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Plant roots release phospholipid surfactants that modify the physical and chemical properties of soil
Abstract
Plant root mucilages contain powerful surfactants that will alter the interaction of soil solids with water and ions, and the rates of microbial processes. •The lipid composition of maize, lupin and wheat root mucilages was analysed by thin layer chromatography and gas chromatography-mass spectrometry. A commercially available phosphatidylcholine (lecithin), chemically similar to the phospholipid surfactants identified in the mucilages, was then used to evaluate its effects on selected soil properties. •The lipids found in the mucilages were principally phosphatidylcholines, composed mainly of saturated fatty acids, in contrast to the lipids extracted from root tissues. In soil at low tension, lecithin reduced the water content at any particular tension by as much as 10 and 50% in soil and acid-washed sand, respectively. Lecithin decreased the amount of phosphate adsorption in soil and increased the phosphate concentration in solution by 10%. The surfactant also reduced net rates of ammonium consumption and nitrate production in soil. •These experiments provide the first evidence we are aware of that plant-released surfactants will significantly modify the biophysical environment of the rhizosphere.
... Although most studies suggested that root exudates can enhance the soil water retention capacity, a few studies presented opposite results (e.g., Read et al., 2003;Whalley et al., 2005;Kroener et al., 2014). Read et al. (2003) measured the SWRCs of Bullionfield soil samples saturated with pure water and 500 mg/l lecithin solution by using the tension table method. ...
... Although most studies suggested that root exudates can enhance the soil water retention capacity, a few studies presented opposite results (e.g., Read et al., 2003;Whalley et al., 2005;Kroener et al., 2014). Read et al. (2003) measured the SWRCs of Bullionfield soil samples saturated with pure water and 500 mg/l lecithin solution by using the tension table method. They found that the water retention capacity of lecithin solution saturated-Bullionfied soil was remarkably lower than that of pure water saturated-Bullionfied soil (see Figure 4g); they attributed this to the reduction of surface tension of pore fluid due to phospholipids. ...
... Furthermore, the dynamic change effect of root exudates with environmental factors is rarely examined. Deng et al. (2015); (g) Read et al. (2003). The relative variation rate was defined as (the parameter of mucilage-treated soilthe parameter of untreated soil) / the parameter of untreated soil ...
... Different mucilage collection methods have been established, each having its own advantages and disadvantages. For example, many studies are from hydroponic (Ahmed et al. 2015;Chaboud 1983;Naveed et al. 2017Naveed et al. , 2019Oburger and Jones 2018;Read et al. 2003), or from percolation systems (Mikutta et al. 2006), where mucilage represents only a small fraction of what is collected besides cell debris, low molecular weight compounds and enzymes (Oburger and Jones 2018). To collect mucilage as a less diluted fraction, seedlings are often grown in aeroponic systems (Brax et al. 2020;Holz et al. 2018;Zickenrott et al. 2016) or on filter paper (Read et al. 2003). ...
... For example, many studies are from hydroponic (Ahmed et al. 2015;Chaboud 1983;Naveed et al. 2017Naveed et al. , 2019Oburger and Jones 2018;Read et al. 2003), or from percolation systems (Mikutta et al. 2006), where mucilage represents only a small fraction of what is collected besides cell debris, low molecular weight compounds and enzymes (Oburger and Jones 2018). To collect mucilage as a less diluted fraction, seedlings are often grown in aeroponic systems (Brax et al. 2020;Holz et al. 2018;Zickenrott et al. 2016) or on filter paper (Read et al. 2003). A benefit of these methods are (semi-)sterile growth conditions, reducing a possible microbial degradation or contamination of mucilage (Chaboud 1983;Morel et al. 1986). ...
... Chemically, up to 97% (w/w) of mucilage are carbohydrates (Carminati and Vetterlein 2013), consisting mainly of neutral and acidic polysaccharides but also monomeric sugars. Minor components include amino acids, organic acids, (poly)uronic acids, phenolic acids, minerals, proteins, glycolipids and other phospholipids (Bacic et al. 1986;Brax et al. 2020;Read et al. 2003). The maize mucilage carbohydrate fraction was extensively analyzed and consists of fucose, galactose, glucose, arabinose, xylose, mannose, rhamnose, ribose as well as acidic galacturonic and glucuronic acid (Amicucci et al. 2019;Bacic et al. 1986;Chaboud 1983;Morel et al. 1986;Nazari et al. 2020;Osborn et al. 1999;Watanabe et al. 2008). ...
Purpose
Mucilage plays crucial roles in root-soil interactions. Collection systems for maize (Zea mays L.) use primary and seminal roots of aeroponically-grown seedlings (CSA), or brace roots of soil-grown plants (CSB). While each method represents specific plant developmental stages, and root types growing in specific (micro-)environments, these factors are rarely considered. It is unclear whether mucilage exhibits distinct physico-chemical properties related to collection system-inherent factors.
