study the effects of various root system architectures on plant development in challenging environments, adding new functionality to expand the capabilities of OpenSimRoot. Our simulations showed that the effect of root loss on plant development depends on nutrient availability, plant species and root system phenotype, varying from very detrimental to slightly beneficial. Simulations of plants under drought implied that parsimonious and deeper rooting phenotypes perform better because of a large reduction in root carbon costs, increasing water uptake efficiency. We also show that machine learning techniques are a useful tool for root trait optimisation over a very large space of possible root system architectures. Our findings show that root system architecture has a large impact on plant development, especially in challenging environments and if we want to breed crops which are suited to deal with the challenges ahead of us we need to think about roots just as much as shoots. Our work also shows the benefits of interdisciplinary approaches by combining mathematical modelling with statistical machine learning in order to increase our understanding of biological systems. We hope our work will lead to increased collaborations across disciplines so that we may gain a better understanding of the hidden half of plants.
... Passioura (1983) indicated that less RLD would be advantageous in the topsoil layer only if more water could be used in deep soil layers. On the other hand, it is worth noting that roots in topsoil often are involved in scavenging phosphorus (Lynch and Brown, 2001). Hence, adapted soil conditions may also be important. ...
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Bambara groundnut [ Vigna subterranea (L.) Verdc.] is grown in rainfed production systems and suffers from periodic drought stress (DS), leading to yield reductions. Natural genotypic variation for root traits is essential for adaptation to water deficit conditions. However, root traits have not been fully utilised as selection criteria to improve DS in bambara groundnut. The present study explored the natural genotypic variation found in single genotypes of bambara groundnut derived from landraces to identify adaptive differences in tap root length (TRL) and root length density (RLD) in response to DS. A diverse core collection of eight bambara groundnut genotypes from various locations (namely, Gresik, LunT, IITA-686, DodR, S19-3, Tiga nicuru, and Ankpa-4, DipC1), were grown for two seasons (2018 and 2019) in polyvinyl chloride (PVC) columns with well-watered (WW) and 30-day DS treatments. Plant samples were collected at 55 days after emergence (DAE) (30 days of DS) and at 105 DAE (30 days of DS plus 50 days of recovery). Under DS, differential TRL among genotypes at 55 DAE was observed, with DodR recording the longest among genotypes with an increase (1% in 2018) in TRL under DS compared to WW, whereas LunT and IITA-686 showed significant ( p < 0.001) decrease in TRL (27 and 25%, respectively, in 2018). Average RLD was observed to have the highest reduction under DS in the 90–110 cm layer (42 and 58%, respectively, in 2018 and 2019). Rainy habitat LunT had limited roots in 2018 and recorded the least (0.06 ± 0.013 cm –3 ) RLD in 2019. However, dry-habitat DodR showed an increase in the RLD (60–90 cm) under DS compared to WW, while dry-habitat S19-3 densely occupied all depths with RLD of 0.16 ± 0.05 and 0.18 ± 0.01 cm cm –3 in the deepest layer in both seasons, respectively. Reduced RLD under DS showed recovery when the plants were re-watered. These plants were additionally observed to have RLD that surpasses the density in WW at all soil depths at 105 DAE. Also, recovery was shown in Tiga nicuru and DodR (0–30 cm) and IITA-686 (90–110 cm) in 2019. Average RLD under DS treatment was associated with substantial grain yield advantage ( R ² = 0.27 and R ² = 0.49, respectively) in 2018 and 2019. An increase in TRL allowed DodR to quickly explore water at a deeper soil depth in response to gradually declining soil water availability. High RLD in genotypes such as DodR, DipC1 and S19-3 also offered adaptive advantage over other genotypes under DS. Variation in intrinsic RLD in deeper soil depths in the studied genotypes determines root foraging capacity when facing DS. This suggests that different agroecological environments to which bambara groundnut is subjected in its natural habitat have promoted a phenotypic differentiation in root systems to adapt to ecotypic conditions, which may help offset the impact of DS. The natural genotypic variation exhibited, especially by DodR, could be exploited to identify potential quantitative trait loci (QTLs) that control deep rooting and root length density.
... Diffusion of water-soluble nutrients such as nitrate, sulfate, calcium, magnesium (for short distances), and mass movement (for longer distances) decreases due to lack of soil moisture (Mackay and Barber 1985). Roots tend to increase their length and surface area so that they can take up more or less mobile nutrients such as phosphorus (Lynch and Brown 2001). Under arid conditions, root growth is reduced and root function impaired, resulting in reduced nutrient uptake capacity of the root systems (Marschner 1995). ...
... Overall nutrient difference between shallow and deeper soil layers is a well-known stable gradient that is almost universal in the field (Jobbágy & Jackson, 2000) and to which species can respond (McNickle & Cahill, 2009; see also, e.g. Lynch & Brown, 2001;Crush et al., 2005;Ros et al., 2018). It is reasonable to assume that the response to this gradient plays a role in shaping the root system in the field owing to the much higher concentrations of all essential nutrients in the shallow layer, but, in our study, individual species responded to this gradient differently. ...
Article
The efficient uptake of nutrients depends on the ability of roots to respond to gradients of these resources. While pot experiments have shown that species differ in their ability to proliferate their roots in nutrient‐rich patches, the role of such differences in determining root shapes in the field is unclear. We used fine‐scale qPCR‐based species‐specific mapping of roots in a grassland community to reconstruct species‐specific root system shapes. We linked them with data from pot experiments on the ability of these species to proliferate in nutrient‐rich patches and their rooting depth. We found remarkable diversity in root system shapes, from cylindrical to conical. Interspecific differences in rooting depths in pots were the main determinant of rooting depths in the field, while differences in foraging ability played only a minor role. While some species with strong foraging ability did place their roots into nutrient‐rich soil layers, it was not a universal pattern. The results imply that although the vertical differentiation of grassland species is pronounced, it is primarily not driven by the differential plastic response of species to soil nutrient gradients. This may constrain the coexistence of species with similar rooting depths and may instead favour coexistence of species differing in their architectural blueprints.
... Roots are the main entry point for P in the plant and as a result plants have developed several root traits to maximize its acquisition (Figure 1.9). Root architecture adaptation to P deficiency is shown to be decisive for P uptake under low P conditions (Lynch and Brown, 2001). These adaptations refer to the root spatial distribution and root disposition in P deficient plants (Lynch, Chapter 1 Review of the literature 2011). ...
Thesis
(EN) Reducing the amount of phosphorus (P) exported from the field at harvest is one of the strategies to sustainably manage P inputs in agriculture. Thus, the grain P concentration is a key parameter in the management of P in agroecosystems. Grain P in wheat originates from two sources: the post-anthesis root P uptake and the remobilization of P from the vegetative parts of the plant. However, the exact contributions of both P sources to the P accumulated in the grains and the effect of P supply remains unclear. The objectives of this thesis are to provide a better understanding of the processes involved in the allocation of P to grains and to determine the contribution of post-anthesis P uptake and P remobilization to grain P in durum wheat (Triticum turgidum. L). The work of this thesis is based on three experiments carried out under controlled conditions in a hydroponic system that allows the controls of P supply to plants. Firstly, the effects of P deprivation during the post-anthesis period on P remobilization and grain P content were analyzed in two durum wheat cultivars. Secondly, the use of P isotope tracing was used to study the dynamics of P during the post-anthesis period. The results show that the P supply does not limit grain growth during the post-anthesis period if the plants are sufficiently supplied with P during the vegetative stage. Indeed, the P deprivation applied from anthesis onwards has no negative effect on grain yield and quality, despite thesignificant decrease in the P content of the grains. We have also shown that P remobilization fluxes increase under P-limiting conditions. The 32P labelling from anthesis to maturity showed that the remobilization of P from vegetative organs represents 81% of grain P in low P plants while it represents 65% for high P plants.The two successive labelling experiments with 32P during grain development reveal that the P absorbed by the roots during the post-anthesis period is not directly allocated to the grains via the xylem. Finally, this work opens up new perspectives for improving P use efficiency by limiting the repeated P export and hence its environmental impact and nutritional consequences. (FR) Une des pistes pour gérer durablement le phosphore (P) dans les agrosystèmes consiste à diminuer les exportations répétées du P avec les récoltes. La teneur en P des produits récoltés, en particulier les grains, est alors un paramètre clé dans la gestion du cycle du P dans les agrosystèmes. Au cours de la période postfloraison, l’accumulation du P dans les grains dépend de l’absorption post-floraison et de la remobilisation du P des organes végétatifs. Cependant les processus écophysiologiques qui régissent le remplissage des grains en P et la contribution des deux sources sont encore mal connus. Les objectifs de cette thèse étaient de mieux comprendre les processus impliqués dans le remplissage des grains en P et de déterminer la contribution de la remobilisation et de l’absorption post-floraison au P des grains chez le blé dur (Triticum turgidum. L). Trois expérimentations ont été réalisées en conditions contrôlées avec un système de culture hydroponique permettant de contrôler l’offre en P. D’abord, les effets de la privation en P pendant la période post-floraison sur la remobilisation et les teneurs en P des grains ont été analysés chez deux cultivars de blé dur. Ensuite, l’utilisation du traçage isotopique du P exogène a permis d’étudier la dynamique fine du P durant la période post-floraison et son transport aux grains. Les résultats obtenus montrent que la croissance des grains n’est pas limitée par l’offre en P pendant la période post-floraison si les plantes sont suffisamment alimentées en P pendant la phase végétative. En effet, la privation en P appliquées à partir de la floraison n’a pas eu d’effet négatif sur le rendement et sur la qualité des grains, malgré la diminution significative dans la teneur en P des grains. Nous avons également montré que les flux de remobilisation augmentent en conditions de limitation en P. Le marquage au 32P de la floraison à la maturité a permis de montrer que la remobilisation du P représente 81 % du P des grains pour les plantes cultivées avec un faible niveau en P tandis qu’elle représente 65 % pour les plantes cultivées avec un niveau élevé en P. Les deux marquages successifs au 32P au cours du développement des grains révèlent que le P absorbé par les racines pendant la période post-floraison n’est pas directement alloué aux grains via le xylème. Enfin, ce travail ouvre de nouvelles perspectives pour améliorer l’efficience d’utilisation du P en limitant les exportations répétées de P et ses conséquences environnementales et sur la qualité nutritionnelle des grains.