Methods
Mucilage of maize genotype B73 was collected from systems CSA and CSB. Chemical composition was assessed by pH, nutrient contents, neutral sugar composition, and polysaccharide polymer length. Viscosity, surface tension and contact angle represented physical properties.
Results
The share of hexoses among total polysaccharides was 11% higher in CSB than in CSA, whereas pentoses were predominant in CSA, together with higher nutrient concentrations and pH values. Mannose was detected only in CSB, which also exhibited higher surface tension, viscosity and contact angle compared to CSA.
Conclusions
Physico-chemical differences between the two mucilages are related to root type functions, environmental root growth conditions, and plant developmental state. Higher fractions of pentoses in CSA mucilage seem related to semi-sterile system conditions. Higher viscosity of CSB mucilage might reflect the need for enhanced water holding capacity of brace roots growing in drier conditions. A strong influence of environmental factors on mucilage properties even for a single genotype might play additional roles e.g. in the attraction of microbiomes. These aspects are relevant when assessing the role of mucilage in the rhizosphere, or when developing models of rhizosphere processes.
... Observations that mucilage turns hydrophobic upon drying (Ahmed et al. 2015) suggest that surface-active substances accumulate at the solid-air interface during drying (Naveed et al. 2019;Read et al. 2003). Their very low concentration makes Plant Soil their quantification challenging (Read et al. 2003) but Naveed et al. (2017) achieved a measure with gas chromatography and mass spectrometer (GC-MS) and comparison of their results with an international database revealed that hydroponic maize root mucilage contains around 120 μg fatty acids (g dry mucilage) −1 . ...
... Observations that mucilage turns hydrophobic upon drying (Ahmed et al. 2015) suggest that surface-active substances accumulate at the solid-air interface during drying (Naveed et al. 2019;Read et al. 2003). Their very low concentration makes Plant Soil their quantification challenging (Read et al. 2003) but Naveed et al. (2017) achieved a measure with gas chromatography and mass spectrometer (GC-MS) and comparison of their results with an international database revealed that hydroponic maize root mucilage contains around 120 μg fatty acids (g dry mucilage) −1 . Also dissolved organic matter in soil, which varies between 15 and 50 mg L −1 for arable soils contains between 30 to 60% little soluble substances such as plant debris (lignocellulose and lignin) and small surface-active humic substances (Blume et al. 2002). ...
... Also dissolved organic matter in soil, which varies between 15 and 50 mg L −1 for arable soils contains between 30 to 60% little soluble substances such as plant debris (lignocellulose and lignin) and small surface-active humic substances (Blume et al. 2002). Read et al. (2003) took lecithin as model for surfaceactive substances in mucilage and measured similar effects of lecithin as wheat RM on surface tension in the concentration range 0-1.0 mg mL −1 . Lipids are further supposed to decrease the surface tension of mucilage at the gas-liquid interface, which may help roots to extract more water from the surrounding soil (Read et al. 2003;Benard et al. 2019). ...
Aims
High viscosity, low surface tension and hydrophobicity are specific properties of maize root mucilage which contribute to modulate the spatial configuration of the liquid phase in soil pores. Several processes in the rhizosphere, in particularly nutrient absorption, root exudation and microbial activity, may cause strong temporal variations in the chemistry of the soil solution of the rhizosphere. Although the physical properties of maize root mucilage have been repeatedly measured in the last years, their variation upon a changing chemical environment and understanding of the chemical mechanisms governing these properties remain unexplored.
Methods
We investigated how flow and surface properties of maize root mucilage varied by changes in pH, calcium chloride (CaCl2) and lecithin concentrations.
Results
The physical properties of mucilage can strongly vary depending on the environmental conditions. Low surface tension of maize root mucilage at pH7 was increased by addition of calcium. Upon pH change and lecithin addition, hydrophobic mucilage turned hydrophilic. High Ca concentration above 0.83 mmol Ca (g dry mucilage)⁻¹, the addition of 167 μg lecithin (g dry mucilage)⁻¹ and a pH rise to 9 decreased the viscosity of mucilage.
Conclusion
Such variations strongly suggest that the role of mucilage in hydraulic processes in the rhizosphere depends on changes of solutes concentration and composition, which themselves vary according to plant growth and soil water content. It seems that mucilage can best serve as a hydraulic bridge only under certain chemical environments, whose spatio-temporal occurrence in the changing rhizosphere remains to be defined.
... In addition to affecting porosity, roots modify the rhizosphere hydraulic properties by exuding mucilage, which absorbs water and therefore increases the water content at a given matric potential (Carminati et al. 2010;McCully and Boyer 1997), while also lowering the surface tension of pore water, sometimes with exudates that turn hydrophobic upon drying onto soil particles (Moradi et al. 2011;Read et al. 2003). Indeed, imaging techniques showed that the soil next to the roots remained wetter than the bulk soil under drying around roots of Lupinus angustifolius (Garrigues et al. 2006), Pinus taeda (MacFall et al. 1990) and Hordeum vulgare (Segal 2008), while after rewetting the rhizosphere remained relatively dry in the proximity of roots in Cicer arietinum, Lupinus albus and Zea mays (Carminati et al. 2010;Moradi et al. 2011;Tumlinson et al. 2008). ...
... Laboratory studies demonstrated that specific root traits associated with the rhizosphere, such as exudate composition and root hairs, influence soil hydro-mechanical properties (Naveed et al. 2018;Read et al. 2003) and plant uptake of water and nutrients (Carminati et al. 2017;George et al. 2014). Treating sandy and clay loam textured soils with root exudates and exudate analogues (chiaseed mucilage) was found to enhance water repellency and mechanical hardness (Naveed et al. 2018). ...
... On a similarly structured soil near to this field experiment, a decrease in < 0.2 µm was observed over the growing season (Geris et al. 2021), despite a widespread assumption that these pores are affected by texture and organic matter so remain relatively static. This could be explained by the physical rearrangement of smaller soil pores to form mesopores, combined with the chemical properties of root exudates (Read et al. 2003). Since the volume of roots increased as the growing season progressed, it can also be hypothesised that, at 124 DAS, the loss of water stored in the roots played an increased influence on the overall water release curve of rooted soil. ...
Aims
Recent laboratory studies revealed that root hairs may alter soil physical behaviour, influencing soil porosity and water retention on the small scale. However, the results are not consistent, and it is not known if structural changes at the small-scale have impacts at larger scales. Therefore, we evaluated the potential effects of root hairs on soil hydro-mechanical properties in the field using rhizosphere-scale physical measurements.
Methods
Changes in soil water retention properties as well as mechanical and hydraulic characteristics were monitored in both silt loam and sandy loam soils. Measurements were taken from plant establishment to harvesting in field trials, comparing three barley genotypes representing distinct phenotypic categories in relation to root hair length. Soil hardness and elasticity were measured using a 3-mm-diameter spherical indenter, while water sorptivity and repellency were measured using a miniaturized infiltrometer with a 0.4-mm tip radius.
Results
Over the growing season, plants induced changes in the soil water retention properties, with the plant available water increasing by 21%. Both soil hardness (P = 0.031) and elasticity (P = 0.048) decreased significantly in the presence of root hairs in silt loam soil, by 50% and 36%, respectively. Root hairs also led to significantly smaller water repellency (P = 0.007) in sandy loam soil vegetated with the hairy genotype (-49%) compared to the hairless mutant.
Conclusions
Breeding of cash crops for improved soil conditions could be achieved by selecting root phenotypes that ameliorate soil physical properties and therefore contribute to increased soil health.
... Building on research exploring the chemistry of root mucilage, Read and Gregory [54] found that these compounds were highly surface active and viscous. By being surface active, root mucilage can decrease the surface tension of water by over 30%, with an expected easing of water capture from surrounding soil [55]. Viscous rhizodeposits, on the other hand, are more resistant to drainage. ...
... The surface activity of other rhizodeposits can help mitigate hydrophobicity, producing greater rewetting rates [43]. Experimental evidence using model rhizodeposits has suggested that they may also decrease water movement rates in dry soil [59], although much of this has been limited to sandy soils where this impact is exacerbated [55]. ...
... So, it is not just the chemical composition of rhizodeposits that improves root-soil interactions but also their physical properties, and this needs to be considered when exploring root traits. The viscosity and surface activity of rhizodeposits varies between plant species [54,55], resulting in different impacts to soil [60]. H. vulgare has a greater proportion of organic acids to sugars in its rhizodeposits than does Zea mays, resulting in a lower viscosity and greater surface activity [50]. ...
Great potential exists to harness plant traits at the root–soil interface, mainly rhizodeposition and root hairs, to ‘build’ soils with better structure that can trap more carbon and resources, resist climate stresses, and promote a healthy microbiome. These traits appear to have been preserved in modern crop varieties, but scope exists to improve them further because they vary considerably between genotypes and respond to environmental conditions. From emerging evidence, rhizodeposition can act as a disperser, aggregator, and/or hydrogel in soil, and root hairs expand rhizosheath size. Future research should explore impacts of selecting these traits on plants and soils concurrently, expanding from model plants to commercial genotypes, and observing whether impacts currently limited to glasshouse studies occur in the field.