... Nutrient diffusion (for short distances) and mass flow (for longer distances) of water-soluble nutrients such as nitrate, sulphate, calcium, magnesium, and silicon decreased due to soil moisture deficit (Mackay and Barber 1985;Barber 1995). Thus, roots tend to increase their length and surface area and modify their architecture to capture the less mobile nutrients like phosphorus (Lynch and Brown 2001). Under drought conditions, the root growth is reduced and root function is impaired, which causes reduction of the nutrient acquisition capacity of root systems (Marschner 1995). ...
Chapter
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The worldwide mean temperature has increased by nearly about 1.1 °C since the preindustrial era and this increase may reach up to 4 °C by the tip of the twenty-first century due to the rising concentration of greenhouse gases. Since soils are related to climate system in a very complex way through nutrient and hydrologic cycles, global climate change is predicted to have a possible impact on soil fertility through the physical, chemical, and biological properties of soil due to rise in temperature, alternation in precipitation pattern, increase in greenhouse gas concentration in the atmosphere, etc. These detrimental effects of global climate change can be minimized by following both adaptation and mitigation measures. This paper reviews the influence of global change in the climate such as rise in temperature, alteration in precipitation pattern, and increase in atmospheric carbon dioxide on soil properties and processes affecting soil fertility.
... Similarly, few of African countries tried to investigate the varietal differences among the common bean genotypes including Ethiopia. Rwandaa climbing bean from Mexico, G2333, has gained great popularity among small farmers and is doubling yields over traditional bush cultivars in the central plateau [49]. They also demonstrate that low-P tolerance is not incompatible with high yield potential. ...
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The most challenging in twenty first century is the cause and consequences of Climate change and variability. It is hastening frequently by over population more in developing countries. Thus, it is affected several sectors, while Agriculture is the most vulnerable for climate change. As a result both biotic and abiotic factors are bottle neck for food security and sustainability. From abiotic factor, soil acidity is the frequently observed at area where received intensive rainfall due to top soil and metallic elements washed out. Soil acidity is arsenic, and causes abortion of expected production, especially susceptible crops such as Common bean. To overcome this problem, agronomic practices such as lime application, soil conservation techniques and appropriate root traits were reported. Root system architectures traits such as higher basal root number between 12 to 16, basal root whorl number of around 4, and shallow root angle of less than 15° were reported suitable to withstand soil acidity. Agronomic practices alone reported as it is tedious and also difficult to get complete genotypes with full of desired acidity resistance root traits. Hence several reported explained that using the desired root traits for acidity resistance genotypes and important agronomic practices obtained more valuable results as form of integration. Therefore, using the appropriate root traits and agronomic practices support with each other to reduces the syndrome of soil acidity impacts.
... Roots are integral in performing a variety of functions, e.g., nutrients and water uptake, serving as a storage organ and helping the plant to anchor in the soil (Smith and De Smet, 2012). The variable interactions of plant roots with the environment depends on root components and root architecture (Lynch and Brown, 2012). ...
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Potato (Solanum tuberosum L) is the third important crop for providing calories to a large human population, and is considered sensitive to moderately sensitive to drought stress conditions. The development of drought-tolerant, elite varieties of potato is a challenging task, which can be achieved through molecular breeding. Recently, the DEEPER ROOTING 1 (DRO1) gene has been identified in rice, which influences plant root system and regulates grain yield under drought stress conditions. The potato StDRO1 protein is mainly localized in the plasma membrane of tobacco leaf cells, and overexpression analysis of StDRO1 in Arabidopsis resulted in an increased lateral root number, but decreased lateral root angle, lateral branch angle, and silique angle. Additionally, the drought treatment analysis indicated that StDRO1 regulated drought tolerance and rescued the defective root architecture and drought-tolerant phenotypes of Atdro1, an Arabidopsis AtDRO1 null mutant. Furthermore, StDRO1 expression was significantly higher in the drought-tolerant potato cultivar “Unica” compared to the drought-sensitive cultivar “Atlantic.” The transcriptional response of StDRO1 under drought stress occurred significantly earlier in Unica than in Atlantic. Collectively, the outcome of the present investigation elucidated the role of DRO1 function in the alternation of root architecture, which potentially acts as a key gene in the development of a drought stress-tolerant cultivar. Furthermore, these findings will provide the theoretical basis for molecular breeding of drought-tolerant potato cultivars for the farming community.
... The nine cores (3 samples × 3 lines) were combined into one composite sample per plot. In agricultural soils, nutrient availability and root uptake decline substantially below the plow layer [38] as a consequence of the upper 20 cm of soil containing a significant proportion of crop roots [39]. On the other hand, long-term trials have shown that fertilizers mainly affect nutrient availability in the plow layer, with very little impact on deeper soil layers [40]. ...
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The current and expected expansion of agriculture in the drylands of Mexico, together with the decrease in precipitation occurring in the country, likely affect ecosystem processes and will bring great challenges for the suitability of rainfed agriculture for smallholder farmers. Here, we assessed metrics of the soil C, N, and P cycles, as well as soil microbial diversity, under rainfed maize and common bean cropping in arid and semiarid regions of central Mexico. The soil enzymatic vector angles of cultivated plots in both regions were above 45°, suggesting P limitation for microbial growth and crop productivity. Although changes were not observed in the intensity of this P-limitation with aridity, we found a negative effect of drought increase on the concentration of soil organic C and total N, with consequences for the C, N, and P balance in soils. Increasing aridity leads to the homogenization of microbial diversity. Considering a scenario in which decreases in mean annual precipitation would uncouple the biogeochemical cycles and homogenize soil biodiversity, the ecological implications could be an increase in the vulnerability of agricultural ecosystems to drought, with negative consequences for the suitability of rainfed agriculture in the drylands of central Mexico.
... Kafkas and Ortaş (2009) indicated that VAM fungi treatments promoted phosphorus uptakes from the soils. Various other researchers also indicated that phosphorus (P) uptakes improved rooting (Lynch and Brown 2001;Walk et al. 2006). Demirkaya et al. (2016) reported that EM treatments increased rooting of snapdragon flowers. ...
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As in Turkey, pepper is among the most widely produced and consumed plant species worldwide. Plant nutrients and fertilization programs have significant effects on seed yield and quality of peppers. Therefore, this study was conducted to investigate the effects of effective microorganism (EM) treatments and nitrogen fertilization on seed yield and quality parameters of peppers. Kandil Dolma and Yalova Çorbacı pepper cultivars commonly grown in Turkey were used as the plant material of the experiments. Three different EM treatments (0, 2 and 3 L da−1 EM) with and without N fertilizer were used. The greatest seed yield of both cultivars (68.57 kg da−1 in Kandil Dolma and 94.90 kg da−1 in Yalova Çorbacı) were obtained from 3 L EM + 2.60 kg da −1 N treatments. EM treatments increased germination ratio of Yalova Çorbacı, but the differences in mean germination times were not found to be significant. Germination index, an indicator of seed performance, increased with EM treatments in both cultivars.
... The taproot classification implies that the taproot is prominent with few, fine lateral roots, while the branched root system also has a taproot, but it may be less prominent and with more thicker lateral roots. We hypothesize that branched alfalfa roots may be especially important for topsoil foraging [58], while the dominant taproot systems may allow more allocation to deeper root systems [59]. ...
Article
Active breeding programs specifically for root system architecture (RSA) phenotypes remain rare; however, breeding for branch and taproot types in the perennial crop alfalfa is ongoing. Phenotyping in this and other crops for active RSA breeding has mostly used visual scoring of specific traits or subjective classification into different root types. While image-based methods have been developed, translation to applied breeding is limited. This research is aimed at developing and comparing image-based RSA phenotyping methods using machine and deep learning algorithms for objective classification of 617 root images from mature alfalfa plants collected from the field to support the ongoing breeding efforts. Our results show that unsupervised machine learning tends to incorrectly classify roots into a normal distribution with most lines predicted as the intermediate root type. Encouragingly, random forest and TensorFlow-based neural networks can classify the root types into branch-type, taproot-type, and an intermediate taproot-branch type with 86% accuracy. With image augmentation, the prediction accuracy was improved to 97%. Coupling the predicted root type with its prediction probability will give breeders a confidence level for better decisions to advance the best and exclude the worst lines from their breeding program. This machine and deep learning approach enables accurate classification of the RSA phenotypes for genomic breeding of climate-resilient alfalfa.
... A variety of root morphological and architectural changes could be used to improve P absorption. Plants improve overall soil exploration through lengthening roots, branching roots, lengthening particular roots, and changing branching angles (Lynch and Brown, 2001;Gahoonia and Nielsen, 2004a;Lynch, 2007). c. ...
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Phosphorus (P) is considered an essential nutrient for all and is also essential from global food security point of view but it is a limited, non-renewable nutrient resource, making its use vitally important. Nowadays, lower productivity in phosphorus availability is major concern. The decreasing ores and suboptimal levels of plant available phosphorus (P) can lead to lower yield. Its interaction with several other plant nutrients makes it very hard for plant availability. Several approaches have been tried and tested and many of them have been found effective, sustainable and cost efficient. However, the need for novel approaches for better phosphorus acquisition like physiological manipulation, better root structure and genetic alteration will help for resource conservation and is environmentally sustainable. But to diagnose environmental impact on excess use of phosphate fertilizers more improvement is required in order so that limited phosphorus stocks can be managed. Thus, there is a need for integrative approach to solve the lower P in soil system.
... As previously explained, Pi is an immobile nutrient that is easily xated in the topsoil, concomitantly, root architecture modi cations are oriented to the development of a shallow root architecture in order to more e ciently explore the top layers of the soil when Pi levels are limiting; this adaptation is known as topsoil Pi foraging (Lynch and Brown 2001). Modi cations of root architecture and morphology have been extensively reviewed and include an increase in the density and length of root hairs (Bates and Lynch 1996), an increase in the emergence of lateral roots with a shallower root angle ( When Pi levels are limiting, PHR1 transcription factors upregulate the expression of high a nity PHT1 transporters ). ...