... Lipids found in root exudates are principally phosphatidylcholines, also called lecithin, (Read et al., 2003) and are supposed to strongly affect surface properties of mucilage. Lecithin possesses a head charged with ammonium and phosphate groups and a tail which can be a variety of fatty acids. ...
... Lecithin possesses a head charged with ammonium and phosphate groups and a tail which can be a variety of fatty acids. Observations that mucilage turns hydrophobic upon drying (Ahmed et al., 2015) suggest that these surface-active substances accumulate at the solid-air interface during drying (Naveed et al., 2019;Read et al., 2003), although their concentration could not be measured until now (Read et al., 2003). Phospholipids are further supposed to decrease the surface tension of mucilage at the gas-liquid interface, which may help roots to extract more water from the surrounding soil . ...
... Lecithin possesses a head charged with ammonium and phosphate groups and a tail which can be a variety of fatty acids. Observations that mucilage turns hydrophobic upon drying (Ahmed et al., 2015) suggest that these surface-active substances accumulate at the solid-air interface during drying (Naveed et al., 2019;Read et al., 2003), although their concentration could not be measured until now (Read et al., 2003). Phospholipids are further supposed to decrease the surface tension of mucilage at the gas-liquid interface, which may help roots to extract more water from the surrounding soil . ...
High viscosity, low surface tension and hydrophobicity are specific properties of maize root mucilage which contribute to modulate the spatial configuration of the liquid phase in soil pores. Being a hotspot for nutrient absorption, root exudation and microbial activity, the rhizosphere soil solution is suspected to chemically vary strongly upon time. Although the physical properties of maize root mucilage have been repeatedly measured in the last years, their variation upon a changing chemical environment and understanding of the chemical mechanisms governing these properties remain unexplored. Therefore, we investigated how flow and surface properties of maize root mucilage varied by changes in pH, CaCl2 and lecithin concentrations.
Results reveal that the physical properties of mucilage can strongly vary depending on the environmental conditions. Low surface tension of maize root mucilage at pH7 was increased by addition of calcium. Upon pH change and lecithin addition, hydrophobic mucilage turned hydrophilic. Viscosity of mucilage decreased with increasing Ca concentration above 2.5 mM, the addition of 0.5 mg/L lecithin and a pH rise to 9. Such variations strongly suggest that the role of mucilage in hydraulic processes in the rhizosphere depends on changes of solutes concentration and composition, which themselves vary according to plant growth and soil water content. It seems that mucilage can best serve as a hydraulic bridge only under certain chemical environments, whose spatio-temporal occurrence in the changing rhizosphere remains to be defined.
... Root tip mucilage is a root exudate mainly composed of polysaccharides, lipids and proteins [16][17][18]. Several studies have characterised the polysaccharidic and lipid component of root mucilage [19][20][21] and both compound classes can facilitate the desorption of Pi from soil particles [20,22]. However, despite the abundance of proteins in mucilage (ca. ...
... Root tip mucilage is a root exudate mainly composed of polysaccharides, lipids and proteins [16][17][18]. Several studies have characterised the polysaccharidic and lipid component of root mucilage [19][20][21] and both compound classes can facilitate the desorption of Pi from soil particles [20,22]. However, despite the abundance of proteins in mucilage (ca. ...
... Mucilage was collected in a laminar-flow cabinet using a drawn Pasteur pipette as described previously [20,26] and stored on ice until a total volume of 1.5 ml of highly viscous mucilage was obtained. For comparison, we also sampled root tips, after mucilage removal. ...
Root secreted acid phosphatases and organic anions are widely perceived as major players of plant phosphorus (P) mobilisation from the rhizosphere under P limiting growth conditions. Previous research indicated that other mechanisms play a role, especially in species with fine roots, such as wheat. In this study we characterised the plant-derived extracellular proteome of wheat roots by profiling root tip mucilage, soluble root secreted and root tip proteomes. Extracellular acid phosphatases and enzymes of the central carbon metabolism were targeted using selected reaction monitoring. More than 140 proteins with extracellular localisation prediction were identified in mucilage. P starvation induced proteins predicted to be localised to the apoplast which are related to cell wall modification and defence in both, root tip and soluble root-secreted proteomes. Glycolytic enzymes were strongly increased in abundance by P limitation in root tips, as were PEPC and plastidial MDH. Soluble acid phosphatases were not identified in extracellular protein samples. Our results indicate that root tip mucilage contains proteins with the functional potential to actively shape their immediate environment by modification of plant structural components and biotic interactions. Wheat acid phosphatases appear to play a minor role in P mobilisation beyond the immediate root surface.