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Due to the importance of Phosphorus (P) on plant development and reproduction, global P security has emerged as a key factor towards global food security. Together with multiple agrochemicals, P-based fertilizers have become the pillars that sustain our food production systems. Therefore, improving the genetics and biology of key crops such as maize, rice, wheat and soybean to develop varieties better adapted to thrive under environments that present low phosphate (Pi) availability and that possess higher Pi-fertilizer use efficiency is imperative. In this review, we summarize the current understanding of Pi nutrition in plants, with particular focus on crops, and provide new perspectives on how to harness the ample repertoire of genetic mechanisms behind plant low-Pi adaptive responses that can be utilized to design smart low-Pi tolerant plants. We discuss on the potential of implementing more integrative, versatile and effective strategies by incorporating genome editing and synthetic biology approaches to reduce Pi-fertilizer input and enable global food security in a more sustainable way.
... In addition to the physiological and morphological changes to roots (Lynch and Brown 2001;Richardson et al. 2009), P deficiency is often associated with an increased release of organic anions (also referred to as organic acids or low molecular weight carboxylates) from roots into the rhizosphere. Organic anions released by roots or microorganisms can increase the availability of P in soil for plant uptake, especially under conditions of P deficiency . ...
Article
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Purpose Organic anions commonly released from plant roots and microorganisms are widely reported to mobilize soil phosphorus (P). We characterized soil organic P that was mobilized by organic anions and assessed its amenability to hydrolysis by phosphatase enzymes. Methods Six soils differing in organic P concentration were extracted with citrate, malate or oxalate solutions and incubated with preparations of phosphomonoesterase, phosphodiesterase, or phytase. Organic P compounds present in these extracts were putatively identified and quantified with solution ³¹P-NMR spectroscopy and the enzyme-labile P fractions were assessed by changes in molybdate-reactive P (MRP) concentration. Results Organic P mobilization varied markedly among the organic anions. Extraction with 10 mM citrate was most effective and extracted 7.8-fold more total P than the water controls across all soils. Approximately 95% of the extracted P was non-MRP. The organic anions increased both the amount of P extracted and the proportion of the total extracted P that was phosphatase-labile. Phytase was generally the most effective enzyme with up to 60% of the total non-MRP being amenable to hydrolysis by phytase across all extracts. The presence of inositol hexakisphosphates in the extracts, as well as other forms of organic P including nucleic acids and phospholipids, was verified by ³¹P-NMR with concentrations dependent on both organic anions and soil type. Conclusion The combination of organic anions and phosphatases represents a key mechanism by which plants and microorganisms can enhance the bioavailability of soil P. This has important implications for understanding P dynamics in natural and managed ecosystems and for ongoing efforts to improve the P-acquisition efficiency of agricultural plants.
... These roots develop at the base of the root-to-shoot junction and together with the tap root and other root types define the root system shape. Despite this simple description, they are defined differently in different studies and are considered to originate, even in the same species, only from hypocotyl (Freschet et al. 2021;Leskovar and Cantliffe 1992;Lynch and Brown 2012;Miguel et al. 2013;Zobel and Waisel 2010;Zobel 2011) or only from a basal portion of the primary root (Basu et al. 2011;González-Sánchez et al. 2021;Lynch and Brown 2001;North et al. 2008). Therefore, the existing definitions of basal roots should be reconsidered. ...
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Purpose Root biology is an actively developing field that includes ecological, morphological, anatomical, developmental, and evolutionary aspects. I focus this opinion paper entirely on the inconsistencies in the usage of various terms in root biology. When terminology is used inconsistently, this may create a confusion in understanding, and the goal of this article is to identify the most common errors and suggest how to avoid them. Identified inconsistencies and proposed suggestions The proposed suggestions are as follows: (1) When definitions are not established or ambiguous, it is recommended to describe what is meant (“basal root” term is discussed); (2) Avoid using ambiguous terms (it is recommended not to use the term “secondary root”); (3) When known, give preference to organogenesis-related terminology (the terms “primary root”, “tap root”, and “main root” are compared); (4) Avoid using terms established for one identity to describe a different identity (inappropriate term selection is discussed, and it is recommended not to use the term “basal meristem” in the context of the root apex longitudinal zonation). Towards better communication Overall, I discuss how to avoid inconsistencies in terminology and achieve better communication among root biologists.
Article
Root architecture can be targeted in breeding programs to develop crops with better capture of water and nutrients. In rich nations such crops would reduce production costs and environmental pollution, and in developing nations they would improve food security and economic development. Crops with deeper roots would have better climate resilience while sequestering atmospheric CO2. Deeper rooting, which improves water and N capture, is facilitated by steeper root growth angles, fewer axial roots, reduced lateral branching, and anatomical phenotypes that reduce the metabolic cost of root tissue. Mechanical impedance, hypoxia, and aluminum toxicity are constraints to subsoil exploration. To improve topsoil foraging for P, K and other shallow resources, shallower root growth angles, more axial roots, and greater lateral branching are beneficial, as are metabolically cheap roots. In high‐input systems, parsimonious root phenotypes that focus on water capture may be advantageous. The growing prevalence of Conservation Agriculture is shifting the mechanical impedance characteristics of cultivated soils in ways that may favor plastic root phenotypes capable of exploiting low resistance pathways to the subsoil. Root ideotypes for many low‐input systems would not be optimized for any one function but would be resilient against an array of biotic and abiotic challenges. Root hairs, reduced metabolic cost, and developmental regulation of plasticity may be useful in all environments. The fitness landscape of integrated root phenotypes is large and complex, hence will benefit from in silico tools. Understanding and harnessing root architecture for crop improvement is a transdisciplinary opportunity to address global challenges.
Chapter
Under global climate change, drought and heat stress combination is often noticed in field conditions. Compared with individual stress, this combination is additive and negatively affects the crops at various growth phases and ultimately decreasees the agricultural crop productivity and grain quality. Existing literature indicates that drought and heat stress combination affects the metabolic activities in an inimitable mode unlike individual stresses. This chapter represents the strategic overview and implications of physiological, biochemical, and molecular aspects related to drought and heat stress combination. The future prospective approaches from breeding, biotechnology, and agronomical management practices to sustain crop productivity under drought and heat stress combination imposed by changing climate are also discussed.
Article
[https://onlinelibrary.wiley.com/doi/epdf/10.1111/jvs.13095] Questions Woody species are crucial biotic components in many of Earth’s terrestrial ecosystems as they support multiple ecosystem functions. The occurrence of woody species (i.e., their likelihood of being present at a single position) is driven by both climate and soil properties. However, empirical evidence is lacking on how multiple environmental factors regulate woody species occurrence across various climate regimes, limiting our ability to predict woody species distribution under different climate change scenarios. Location Woody plant species in eastern Australia; 1500 km gradient. Methods We surveyed 6353 mature (height > 4 m) woody plants from 62 woody species at 150 sites along a 1500 km climatic (i.e., aridity) gradient in eastern Australia, and used a generalized linear mixed model to explore the impact of summer rainfall, available soil phosphorus in the surface layer (0-10 cm), subsurface (100-200 cm) soil moisture and clay content, and interactions between aridity and the three soil variables on woody species occurrence (presence/absence). Results The overall occurrence of Australian woody species declined as the concentration of available soil phosphorus in the surface layer increased, with the impacts varying with increasing aridity. Subsurface soil moisture had strong positive effects on woody species occurrence in mesic areas, but the effect was attenuated with increasing aridity. Subsurface soil clay regulated the distribution of woody species, with finer soils promoting the likelihood of species occurrence in dry subhumid zone, but coarser soils supported more woody species in arid zone. Conclusions Our study provides empirical evidence that the distribution of Australian woody species is regulated by large-scale shifts in soil moisture and available soil phosphorus, and local-scale heterogeneity in soil texture. Our results suggest that forecasted climate change may restrict the distribution of woody species preferring particular soils, but expand the range of woody species that occur on dry or infertile soils.
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We analysed nine traits of the root system of 223 genotypes of Triticum turgidum (2n = 4x = AABB) subspecies dicoccoides, dicoccum, turgidum, durum and polonicum, finding a large intra and interspecific variability in both the number and size of roots, as well as in their spatial distribution. We studied the presence of an incomplete MITE (Miniature Inverted-repeat Transposable Element) inserted in the TtDro1B gene, which is present in some genotypes of dicoccoides, dicoccum, and turgidum, but not in polonicum and the 97.9% of the durum accessions. Comparison between genotypes shows that genotypes with the MITE element have smaller and shallower roots. Since Aegilops is considered to be the donor of the wheat B genome, the presence of the same MITE element was analysed in 55 accessions of the species Aegilops speltoides, searsii, bicornis and longissima, and in no case was it detected. We propose that after the emergence of T. turgidum subsp. dicoccoides, the insertion of the MITE element probably occurred in a single plant. Subsequent domestication resulted in genotypes of dicoccum with and without the MITE element, which after selection gave rise to the subspecies turgidum, and durum and polonicum, respectively. The MITE element can be used to differentiate turgidum from the durum and polonicum with high reliability.
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In the context of a recent massive increase of research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on generating cutting edge, meaningful and integrated knowledge. Consideration of the belowground components in plant and ecosystem studies has been consistently called for in recent decades, but methodology is disparate and sometimes inappropriate. This handbook, based on the collective effort of a large team of experts, will improve trait comparisons across studies and integration of information across databases by providing standardized methods and controlled vocabularies. It is meant to be used not only as starting point by students and scientists who desire working on belowground ecosystems, but also by experts for consolidating and broadening their views on multiple aspects of root ecology. Beyond the classical compilation of measurement protocols, we have synthesized recommendations from the literature to provide key background knowledge useful for (i) defining belowground plant entities and giving keys for their meaningful dissection, classification and naming beyond the classical fine- versus coarse-root approach, (ii) considering the specificity of root research to produce sound laboratory and field data, (iii) describing typical, but overlooked steps for studying roots (e.g., root handling, cleaning and storage) and (iv) gathering meta-data necessary for the interpretation of results and their re-use. Most importantly, all root traits have been introduced with some degree of ecological context that will be a foundation for understanding their ecological meaning, their typical use and uncertainties, and some
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Aims Common bean is an important source of food and fodder and is used to improve soil fertility when integrated in cropping systems through intercropping or rotation. Although widely grown by smallholders in Ethiopia, its productivity is constrained by several factors including soil acidity, which limits nutrient availability and uptake. The negative effects of soil acidity may be mediated by root system morphology and structure. Therefore, the aim of this study was to determine if root traits in common bean are associated with tolerance to soil acidity. Methodology The study was laid out in a split-plot design, whereby soil amendment practices with phosphorus (+P or -P) and liming (+lime or –lime) and their combinations were assigned to main plots and varieties to subplots. Two improved varieties (Nasir and Deme) and two farmers’ varieties (Polpole and Pantarkin) of common bean were evaluated under each soil amendment practice. Results Farmers’ variety Polpole produced significantly (p< 0.05) more hypocotyl roots (14.17) with a wider hypocotyls angle (5.87°) than other varieties. In addition, Polpole had a larger tap root diameter (2.57 mm) and length (34.10 cm) in plots amended with P. The improved variety Deme showed a higher number of basal roots (14.0) and basal root whorls (4.20) as well as lower basal root angle (0-15°) in plots amended with P and lime. Analyses demonstrate that common bean varieties differ in their ability to tolerate soil acidity due to differences in their root morphological and structural traits. Conclusion The results suggest an opportunity to identify and further develop acid tolerant varieties for low input farming systems to improve and enhance bean productivity and efficiency of the agro ecosystem at large.