Significance
Phosphorus (P) is a plant growth limiting nutrient in many agricultural situations and the development of phosphorus efficient crops is of paramount importance for future agricultural management practices. As P is relatively immobile in soils, processes occurring at the root-soil interface, the rhizosphere, are suspected to play a key role in plant-induced P mobilisation. According to the current view, the secretion of extracellular acid phosphatases and organic anions enhances P mobilisation within several millimetres beyond the root surface, either directly or indirectly through the selection and appropriate soil microbes. However, the mechanisms of P mobilisation in species with fine roots, such as wheat, and the role of other secreted root proteins are poorly understood. Here, we carried out the profiling of wheat root tip mucilage, soluble root secreted and root tip proteomes. We analysed proteome changes in response to P starvation. We found that proteins with a predicted localisation to the apoplast made up a major proportion of stress-responsive proteins. Acid phosphatases were not identified within extracellular protein samples, which were enriched in proteins with predicted extracellular localisation. The absence of extracellular APases was further validated by multiple reaction monitoring. Our data indicates that wheat acid phosphatases play a minor role in P mobilisation beyond the immediate root surface and provides a resource for breeding strategies and further investigations of the functional roles of root tip-released proteins in the rhizosphere under P limitation.
... Previous studies have showed that higher concentrations of root mucilage resulted in greater viscosity (Benard et al., 2021;Naveed et al., 2019;Read & Gregory, 1997). Similar to synthetic hydrogels, soil-borne hydrogels form interconnected 3D networks thus can absorb and hold a large amount of water (Read et al., 2003;Roberson & Firestone, 1992). The hygroscopic property of hydrogels is also characterized by the more negative water potential of hydrogel solutions compared to DI water (Benard et al., 2021). ...
... The increased viscosity and more negative water potential were also reported for surrogates of bacterial EPS, for example, xanthan (Chenu, 1993), agarose (Strathmann et al., 2000), and PGA (Gessa & Deiana, 1990). On the other hand, soil-borne hydrogels often contain surface active substances such as phospholipids (Naveed et al., 2019;Read et al., 2003), nucleic acids, and lipids (Flemming & Wingender, 2010), which reduce the surface tension of soil solution and change the contact angle between the solid, liquid, and air phases. The magnitude of reduction in surface tension largely depends on the type of plant or bacteria strain that secretes such substances. ...
Soil‐borne hydrogels (e.g., bacterial extracellular polymeric substances, root‐derived mucilage) contain polymeric (i.e., gel‐like) substances as well as surface active substances, which modify the viscosity of soil fluid and wetting properties of soil particles, respectively. In this study, the individual and coupled effects of gel and surfactant on soil water retention were investigated experimentally using xanthan and Triton X‐100 as surrogates for gel and surfactant, respectively. Results show opposing effects that xanthan enhanced soil water retention because of increased solution viscosity and its hygroscopic property while surfactant reduced soil water retention due to reduced surface tension. We applied the modified Kosugi model that considers non‐capillary adsorption to describe the soil water retention curves measured in this study as well as additional curves reported in the literature. The model effectively captured the gel and surfactant effects using two model parameters, ω and μm, representing the weighting factor for capillary retention and the matric potential required to drain the effective mean pore diameter, respectively. Compared with the controls, gel‐dominated samples increased non‐capillary adsorption whereas surfactant‐dominant samples had a smaller (i.e., less negative) matric potential at the onset of drainage. Finally, we provided an explanation for enhanced water retention due to increased solution viscosity considering fluid displacement process. Our study improved the mechanistic understanding of soil water retention as affected by viscosity and surface tension individually as well as their coupled effects.
... Biosurfactants produced by microorganisms and plants are reported to modify the physical properties of soil. A team of researchers have (Read et al., 2003) measured the moisture release and hydraulic conductivity of Bullionfield soil (sandy loam, slightly acidic) after application of biosurfactant (lecithin). They found that the presence of biosurfactant reduced the equilibrium water content of the soil indicating higher release of water from soil colloids with subsequently increased absorption by the plant roots. ...
... When lecithin was exploited in two different types of soil, it was observed that lecithin decreased the amount of phosphate adsorption on soil and enhanced phosphate concentration in solution by 10%. Read et al. (2003) Nitrogen mineralization ...