Article
Root hairs represent a beneficial agronomic trait to potentially reduce fertiliser and irrigation inputs. Over the past decades, research in the plant model Arabidopsis thaliana has provided insights about root hair development, the underlying genetic framework, and the integration of environmental cues within this framework. Recent years have seen a paradigm shift, where studies are now highlighting conservation and diversification of root hair developmental programs in other plant species and the agronomic relevance of root hairs in a wider ecological context. In this review, we specifically discuss the molecular evolution of RSL (RHD Six-Like) pathway that controls root hair development and growth in land plants. We also discuss how root hairs contribute to plant performance as an active physiological rooting structure by performing resource acquisition, providing anchorage, and constructing the rhizosphere with desirable physical, chemical, and biological properties. Finally, we outline future research directions that can help achieve the potential of root hairs in developing sustainable agroecosystems. This article is protected by copyright. All rights reserved.
Article
Enhancing the acquisition of belowground resources has been identified as an opportunity for improving soybean productivity worldwide. Root system architecture is gaining interest as a selection criterion in breeding programs for enhancing soil resource acquisition and developing climate-resilient varieties. Here we are presenting two novel characteristics of soybean root system architecture that improve aboveground growth and yield. Eleven selected soybean genotypes were tested under rain-fed conditions in 2019 and 2020 at two locations in South Carolina, in which one of the locations was characterized by compacted soils. The elite SC breeding line SC07-1518RR, exotic pedigree line N09-12854, and slow wilting line N09-13890 were superior genotypes in terms of biomass production, seed yield, and/or water use efficiency. Genotypes N09-12854 and N09-13890 demonstrated reduced root development (based on total root count and length), likely to restrict belowground growth and allocate more resources for shoot growth. This characteristic, which can be referred as a parsimonious root phenotype, might be advantageous for soybean improvement in high-input production systems (characterized by adequate fertilizer application and soil fertility) that exist in many parts of the world. Genotype SC07-1518RR exhibited a similar strategy: while it maintained its root system at an intermediate size through reduced levels of total root count and length, it selectively distributed more roots at deeper depths (53–70 cm). The increased root distribution of SC07-1518RR at deeper depths in compacted soil indicates its root penetrability and suitability for clayey soils with high penetration resistance. The beneficial root phenotypes identified in this study (parsimonious root development and selective root distribution in deeper depths) and the genotypes that possessed those phenotypes (SC07-1518RR, N09-12854, and N09-13890) will be useful for breeding programs in developing varieties for optimal, drought, and compacted-soil conditions.
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Potato (Solanum tuberosum) is the third and fourth most important tuberous crop in terms of human consumption and production, respectively. However, its growth and development are affected by drought, which is an emerging threat to agriculture especially in arid and semiarid areas. Potassium (K) is a well-known macronutrient that improves the performance of crops under drought. Therefore, the present study was enacted with the aim of evaluating the impact of K fertilizer on potato crop growth, productivity, and drought tolerance under full root irrigation (FRI) and partial root irrigation (PRI) conditions. Two potato cultivars (Lady Rosetta and Hermes) were grown under normal field conditions followed by FRI and PRI applications. Potassium sulfate was applied in three doses (T0 = 50 kg·ha −1 , T1 = 75 kg·ha −1 , and T2 = 100 kg·ha −1). The experiment was laid out under randomized complete block design (RCBD) with split plot arrangement. The main plot was allocated to irrigation, along with a subplot to potassium and a sub-subplot to potato cultivars. The results indicated that K application significantly improved the plant growth and yield by exhibiting better performance in morpho-physiological and biochemical attributes under FRI and PRI conditions; however, a more remarkable change was noticed under PRI compared with FRI. K application alleviated drought stress regardless of cultivars. This study suggests that K application at the rate of 100 kg·ha −1 is an effective approach for inducing drought tolerance in potato crops.
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Low phosphorus (P) availability in acid soils is one of the main limiting factors in sugarcane ( Saccharum officinarum L.) production. Reconstruction of the root system architecture (RSA) is a vital mechanism for crop low P adaption, while the RSA of sugarcane has not been studied in detail because of its complex root system. In this study, reconstruction of the RSA and its relationship with P acquisition were investigated in a P-efficient sugarcane genotype ROC22 (R22) and two P-inefficient genotypes Yunzhe 03-103 (YZ) and Japan 2 (JP). An efficient dynamic observation room was developed to monitor the spatiotemporal alternation of sugarcane root length density (RLD) and root distribution in soil with heterogeneous P locations. The sugarcane RSA was reconstructed under P deficiency, and R22 had an earlier response than YZ and JP and presented an obvious feature of root shallowness. Compared with the normal P condition, the shallow RLD was increased by 112% in R22 under P deficiency while decreased by 26% in YZ and not modified in JP. Meanwhile, R22 exhibited a shallower root distribution than YZ and JP under P deficiency, supported by 51 and 24% greater shallow RLD, and 96 and 67% greater shallow root weight, respectively. The ratio of shallow RLD to total RLD in R22 was 91% greater than YZ, and the ratio of shallow root weight to total root weight in R22 was greater than that of YZ and JP by 94 and 30%, respectively. As a result, R22 had a higher shoot P accumulation than YZ and JP, which thereby increased the relative leaf sheath inorganic P concentration (RLPC) by 47 and 56%, relative shoot biomass (RSB) by 36 and 33%, and relative cane weight (RCW) by 31 and 36%, compared with YZ and JP under P deficiency, respectively. We verified the reliability and efficiency of a dynamic observation room and demonstrated that a shallower root distribution contributed to improving topsoil foraging, P acquisition, and low P adaption under P deficiency in sugarcane. Therefore, a shallower root distribution merits consideration as an evaluation trait for breeding P efficient sugarcane genotypes and genetic improvement.
Article
Ubiquitination-mediated post-translational modification of proteins is a pivotal regulatory mechanism involved in the growth and development of the plant. The Arabidopsis Tóxicos en Levadura (ATL) family is a group of RING-type ubiquitin ligases (E3) and ATL8 is a membrane-localized protein. Here, a reverse genetics approach was used to elucidate the role of ATL8 in phosphate (Pi) homeostasis. Deficiencies of Pi and sucrose (Suc) enhanced the relative expression level of ATL8 in different tissues of the wild-type (Wt). The relative expression level of ATL8 was attenuated and augmented in the mutant (atl8) and overexpression lines (Oe1 and Oe2), respectively. There were significant reductions in different root traits, root hairs, root to shoot ratio, and total Pi content in atl8 compared with the Wt under different Pi regimes. On the contrary, Oe1 and Oe2 lines exhibited enhancement in some of these traits. Noticeably, anthocyanin content was significantly reduced in Oe1 and Oe2 compared with the Wt and atl8 under P- condition. Abscisic acid (ABA) treatment led to an increase in the primary root length of atl8 compared with the Wt, suggesting a cross-talk between ABA and ATL8 on root growth. Furthermore, the relative expression levels of the genes involved in the maintenance of Pi homeostasis (WRKY75, RNS1, E3L, and ACP5) were differentially modulated in atl8, Oe1, and Oe2 compared with the Wt under different Pi regimes. The results revealed the pivotal role of ATL8 in mediating morphophysiological and molecular adaptive responses to Pi deficiency.
Article
Most terrestrial plants experience multiple edaphic (i.e. soil-related) stresses concurrently. Interactions among edaphic stresses may be characterized by dominance of one stress over others, neutral interactions whose effects are additive, mitigating interactions in which the effects of multiple stresses are less than additive, potentiating interactions in which the effects of multiple stresses are greater than additive, and dynamic equilibria in which plant responses seek to minimize all stresses concurrently. Existing theoretical paradigms, notably the Law of the Minimum and the Multiple Limitation Hypothesis, do not adequately account for interactions of edaphic stresses. Reduced growth resulting from edaphic stress is itself an important modulator of stress interactions by reducing resource demand as well as reducing soil resource capture. Root adaptations to one edaphic stress may either mitigate or potentiate other edaphic stresses, as evidenced by the examples of root architectural tradeoffs for water and P capture, in contrast to root architectural synergies for water and nitrate capture. In high-input agroecosystems, edaphic stress complexes of global importance include mechanical impedance/poor soil structure/drought stress, and salinity/drought stress. In low-input agroecosystems, globally important edaphic stress complexes include drought and low soil P availability, as well as the acid soil complex, characterized by toxicity of Al and possibly Mn, combined with low availability of P, K, Ca, and Mg. Global climate change is likely to exacerbate edaphic stress, by increasing the severity and frequency of drought and flooding, accelerating soil degradation, and altering plant phenology. Many of the edaphic stresses exacerbated by global climate change have potentiating interactions with each other, which is likely to make them more harmful than anticipated from their direct effects. Edaphic stresses linked to climate change are likely to have severe impacts in developing nations, which generally have more problematic soils and more limited management options than developed nations. The potential benefits of CO2 fertilization for plants in future climates may be mitigated by edaphic stress. Edaphic stress interactions are a challenge for crop breeding programs, because resistance to edaphic stress is generally quantitative and may display fitness tradeoffs with other edaphic stresses. Germplasm evaluation in farmer’s conditions, ideotype breeding, and deployment of root phenotypes with utility for multiple edaphic stresses, such as long, dense root hairs, and root cortical aerenchyma, would be helpful in this context. Interactions among edaphic stresses are key drivers of plant growth in the majority of global soils. Improved understanding of edaphic stress interactions is needed to develop the more resilient, stress tolerant crops and cropping systems that are urgently needed in global agriculture.
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Efficient acquisition and use of available phosphorus from the soil is crucial for plant growth, development, and yield. With an ever‐increasing acreage of croplands with suboptimal available soil phosphorus, genetic improvement of sorghum germplasm for enhanced phosphorus acquisition from soil is crucial to increasing agricultural output and reducing inputs, while confronted with a growing world population and uncertain climate. Sorghum bicolor is a globally important commodity for food, fodder, and forage. Known for robust tolerance to heat, drought, and other abiotic stresses, its capacity for optimal phosphorus use efficiency (PUE) is still being investigated for optimized root system architectures (RSA). Whilst a few RSA‐influencing genes have been identified in sorghum and other grasses, the epigenetic impact on expression and tissue‐specific activation of candidate PUE genes remains elusive. Here, we present transcriptomic, epigenetic, and regulatory network profiling of RSA modulation in the BTx623 sorghum background in response to limiting phosphorus (LP) conditions. We show that during LP, sorghum RSA is remodeled to increase root length and surface area, likely enhancing its ability to acquire P. Global DNA 5‐methylcytosine and H3K4 and H3K27 trimethylation levels decrease in response to LP, while H3K4me3 peaks and DNA hypomethylated regions contain recognition motifs of numerous developmental and nutrient responsive transcription factors that display disparate expression patterns between different root tissues (primary root apex, elongation zone, and lateral root apex).
Article
The management of agricultural soils during crop establishment can affect root development due to changes in the soil structure. This paper assesses the influence of tillage depth (250 mm, 100 mm, and zero tillage) and traffic management (conventional tyre pressure, low tyre pressure, and no traffic) on wheat root system architecture during winter wheat (Triticum aestivum L.) tillering and flowering growth stages (GS) at a long-term tillage trial site. The study revealed that zero-tillage systems increased crop yield through significantly greater root biomass (P<0.001), root length density, and deeper seminal rooting analysed using X-ray computed tomography (CT) (P<0.001) compared with trafficked treatments. In general, conventional-pressure traffic had a significant negative influence on the crop yield (P<0.01), root development (0.001), bulk density (P<0.05), and total soil porosity (P<0.05) of deep- and shallow-tillage conventional-pressure systems compared with no-traffic zero- and deep-tillage systems. Visual improvements in soil structure under zero-tillage conditions may have improved crop rooting in zero-tillage treatments through vertical pore fissures (biopores), enhancing water uptake during the crop flowering period. This study highlights the increasing implications of soil structural damage on root system architecture created by machinery traffic in crop production. Although the tillage method was less important, the constricted root systems were more pronounced in conventional-pressure shallow-tillage and deep-tillage systems, emphasizing the importance of using controlled-traffic farming methods to improve soil management and reduce the trafficked areas of agricultural fields.
Article
Phosphate (Pi) limitation represents a primary constraint on crop production. To better cope with Pi deficiency stress, plants have evolved multiple adaptive mechanisms for phosphorus acquisition and utilization, including the alteration of growth and the activation of Pi starvation signaling. However, how these strategies are coordinated remains largely unknown. Here, we found that the alternative splicing (AS) of REGULATOR OF LEAF INCLINATION 1 (RLI1) in rice (Oryza sativa) produces two protein isoforms: RLI1a, containing MYB DNA binding domain; and RLI1b, containing both MYB and coiled-coil (CC) domains. The absence of a CC domain in RLI1a enables it to activate broader target genes than RLI1b. RLI1a, but not RLI1b, regulates both brassinolide (BL) biosynthesis and signaling by directly activating BL-biosynthesis and signaling genes. Both RLI1a and RLI1b modulate Pi starvation signaling. RLI1 and PHOSPHATE STARVATION RESPONSE 2 function redundantly to regulate Pi starvation signaling and growth in response to Pi deficiency. Furthermore, the AS of RLI1-related genes to produce two isoforms for growth and Pi signaling is widely present in both dicots and monocots. Together, these findings indicate that the AS of RLI1 is an important and functionally conserved strategy to orchestrate Pi starvation signaling and growth to help plants adapt to Pi-limitation stress.
Article
It has been shown that the joint application of phosphorus (P) and ammonium (N-NH+4) increases maize root proliferation and P acquisition by maize in alkaline soils, but this has not been shown in acidic soils for legumes. A greenhouse experiment was conducted to assess the effect of the joint application of P and NH4+ on soybean root growth and P acquisition. Soybean was grown in glass-walled pots without P, with monoammonium phosphate (MAP) and triple super phosphate (TSP) applied on the soil surface or localized. The soil P increased irrespective of the P source and localization. The rhizosphere pH was decreased by MAP, while the soil bulk pH was not affected. The TSP increased the root length by 55% and MAP by 76% over the control, and the number of root tips increased by 21% with TSP, 58% with MAP applied on the soil surface, and 78% with MAP localized. The soybean dry matter, N and P uptake, and P use efficiency were increased by P fertilization, mainly with MAP localized. The joint application of P and ammonium decreases the soybean rhizosphere pH, which results in root proliferation early in the cycle, and eventually in higher P uptake and use efficiency.
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This study characterized growth characteristics and cellular details employing microscopy techniques in hydroponically-grown Ca2+-sufficient and Ca2+-deficient grapevines (Vitis vinifera) in a glasshouse. The Ca2+-deficient vines exhibited significant reductions in shoot length, shoot and trunk fresh weights, leaf area, chlorophyll, which eventually led to drooping, yellowing, and chlorosis of leaves. Roots were less dense and primarily dark and necrotic. Furthermore, their xylem vessels were small, polygonal, and appeared to be collapsed yet increased in number and developed lateral roots. Despite such alterations, the anatomical organization of leaves was not affected, yet they developed with more xylem vessels with thick walls and lignin in their mesophyll and vascular tissues. The chloroplasts in internodes’ chlorenchyma, phloem, and cambium underwent significant ultrastructural modifications. The concentrations of macro and micronutrients varied significantly among the roots, trunk, canes, and leaves, including the growth characteristics. These structural and growth modifications of calcium deficiency enable us to understand better the link between the symptoms and functions and for a holistic understanding of Ca2+ functionalities.
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Inundation that gives rise to soil flooding, or more complete submergence, is the most common environmental cause of oxygen deprivation for vascular plants. Species differ considerably in their susceptibility to the stress. Tolerance can vary from only a few hours to many days or weeks depending on species, the organs directly affected, stage of development, and external conditions such as temperature. Mechanisms that underlie short-and long-term tolerance to external anaerobic conditions are reviewed. For roots, these include metabolic adaptations such as avoidance of self poisoning and cytoplasmic acidosis, maintenance of adequate supplies of energy and sugar, modifications to gene expression and metabolic acclimation to tissue anoxia by previous exposure to partial oxygen shortage. Morphological escape mechanisms based on aerenchyma development and internal aeration pathways are emphasised. Shoots are often less susceptible to oxygen deficiency than roots. Their mechanisms of tolerance can include metabolic adaptations and developmentally passive tolerance such as that seen in overwintering rhizomes of many wetland species. Escape mechanisms for shoots are based on active and, sometimes, increasingly rapid shoot extension in the presence or absence of oxygen, and formation of replacement roots through adventitious rooting at the shoot base. Systemic signalling between roots and shoots integrates root and shoot physiology and limits indirect damage to shoot tissues by soil flooding. The review is completed by an assessment of prospects for future research.
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Phosphorus deficiency is a major yield limiting constraint in wheat cultivation on acid soils. The plant factors that influence P uptake efficiency (PUPE) are mainly associated with root characteristics. This study was conducted to analyze the genotypic differences and relationships between PUPE, root length density (RLD), colonization by vesicular arbuscular and arbuscular mycorrhizal (V)AM fungi and root excretion of phosphatases in a P-deficient Andisol in the Central Mexican Highlands. Forty-two semidwarf spring-bread-wheat (Triticum aestivum L.) geno-types from CIMMYT were grown without (−P) and with P fertilization (+P), and subsequently in subsets of 30 and 22 genotypes in replicated field trials over 2 and 3 years, respectively. Acid phosphatase activity at the root surface (APASE) was analyzed in accompanying greenhouse experiments in nutrient solution. In this environment, PUPE contributed more than P utilization efficiency, in one experiment almost completely, to the variation of grain yield among genotypes. Late-flowering genotypes were higher yielding, because the postanthesis period of wheat was extended due to the cold weather at the end of the crop cycles, and postanthesis P uptake accounted for 40–45% of total P uptake. PUPE was positively correlated with the numbers of days to anthesis (at −P r=0.57 and at +P r=0.73). The RLD in the upper soil layer (0–20 cm) of the wheat germplasm tested ranged from 0.5 to 2.4 cm cm −3 at –P and 0.7 to 7.7 at +P. RLD was the most important root trait for improved P absorption, and it was positively genetically correlated with PUPE (at –P r=0.42 and at +P r=0.63) and the number of spikes m −2 (at –P r=0.58 and at +P r=0.36). RLD in the upper soil layer was more important with P fertilizer application. Without P fertilization, root proliferation in the deeper soil profile secured access to residual, native P in the deeper soil layer. (V)AM-colonisation and APASE were to a lesser degree correlated with PUPE. Among genoptypes, the level of (V)AM-colonisation ranged from 14 to 32% of the RLD in the upper soil layer, and APASE from 0.5 to 1.1 nmol s −1 plant −1 10 −2 .