Beverage or “drinks” inclusively stands for various consumable liquids, such as soft drinks, juices, milk derivatives, alcohols, beers, and hot liquids such as coffee, tea, hot chocolate, caffeinated drinks, etc. In contemporary times, various other types of beverages are produced for purposes such as immunization, personal care, and social celebrations, etc., with the main aim of compensation of thirst and balancing electrolytic equilibrium in the body system. With the evolution of health science, more consciousness regarding health and wellness has spread among people, and modification of traditional beverages is now a demand. Consumers around the globe are looking for drinks that are more sophisticated, more natural, and authentic. The definition of new generation food invokes more “green” and less synthetic foods for consumption, which includes lowering of addition of sugars, synthetic flavors, and chemicals. The demand for add-on benefits such as promotion of immunity, fitness, and overall wellness without altering taste is on top. To fulfill all these demands, the formulation of beverages needs a holistic understanding of science and technology of production and also alternative biomolecules that may replace the synthetic chemicals used in beverages. In this context, the most potential candidate in modern science has recognized is biosurfactants (BSs). BSs are termed as “new generation bio-chemicals,” because of their excellent properties, which include the least toxicity, effective environmental compatibility, efficient biodegradability, enhanced foaming property, advanced selectivity, specificity at exciting temperature, pH, and salinity; also, for their facility to be produced from renewable feedstock. In this book chapter, we have extensively discussed the application of both microbial-assisted BSs and plant-based BSs, in the production of various types of beverages. BSs act as emulsifiers, antioxidants, antimicrobials, and antiadhesives. Each application is discussed with scientific documentation and examples. The real-time commercial applications and their add-on benefits to promote human health have also been discussed in this book chapter. The recent market reports along with prospects of these BSs in the beverage industry have also been reported for the benefit of readers.
... Biosurfactants produced by microorganisms and plants are reported to modify the physical properties of soil. A team of researchers have (Read et al., 2003) measured the moisture release and hydraulic conductivity of Bullionfield soil (sandy loam, slightly acidic) after application of biosurfactant (lecithin). They found that the presence of biosurfactant reduced the equilibrium water content of the soil indicating higher release of water from soil colloids with subsequently increased absorption by the plant roots. ...
... When lecithin was exploited in two different types of soil, it was observed that lecithin decreased the amount of phosphate adsorption on soil and enhanced phosphate concentration in solution by 10%. Read et al. (2003) Nitrogen mineralization ...
Biosurfactants may be defined as molecules of amphiphilic nature having the capacity of decreasing both surface as well as interface tension between nonmiscible fluids. These are generally classified on the basis of their microbial origin and chemical composition. These are environment friendly molecules with lesser toxic effect and having higher biodegradation. These have full potential of replacing the chemical surfactants. The most important biosurfactants include Glycolipids and lipopeptides. Some other classes of biosurfactants are Phospholipids, fatty acids and Polymeric. These biosurfactants are manufactured throughout the microbial growth on both water soluble or insoluble substrates. These have widespread usage in bioremediation, pathogens management, medicines, cosmetics, and petroleum industry. Although, their usage is encouraging in bioremediation practices, their mass production on industrial basis is a tough task because of costly inputs low manufacturing outputs.
... Biosurfactants produced by microorganisms and plants are reported to modify the physical properties of soil. A team of researchers have (Read et al., 2003) measured the moisture release and hydraulic conductivity of Bullionfield soil (sandy loam, slightly acidic) after application of biosurfactant (lecithin). They found that the presence of biosurfactant reduced the equilibrium water content of the soil indicating higher release of water from soil colloids with subsequently increased absorption by the plant roots. ...
... When lecithin was exploited in two different types of soil, it was observed that lecithin decreased the amount of phosphate adsorption on soil and enhanced phosphate concentration in solution by 10%. Read et al. (2003) Nitrogen mineralization ...
The biosurfactants produced by the microbes are mostly glycolipid which
owing to their low toxicity, biodegradability shows a greater utility than
the chemical surfactants. With the enhanced use of biosurfactants the envi�ronment gets converted into a green environment. They apart from hav�ing these properties have the emulsifying capability, pore-forming ability and
antibiofilm forming abilities which have made them potent biopesticides.
Numerous reviews showed the efficiency of glycolipids. The antiphyto�pathogenic activities of glycolipids, the insecticidal and larvicidal activities
their antiadhesive properties have attracted the industries in producing
amplified amounts of these green compounds
... The intensive fluxes of organic compounds from roots into the rhizosphere stimulate microbial activity, and drive changes in microbiome structure and function, which, in turn, facilitate the availability of mineral nutrients by exuding low-and high-molecular-weight substances such as organic acids and viscous exopolysaccharides, commonly known as mucilage (Paterson et al., 2007;Jones et al., 2009;Gunina et al., 2014). These exudates, especially mucilage, not only modify nutrient availability in the rhizosheath, but also improve the mechanical stability of rhizosheaths by influencing their hydraulic properties (Read et al., 2003). Therefore, plants with rhizosheaths often form associations with soil microorganisms by exchanging greater amounts of substances between their roots and soil, thus allowing them to better cope with stress environments (Rabbi et al., 2018;Basirat et al., 2019). ...