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Sitka spruce [Picea sitchensis (Bong.) Carr.] plants were grown under controlled conditions in specially designed boxes in which the plagiogravitropic lateral roots grew out from moist peat into an air space at one side. In one treatment the air was saturated with water vapour. In the other a linear horizontal gradient of water vapour pressure was maintained between the vertical peat surface and a porous membrane made from black cloth. Near the peat surface the vapour pressure gradient was equivalent to a soil water-potential gradient of about 0.48 MPa mm−1. The angles of the roots before and after emergence from the peat were measured in different planes. In the saturated air treatment there was no significant change of angle after emergence but roots which emerged into the air with the humidity gradient grew more slowly and showed a marked change of direction. This deflection had two components; there was some tendency for the roots to be attracted to the moist peat surface (hydrotropism), and there was also a downward component (gravitropism). Mean deviation from the angle of emergence which would have resulted from a hydrotropic response alone was 58°, while deviation from the true gravitropic position was only 31°. Therefore under the conditions of the experiment the gravitropic response was considerably stronger than the hydrotropic one. The implications of these results are discussed for the growth of roots in soils, where water potential gradients generally do not seem to be sufficient to cause hydroptropic curvature. However, there was an enhanced gravitropic response of the tips of lateral roots subject to water stress and this may help control the orientation of roots near the soil surface.
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This book is intended for a wide range of individuals, including scientists, students and informed laypersons who are interested in agricultural biotechnology, alternative agriculture, bioremediation of the environment and decreasing our reliance on pesticides and fungicides. It will deal primarily with understanding, at a biochemical and molecular biological level, how certain free living bacteria are able to promote plant growth; symbiotic bacteria such as Rhizobia will be mentioned only briefly. The assumption underlying the entire endeavour will be that a more profound understanding of these fundamental mechanisms will eventually permit scientists to manipulate these bacteria and use them more efficiently as a regular component of agricultural and/or horticultural practice. Therefore, while all the topics are discussed in as comprehensive a manner as possible, the book emphasizes a critical overview of the field rather than a mere compendium of data.
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Phosphorus deficiency is a major yield limiting constraint in wheat cultivation on acid soils. The plant factors that influence P uptake efficiency (PUPE) are mainly associated with root characteristics. This study was conducted to analyze the genotypic differences and relationships between PUPE, root length density (RLD), colonization by vesicular arbuscular and arbuscular mycorrhizal (V)AM fungi and root excretion of phosphatases in a P-deficient Andisol in the Central Mexican Highlands. Forty-two semidwarf spring-bread-wheat (Triticum aestivumL.) genotypes from CIMMYT were grown without (−P) and with P fertilization (+P), and subsequently in subsets of 30 and 22 genotypes in replicated field trials over 2 and 3 years, respectively. Acid phosphatase activity at the root surface (APASE) was analyzed in accompanying greenhouse experiments in nutrient solution. In this environment, PUPE contributed more than P utilization efficiency, in one experiment almost completely, to the variation of grain yield among genotypes. Late-flowering genotypes were higher yielding, because the postanthesis period of wheat was extended due to the cold weather at the end of the crop cycles, and postanthesis P uptake accounted for 40–45% of total P uptake. PUPE was positively correlated with the numbers of days to anthesis (at −P r=0.57 and at +P r=0.73). The RLD in the upper soil layer (0–20 cm) of the wheat germplasm tested ranged from 0.5 to 2.4 cm cm-3 at –P and 0.7 to 7.7 at +P. RLD was the most important root trait for improved P absorption, and it was positively genetically correlated with PUPE (at –P r=0.42 and at +P r=0.63) and the number of spikes m-2 (at –P r=0.58 and at +P r=0.36). RLD in the upper soil layer was more important with P fertilizer application. Without P fertilization, root proliferation in the deeper soil profile secured access to residual, native P in the deeper soil layer. (V)AM-colonisation and APASE were to a lesser degree correlated with PUPE. Among genoptypes, the level of (V)AM-colonisation ranged from 14 to 32% of the RLD in the upper soil layer, and APASE from 0.5 to 1.1 nmol s-1 plant-1 10-2.
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Roots represent a considerable carbon cost for plants. Furthermore, plants vary considerably in how carbon is expended for belowground processes. One attribute that varies widely among species is the investment of root biomass in the production of root length. Relatively thin roots have a high specific root length (SRL) or length: dry weight ratio. Since water and nutrient uptake is based more upon root length than mass, one might conclude that species of high SRL invest their root biomass more efficiently than species of low SRL. This, however, ignores many other functional attributes of roots that may permit coarse lateral roots to be more adaptive than fine lateral roots under certain environmental conditions. In leaves, studies on the relationship of structure and function suggest that evergreen plants with greater leaf longevity commonly have thicker leaves, lower photosynthetic capacity, and lower respiration rates than deciduous plants. These kinds of relationships may also be true for thick roots (low SRL). Limited evidence suggests that species of high SRL tend to have greater plasticity in root growth, greater physiological capacity for water and nutrient uptake, but less root longevity and less mycorrhizal dependency than species of low SRL. More study is needed before the physiological traits associated with variation in length-biomass ratio are understood.
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Although ethylene is known to be involved in plant response to a number of biotic and abiotic stresses, relatively little is known concerning its role in nutritional stress arising from nutrient deficiency or mineral toxicity. There is clear evidence for involvement of ethylene in the symbiosis between Rhizobium and legumes, and in the ‘Strategy 1’ response to Fe deficiency. Ethylene may also be generated during tissue necrosis induced by severe toxicities and deficiencies. Metal toxicity may generate ethylene through oxidative stress. Evidence for a more general role for ethylene in regulating plant responses to macronutrient deficiency is suggestive but incomplete. Few studies have addressed this interaction, and most published reports are difficult to interpret because of the unrealistic way that nutrient treatments were imposed. Deficiency of N and P appear to interact with ethylene production and sensitivity. A role for ethylene in mediating adaptive responses to P stress is suggested by the fact that P stress can induce a variety of morphological changes in root systems that are also affected by ethylene, such as gravitropism, aerenchyma formation, and root hair development. Other adaptive responses include senescence or abscission of plant parts which cannot be supported by the plant. Ethylene and other plant hormones may be involved in mediating the stress signal to generate these responses. Although existing literature is inconclusive, we speculate that ethylene may play an important role in mediating the morphological and physiological plasticity of plant responses to nutrient patches in time and space, and especially root responses to P stress.
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We have observed that low soil phosphorus availability alters the gravitropic response of basal roots in common bean (Phaseolus vulgaris L.), resulting in a shallower root system. In this study we use a geometric model to test the hypotheses that a shallower root system is a positive adaptive response to low soil P availability by (1) concentrating root foraging in surface soil horizons, which generally have the highest P availability, and (2) reducing spatial competition for P among roots of the same plant. The growth of nine root systems contrasting in gravitropic response over 320 h was simulated in SimRoot, a dynamic three-dimensional geometric model of root growth and architecture. Phosphorus acquisition and inter-root competition were estimated with Depzone, a program that dynamically models nutrient diffusion to roots. Shallower root systems had greater P acquisition per unit carbon cost than deeper root systems, especially in older root systems. This was due to greater inter-root competition in deeper root systems, as measured by the volume of overlapping P depletion zones. Inter-root competition for P was a significant fraction of total soil P depletion, and increased with increasing values of the P diffusion coefficient (De), with root age, and with increasing root gravitropism. In heterogenous soil having greater P availability in surface horizons, shallower root systems had greater P acquisition than deeper root systems, because of less inter-root competition as well as increased root foraging in the topsoil. Root P acquisition predicted by SimRoot was validated against values for bean P uptake in the field, with an r 2 between observed and predicted values of 0.75. Our results support the hypothesis that altered gravitropic sensitivity in P-stressed roots, resulting in a shallower root system, is a positive adaptive response to low P availability by reducing inter-root competition within the same plant and by concentrating root activity in soil domains with the greatest P availability.
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Root gravitropism may be an important element of plant response to phosphorus availability because it determines root foraging in fertile topsoil horizons, and thereby phosphorus acquisition. In this study we seek to test this hypothesis in both two dimensional paper growth pouch and three-dimensional solid media of sand and soil cultures. Five common bean (Phaseolus vulgaris L.) genotypes with contrasting adaptation to low phosphorus availability were evaluated in growth pouches over 6 days of growth, and in sand culture and soil culture over 4 weeks of growth. In all three media, phosphorus availability regulated the gravitropic response of basal roots in a genotype-dependent manner. In pouches, sand, and soil, the phosphorus-inefficient genotype DOR 364 had deeper roots with phosphorus stress, whereas the phosphorus-efficient genotype G19833 responded to phosphorus stress by producing shallower roots. Genotypes were most responsive to phosphorus stress in sand culture, where relative root allocation to the 0–3- and 3–6-cm horizons increased 50% with phosphorus stress, and varied 300% (3–6 cm) to 500% (0–3 cm) among genotypes. Our results indicate that (1) phosphorus availability regulates root gravitropic growth in both paper and solid media, (2) responses observed in young seedlings continue throughout vegetative growth, (3) the response of root gravitropism to phosphorus availability varies among genotypes, and (4) genotypic adaptation to low phosphorus availability is correlated with the ability to allocate roots to shallow soil horizons under phosphorus stress.
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Accumulation of the gaseous plant hormone ethylene is very important for the induction of several responses of plants to flooding. However, little is known about the role of this gas in the formation of flooding-induced adventitious roots. Formation of adventitious roots in Rumex species is an adaptation of these plants to flooded soil conditions. The large air-spaces in these roots enables diffusion of gases between shoot and roots. Application of ethylene to non-flooded Rumex plants resulted in the formation of adventitious roots. In R. palustris Sm. shoot elongation and epinasty were also observed. The number of roots in R. thyrsiflorus Fingerh. was much lower than in R. palustris, which corresponds with the inherent difference in root forming capacity between these two species. Ethylene concentrations of 1.5–2μI I− 1 induced a maximum number of roots in both species. Quantification of ethylene escaping from root systems of Rumex plants that were de-submerged after a 24 h submergence period showed that average ethylene concentrations in submerged roots reached 1.8 and 9.1 μl I−1 in R. palustris and R. thyrsiflorus, respectively. Inhibition of ethylene production in R. palustris by L-α-(2-aminoethoxyvinyl)-glycine (AVG) or α-aminobutyric acid (AIB) decreased the number of adventitious roots induced by flooding, indicating that high ethylene concentrations may be a prerequisite for the flooding-induced formation of adventitious roots in Rumex species.
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Plants respond strongly to environmental heterogeneity, particularly below ground, where spectacular root proliferations in nutrient-rich patches may occur. Such 'foraging' responses apparently maximize nutrient uptake and are now prominent in plant ecological theory. Proliferations in nitrogen-rich patches are difficult to explain adaptively, however. The high mobility of soil nitrate should limit the contribution of proliferation to N capture. Many experiments on isolated plants show only a weak relation between proliferation and N uptake. We show that N capture is associated strongly with proliferation during interspecific competition for finite, locally available, mixed N sources, precisely the conditions under which N becomes available to plants on generally infertile soils. This explains why N-induced root proliferation is an important resource-capture mechanism in N-limited plant communities and suggests that increasing proliferation by crop breeding or genetic manipulation will have a limited impact on N capture by well-fertilized monocultures.
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The development of plant root systems is sensitive to the availability and distribution of nutrients within the soil. For example, lateral roots proliferate preferentially within nitrate (NO3 –)-rich soil patches. A NO3 −-inducible Arabidopsis gene (ANR1), was identified that encodes a member of the MADS box family of transcription factors. Transgenic plants in whichANR1 was repressed had an altered sensitivity to NO3 – and no longer responded to NO3 –-rich zones by lateral root proliferation, indicating that ANR1 is a key determinant of developmental plasticity in Arabidopsis roots.
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The roots of most plants are colonized by symbiotic fungi to form mycorrhiza, which play a critical role in the capture of nutrients from the soil and therefore in plant nutrition. Mycorrhizal Symbiosis is recognized as the definitive work in this area. Since the last edition was published there have been major advances in the field, particularly in the area of molecular biology, and the new edition has been fully revised and updated to incorporate these exciting new developments. . Over 50% new material . Includes expanded color plate section . Covers all aspects of mycorrhiza . Presents new taxonomy . Discusses the impact of proteomics and genomics on research in this area.
Book
An understanding of the mineral nutrition of plants is of fundamental importance in both basic and applied plant sciences. The Second Edition of this book retains the aim of the first in presenting the principles of mineral nutrition in the light of current advances. This volume retains the structure of the first edition, being divided into two parts: Nutritional Physiology and Soil-Plant Relationships. In Part I, more emphasis has been placed on root-shoot interactions, stress physiology, water relations, and functions of micronutrients. In view of the worldwide increasing interest in plant-soil interactions, Part II has been considerably altered and extended, particularly on the effects of external and interal factors on root growth and chapter 15 on the root-soil interface. The second edition will be invaluable to both advanced students and researchers.
Chapter
Gregor Mendel (1866) conducted the first genetic analysis of common beans. Mendel studied the inheritance of growth habit, and pod color and shape in a progeny between P. vulgaris and P. nanus (= P. vulgaris, bush type) in order to confirm his findings with peas. Unfortunately, further studies on the inheritance of flower and seed coat color were hampered by his use of interspecific hybrids between P. nanus and P. multiflorus (= P. coccineus), which are now known to yield aberrant ratios. Later, Shaw and Norton (1918) used intraspecific crosses and determined that pigmentation and pigmentation patterns of the seed coat are controlled by multiple independent factors. A few years later Sax (1923) began to identify the multiple components that determine the inheritance of these traits. A single factor was identified as responsible for pigmentation, while two linked factors were identified to control mottling; this appears to be the first report of linkage in beans. Furthermore, Sax (1923) was the first to report linkage between a Mendelian character (seed coat pigmentation) and a QTL (for seed size). Although the common bean was used as experimental material at the inception of genetics, its genetic characterization has lagged behind that of many other crop species.
Chapter
Bean (Phaseolus vulgaris L.) is a staple food in Latin America and is frequently grown by small farmers on acid infertile soils. Average yields are very low (±500 kg ha-1) when compared to those in temperate zones (1500 kg ha-1). On acid, infertile soils, the main limiting factor for bean production is low available phosphorus. These soils have a high P-fixing capacity, so additional P from fertilizers is soon unavailable. Furthermore, the residual effect of P fertilizer is practically nil. P fertilizer is expensive and many farmers cannot afford to apply it. Other soil constraints (e.g. pH, Al toxicity, and low organic matter) impede P use efficiency and response to applied P. Also, fertilizer management influences the response of beans to P application. Differential yield response of beans in the field and in the greenhouse to P fertilization has been observed, but poor correlation is found as to dry matter production, leaf area index, P content in the tissue, and P uptake by the plant. Screening techniques for controlled environments do not give satisfactory results when tested in the field. The difficulties of these methods proposed the simple field screening method but the efficiency gain is small. Some other potential mechanisms for screening are discussed.
Article
Ethylene in Plant Biology, Second Edition provides a definitive survey of what is currently known about this structurally simplest of all plant growth regulators. This volume contains all new material plus a bibliographic guide to the complete literature of this field. Progress in molecular biology and biotechnology as well as biochemistry, plant physiology, development, regulation, and environmental aspects is covered in nine chapters co-authored by three eminent authorities in plant ethylene research. This volume is the modern text reference for all researchers and students of ethylene in plant and agricultural science. Key Features * Completely updated * Concise, readable style for students and professional * Contains an extensive bibliographic guide to the original literature * Well illustrated with diagrams and photographs * Thorough coverage of: * ethylene and ethephon roles and effects * stress ethylene * biosynthesis of ethylene * molecular biology of ethylene * action of ethylene * agricultural uses of ethylene.
Article
The effects on elongation of applying a continuous flow of ethylene in moist air to the roots of intact seedlings of rice (Oryza sativa L. cv. «IR 20»), white mustard (Sinapis alba L.) and tomato (Lycopersicon esculentumMill., CV's «Ailsa Craig», «Moneymaker» and «Diageotropica») were studied in relation to their rates of endogenous ethylene production.
Article
Variation in root system architecture among plants and its ecological correlates are discussed. The need to quantify costs and benefits of different root system architectures is emphasized. The components of root system architecture are magnitude, topology, angle of branching and radial angle, link lengths and link radii. The costs of root systems are affected by all of these angles. Root systems perform two primary functions for plants: anchorage and resource acquisition. Both depend on architectural charcteristics. Architecture appears to be a component of the underlying morphological plan or strategy of many plants, and plasticity in architecture is discussed. -from Author
Article
Nutrient-efficient crops have an important role in modern agriculture. In the low-input systems that characterize most of world agriculture, nutrient-efficient crops improve crop productivity. In high-input systems of the developed world, nutrient-efficient crops are valuable in reducing pollution of surface and ground water resources from intense fertilization. Recent developments in molecular biology, root biology, rhizosphere interactions, and modeling present new opportunities for the understanding and improvement of crop nutrient efficiency. The degree and extent of nutritional limitations to crop productivity, and the economic and ecological liabilities of intensive fertilization, are such that eventually nutrient-efficient crops will be an important part of integrated nutrient management of cropping systems.
Article
Remobilization of P from vegetative tissues can be an important source of grain P in common bean (Phaseolus vulgaris L.). Yet, data on the extent of remobilization of P from roots is scarce. We measured P remobilization from roots and leaves and examined the influence of P nutrition on remobilization patterns and tissue longevity. A split-root system was used to expose a portion of the root system (compartment roots) to low-P treatments independently from those imposed on the main root system. Phosphorus content of retained leaves, abscised leaves, stems, pods, seeds, and roots were observed over time. Leaf remobilization supplied over half of the pod plus seed P. Flux analysis suggested that leaf remobilization occurred earlier in low-P than it did in high-P plants. Root P content of compartment roots did not decrease with ontogeny. We compared remobilization patterns of root P in low-P and high-P plants in a short-term experiment using 32P applied to roots grown in either low-P or high-P compartments. Compartment roots retained over 80% of absorbed 32P almost all cases. The exception was roots of high-P plants grown in a high-P compartment, which retained only 20% of absorbed 32P. Together these results indicate common bean roots retain P when soil P levels are low. This behavior is in contrast to that of leaves and stems which remobilize P to the grain at both low and high levels of soil P fertility.
Article
The effect of available P in subsoil on plant uptake of P from surface soil layers and consequently on fertilizer recommendations is largely unknown. Our objective was to determine how much P may be removed by crops from subsurface soil layers and how the subsurface P level influences crop drawdown of P from the surface layer. Soil columns were prepared that contained all four possible combinations of low or high P levels in the surface soil and low or high P levels in the subsurface layer. The soil used was a Farnum fine sandy loam (fine-loamy, mixed, thermic Pachic Argiustoll). Phosphorus removal was monitored by growing hybrid sorghum-sudangrass [Sorghum bicolor (L.) Moench cv. ‘Honey Chow’] 84 days with an intermediate harvest after 35 days. Compared to when both the surface and subsurface layers had low P levels, P uptake increased when either or both layers had high P levels. Root growth and morphology responded to the P level of the soil layers. For all conditions, P uptake demonstrated a high correlation (R²=0.94) to an accessibility factor defined as the product of the available P level and root length summed for both soil layers. Comparing ratios of crop P removal to the change in soil test showed that P drawdown from the surface layer was reduced when the P-supplying capacity of the subsurface layer was high. We concluded that available P in the subsurface layer can supply P to the crop, especially if the surface layer cannot supply sufficient P to meet crop demand. These results suggest that subsoil fertility should be considered in evaluating P available to crops and in estimating crop drawdown of available P. Please view the pdf by using the Full Text (PDF) link under 'View' to the left. Copyright © . .
Article
1 This paper demonstrates the role played by plant hormones in linking environmental signals with plant responses. It concentrates on two strategies for a sessile organism as a plant to cope with changing environmental conditions: life cycle timing and phenotypic adjustment. 2 The significance of abscisic acid and gibberellins for dormancy and germination, respectively, is discussed in relation to life cycle timing. Cytokinins are presented in relation to a possible role in carbon allocation. The gaseous plant hormone ethylene is discussed in relation to its involvement in wind- and water-induced changes in shoot growth. 3 Evidence for the role played by plant hormones in developmental processes and plastic responses comes from only a very few plant species. It will be a task for ecologists to come to a more generalized understanding of the involvement of plant hormones in ecological processes by applying the existing knowledge to a much wider range of species.