... Although grassland types affected the soil physicochemical property, they hardly affected on the soil biological and stress-avoiding properties (Table S4). Mucilage, as a key parameter of stress-avoiding property, is a polymeric gel composed of polysaccharides and a small amount of lipids exuded by both roots and microorganisms (Read et al., 2003). The mucilage not only maintains a higher hydraulic conductivity at the root-soil interface, but also decreases P sorption by soil particles (Gaume et al., 2000). ...
Belowground plant traits play important roles in plant diversity loss driven by atmospheric nitrogen (N) deposition. However, the way N enrichment shapes plant microhabitats by patterning belowground traits and finally determines aboveground responses is poorly understood. Here, we investigated the rhizosheath trait of 74 plant species in seven N‐addition‐simulation experiments across multiple grassland ecosystems in China. We found that rhizosheath formation differed among plant functional groups and contributed to changes in plant community composition induced by N enrichment. Compared with forb species, grass and sedge species exhibited more distinct rhizosheaths; moreover, grasses and sedges expanded their rhizosheaths with increasing N‐addition rate which allowed them to colonize belowground habitats. Grasses also shaped a different microenvironment around their roots compared with forbs by affecting the physicochemical, biological and stress‐avoiding properties of their rhizosphere soil. Rhizosheaths act as a “biofilm‐like shield” by accumulating of protective compounds, carboxylic anions and polysaccharides, determined by both plants and microorganisms. This enhanced the tolerance of grasses and sedges to stresses induced by N enrichment. Conversely, forbs lacked the protective rhizosheaths which renders their roots sensitive to stresses induced by N enrichment, thus contributing to their disappearance under N‐enriched conditions. This study uncovers the processes by which belowground facilitation and trait matching affects aboveground responses under conditions of N enrichment, which advances our mechanistic understanding of the contribution of competitive exclusion and environmental tolerance to plant diversity loss caused by N deposition.
... Mucilage is a polymeric gel, mostly made up of polysaccharides from plants and microorganisms, which also contains a trace amount of lipids (Vermeer and McCully 1982;Read et al. 2003). McCully and Boyer (1997) reported that mucilage has a large water holding capacity. ...
Aims
Root exudates contain polymers that form crosslinks and can create a jelly like substance known as mucilage, which adheres to soil and thus promotes the formation of rhizosheaths, i.e. soil that remains attached to the roots after gentle shaking. We hypothesized that rhizosheath formation is optimal at an intermediate chia seed mucilage concentration and water content, but that its formation is limited at both a high concentration of chia seed mucilage and under dry conditions as well as at a low concentration of chia seed mucilage and under wet conditions. We used an artificial root soil system in which soil moisture and mucilage concentrations could be varied independently from one another with respect to their effect on rhizosheath formation.
Methods
Jute cords were disposed in sandy loam soil and in quartz sand. In a subsequent study, they were also amended to different moisture contents with five different concentrations of mucilage (from 0 to 0.2 g dry mucilage g⁻¹ water), before being isolated from chia and flaxseed mucilage after swelling of the respective seeds in distilled water for 15 min.
Results
We found that in dry soil, rhizosheath formation peaked at an intermediate chia seed mucilage concentration. This behavior was supported by our conceptual model of mucilage spreading and rhizosheath formation, which relies on a radial diffusion equation and assumes that at low mucilage concentration, molecule numbers are insufficient to support polymer-like networks that stick soil particles together. In a very concentrated gel, however, mucilage is too sticky to diffuse far into the soil. Increasing soil moisture promotes rhizosheath formation both in a low and a high mucilage concentration range, although only up to an intermediate volumetric water content of 0.15cm³ cm–3.
Conclusions
We conclude that both water and chia seed mucilage concentration are important drivers of rhizosheath formation. The effects are not additive but can combine to an optimum range, with a maximum formation of rhizosheaths observed in this study at 0.12 g mucilage g⁻¹ rhizosphere water.
... In our study, between the two time points, CK showed no change while CF showed improvement ( Fig. 2a; Tables S2 and S3), in which the changes in pH and AP were particularly pronounced. Studies have shown that stands with higher diversity of vegetation can accelerate the availability of N, P and K and promote the accumulation of AN, AP and AK in the soil (Read et al. 2003; Dybzinski et al. Table 1 Pearson's correlation analysis between soil physiochemical properties, biomass, enzyme activities and gene abundance * represents a significant correlation at the P < 0.05 level, ** represents a significant correlation at the P Plant Soil 2008). ...