Article
Soybean [Glycine max (L.) Merr.] acclimation to flooding has been investigated at biochemical and physiological levels, but long-term acclimation to flooding may be associated with morphological changes. Supplementing legumes with nitrate ameliorates flooding stress, compared with plants dependent on N2 fixation. Two experiments evaluated morphological and anatomical changes induced by flooding for plants relying on N2 fixation or supplemented with nitrate. An additional experiment assessed the role of aerenchyma in flooding acclimation by blocking aerenchyma formation with silver. Flooding soybean for 21 d with nutrient solution without N increased biomass allocation to roots. No aerenchyma was observed in roots of non-flooded plants; however, it was abundant in roots of flooded plants. Porosity measurements of root and stem-base tissues indicated that 10 to 15% of the volume was gas filled in flooded plants, while gas-filled volume was negligible in the non-flooded controls. Flooding in the presence of nitrate decreased aerenchyma and increased adventitious roots, compared with plants flooded with nutrient solution without N. Increased growth under flooding of nitrate-supplemented plants, therefore, was not directly associated with increased root porosity. For flooded plants dependent on N2 fixation, silver prevented aerenchyma and adventitious root development and decreased biomass accumulation and N2 fixation by approximately one-half compared with plants flooded in the absence of silver. These results indicate that acclimation to flooding in soybean involves preferential allocation of photosynthates to development of porous adventitious roots and that aerenchyma formation for soybean relying on N2 fixation is critical for acclimation to flooding.
Article
Published data for the most widely used bioassays for auxins as. well as other auxin-induced responses, have been analyzed to determine whether the response is ultrasensitive, hyperbolic or subsensitive, i.e. whether the change from 10 to 90% of maximal response requires less than, equal to or more than an 81-fold increase in external auxin concentration. Auxin-dependent callus growth is most often ultrasensitive. Other responses, including the curvature of Avena coleoptiles, may show simple Mi-chaelis-Menten kinetics. In the majority of cases, however, the response is sub-sensitive, often very markedly so. The finding of subsensitive responses is consistent with the proposal by A. J. Trewavas that plant development is regulated by changes in the sensitivity to plant growth substances. Detailed data for subsensitive dose responses, particularly auxin-induced inhibition of root growth, can often be precisely represented by bi- or multiphasic isotherms, with the phases separated by sharp transitions. The relationship to auxin uptake and the relationship, if any, between carriers and receptors for auxins remain to be elucidated.
Article
Low phosphorus availability stimulates root hair elongation in many plants, which may have adaptive significance in soil phosphorus acquisition. We investigated the effect of low phosphorus on the elongation of Arabidopsis thaliana root hairs. Arabidopsis thaliana plants were grown in plant culture containing high (1000 mmol m−3) or low (1 mmol m−3) phosphorus concentrations, and root hair elongation was analysed by image analysis. After 15d of growth, low-phosphorus plants developed root hairs averaging 0.9 mm in length while high-phosphorus plants of the same age developed root hairs averaging 0.3 mm in length. Increased root hair length in low-phosphorus plants was a result of both increased growth duration and increased growth rate. Root hair length decreased logarithmically in response to increasing phosphorus concentration. Local changes in phosphorus availability influenced root hair growth regardless of the phosphorus status of the plant. Low phosphorus stimulated root hair elongation in the hairless axr2 mutant, exogenously applied IAA stimulated root hair elongation in wild-type high-phosphorus plants and the auxin antagonist CM PA inhibited root hair elongation in low-phosphorus plants. These results indicate that auxin may be involved in the low-phosphorus response in root hairs.
Article
The involvement of ethylene in root architectural responses to phosphorus availability was investigated in common bean (Phaseolus vulgaris L.) plants grown with sufficient and deficient phosphorus. Although phosphorus deficiency reduced root mass and lateral root number, main root length was unchanged by phosphorus treatment. This resulted in decreased lateral root density in phosphorus-deficient plants. The possible involvement of ethylene in growth responses to phosphorus deficiency was investigated by inhibiting endogenous ethylene production with amino-ethoxyvinylglycine (AVG) and aerating the root system with various concentrations of ethylene. Phosphorus deficiency doubled the root-to-shoot ratio, an effect which was suppressed by AVG and partially restored by exogenous ethylene. AVG increased lateral root density in phosphorus- deficient plants but reduced it in phosphorus-sufficient plants. These responses could be reversed by exogenous ethylene, suggesting ethylene involvement in the regulation of main root extension and lateral root spacing. Phosphorus-deficient roots produced twice as much ethylene per g dry matter as phosphorus-sufficient roots. Enhanced ethylene production and altered ethylene sensitivity in phosphorus-deficient plants may be responsible for root responses to phosphorus deficiency.
Article
We investigated whether the capacities of Lolium perenne L. and Poa pratensis L. roots to proliferate locally and to alter local nitrogen (N) inflows in a decomposing organic matter patch were important in their capture of N when grown together. In the presence of a patch, plants of both species were significantly heavier and contained more N. Root length and weight densities increased in the patch, but specific root length was unaltered. Although both species proliferated roots in the patch, L. perenne produced greater root length densities than P. pratensis, and also captured more N from the patch. Indeed, total N uptake from the patch was related to root length density within the patch. N inflows (rate of N uptake per unit root length) in the patch were no faster than in the whole root system for both species. Under the conditions of this study, root proliferation in an organic patch was more important for N capture from the patch than alterations in N inflows. Local proliferation of roots may be a key factor in interspecific competition for non-uniformly distributed supplies of N in natural habitats, so resolving the previous uncertainty as to the ‘adaptive’ nature of root proliferation.
Article
The paper examines some of the physical and chemical properties of a deep red friable clay profile (Nitosol) developed in the Kenya highlands over trachyte. This soil is widely planted to coffee. The profile is remarkable for the depth and uniformity of the B horizon and for its stable microaggregation. Despite a clay content of 50–60% throughout the deep solum, the soil feels and handles like a loam. Work with radioactive phosphorus on samples from the profile and from nearby experimental plots under coffee showed that the activity, and the amounts, of surface phosphate fractions decreased in the order Fe-P, Al-P and Ca-P, but the order of decrease of specific surface activity was Al-P, Fe-P, Ca-P. The relative availabilities of these forms is discussed.
Article
Low phosphorus availability is often a primary constraint to plant productivity in native soils. Here we test the hypothesis that root carbon costs are a primary limitation to plant growth in low P soils by assessing the effect of P availability and mycorrhizal infection on whole plant C budgets in common bean (Phaseolus vulgaris L.). Plants were grown in solid-phase-buffered silica sand providing a constant supply of low (1 μm) or moderate (10 μm) P. Carbon budgets were determined weekly during the vegetative growth phase. Mycorrhizal infection in low-P plants increased the root specific P absorption rate, but a concurrent increase in root respiration consumed the increased net C gain resulting from greater P uptake. The energy content of mycorrhizal and non-mycorrhizal roots was similar. We propose that the increase in root respiration in mycorrhizal roots was mainly due to increased maintenance and growth respiration of the fungal tissue. Plants grown with low P availability expended a significantly larger fraction of their total daily C budget on below-ground respiration at days 21, 28 and 35 after planting (29–40%) compared with plants grown with moderate P supply (18–25%). Relatively greater below-ground respiration in low P plants was mainly a result of their increased root∶shoot ratio, although specific assimilation rate was reduced significantly at days 21 and 28 after planting. Specific root respiration was reduced over time by low P availability, by up to 40%. This reduction in specific root respiration was due to a reduction in ion uptake respiration and growth respiration, whereas maintenance respiration was increased in low-P plants. Our results support the hypothesis that root C costs are a primary limitation to plant growth in low-P soils.
Article
Adventitious rooting in Rumex plants, in which the root systems were in hypoxic conditions, differed considerably between two species. R. palustris, a species from frequently flooded river forelands, developed a large number of adventitious roots during hypoxia, whereas adventitious root formation was poor in R. thyrsiflorus, a species from seldom flooded dykes and river dunes. Adventitious rooting could also be evoked in aerated plants of both species by application of auxin (1-naphthaleneacetic acid or indoleacetic acid) to the leaves. The response to auxin was dose-dependent, but even high auxin doses could not stimulate R. thyrsiflorus to produce as many adventitious roots as R. palustris. Consequently, the difference between the species in the amount of adventitious root formation was probably genetically determined, and not a result of a different response to auxin.A prerequisite for hypoxia-induced adventitious root formation is the basipetal transport of auxin within the shoot, as specific inhibition of this transport by N-1-naphthylphthalamic acid severely decreased the number of roots in hypoxia-treated plants. It is suggested that hypoxia of the root system causes stagnation of auxin transport in the root system. This can lead to an accumulation of auxin at the base of the shoot rosette, resulting in adventitious root formation.
Article
SimRoot, a geometric simulation model of plant root systems, is described. This model employs a data structure titled the Extensible Tree, which is well suited to the type of data required to model root systems. As implemented on Silicon Graphics workstations, the data structure and visualization code provides for continuous viewing of the simulated root system during growth. SimRoot differs from existing models in the explicit treatment of spatial heterogeneity of physiological processes in the root system, and by inclusion of a kinematic treatment of root axes. Examples are provided of the utility of the model in estimating the fractal geometry of simulated root systems in 1, 2, and 3 dimensional space. We envision continued development of the model to incorporate competition from neighboring root systems, linkage with crop simulation models to simulate root-shoot interactions, explicit treatment of soil heterogeneity, and plasticity of root responses to soil factors such as presence of mycorrhizal associations.
Article
Recent work on root distribution, growth angles and gravitropic responses in Japanese cultivars of winter wheat are reviewed. Vertical distribution of roots, which influences the environmental stress tolerance of plants, was observed in the 12 Japanese cultivars in the field. The root depth index (RDI: the depth at which 50% of the root length has been reached) differed among the cultivars at the stem elongation stage. Since the RDI was closely related to the growth angle of seminal roots obtained in a pot experiment, it was assumed that growth angle is useful for predicting vertical root distribution among wheat genotypes. Gravitropic responses of the primary seminal root of 133 Japanese wheat cultivars assessed by measuring the growth angle in agar medium, were larger in the northern Japanese cultivars and smaller in the southern ones. It was also found that the geographical variation resulted from the wheat breeding process, i.e. genotypes with limited gravitropic responses of roots had been selected in the southern part of Japan where excessive soil moisture is one of the most serious problems.