Aims
Elucidating the mechanisms of understory vegetation response to forest management practices is critical to the impact of soil nutrient cycling. However, as common management practices, how thinning leading to the restoration of understory vegetation affects soil biochemical properties and microbial-mediated belowground processes and their main drivers remains unclear.
Methods
We investigated the differences of 16 S rRNA, nifH gene abundance, diversity and composition of vegetation restoration using a 5-year in situ experiment including Pinus massoniana-dominated forests (CK), virgin Cunninghamia lanceolata plantations (CF) and C. lanceolata plantations that have undergone a cycle of cutting and replanting (CS) in southern China.
Results
We found that understory vegetation restoration improved some soil physicochemical properties (e.g., AP) and enzyme activity. The abundance of nifH were higher in CF than in CK or CS. As a result of vegetation restoration, α-diversity of soil diazotrophic communities significantly increased in CF. These communities were clustered into different groups based on the forest type. The number of potential keystone species of diazotrophs was highest in CF, including several beneficial taxa (e.g., Paraburkholderia and Pelomonas). Most of the bacterial functional groups related to soil nitrogen cycling were also significantly elevated in CF, such as nitrogen fixers. Partial least squares path modelling (PLS-PM) further showed that soil diazotrophic communities were clearly shaped by soil nutrient content.
Conclusions
Overall, these results highlight the effects of vegetation restoration on soil diazotrophic communities and clarify the contributions of potential for improving edaphic conditions in areas of southern China with acidic red soils.
... However, Carminati et al. (2010) attributed this reduction to the dry mucilage, which induces hydrophobic properties in dry soils. Read et al. (2003) observed increased drying of the rhizosphere due to the smaller surface tension of the mucilage compared to water. It is argued that the slight increase in water repellency by maize root exudates may have little in uence on the ability of plants to extract water from the soil; however, this might vary depending on soil type and initial soil water content . ...
Background and Aim
To investigate the independent effects of soil mechanical resistance (SMR) on the properties of two maize cultivars (703 and 704), an experiment was conducted.
Methods
The SMR was manipulated by inducing soil compaction at five levels, resulting in different bulk densities (1.56, 1.6, 1.66, 1.69, and 1.71 Mg.m-3), along with cementation in a loamy sand soil. These manipulations were carried out while maintaining a constant matric potential.
Results
The studied plants responded to the increased soil mechanical resistance (SMR) by exuding mucilage, which effectively reduced the resistance to root penetration. However, when SMR was increased through compaction, it ultimately resulted in a decrease in root water uptake, particularly at bulk densities exceeding 1.6 Mg.m-3 (equivalent to SMR > 1.8 MPa). Furthermore, the cementation treatment and the increase in SMR consistently impacted plant function and properties, resulting in a continuous decrease in plant growth. The findings revealed a significant correlation between SMR, mucilage exudation by roots, and Total Root Water Uptake (TRWU) in the treatments examined.
Conclusions
As this study did not identify a specific threshold value of SMR that triggered an abrupt response in plant behavior, the exudation weight and water content of root mucilage can be considered as potential criteria for determining the critical limit of SMR. Since root exuded mucilage is influenced by both soil properties and plant conditions, analyzing its properties can provide valuable insights into SMR thresholds.
... It depends on the removal and reduction of petroleum hydrocarbons [34], as it depends on the presence of microorganisms in the area surrounding the root, which are stimulated by the root secretions on the bio-breaking process [35], and root secretions have a direct role in the breakdown of hydrocarbon compounds, as the root secretes external enzymes such as lactase and peroxidases, which break down polycyclic aromatic compounds found in fuel [36]. The root secretes lipophilic substances that increase the solubility of petroleum hydrocarbons and thus make petroleum hydrocarbons more available to microorganisms in the root [37]. Also, some microorganisms in the root have the ability to produce bio emulsifiers such as rhamnolipids, which lead to Increasing the solubility of organic pollutants so that microorganism cells can stick to organic pollutants [38][39][40]. ...
Water pollution with Kerosene is considered as a serious environmental problem in Basrah. Typha domingensis was used to treat kerosene-contaminated water in the construction wetland system. The Isolated bacteria was the most efficient in degradation Kerosene. Bacteria isolated from contaminated water by hydrocarbons of four stations in Basrah, these isolates were tested to show efficiency in degrading hydrocarbons to be used in phytoremediation of water contamination with hydrocarbons. Twelve bacterial isolates were identified as Bacillus sp. The Isolated bacteria (B11) was the most efficient in degradation oil hydrocarbons based on these used in the phytotoxicity test. The phytotoxicity experiment was conducted for 72 days in two glass basins containing water contaminated with 5% kerosene concentration with T. domingensis . The Isolate bacterial were added to two glass basins to test the interaction of the bacterial degradation with the plant and two contaminated ponds without plant and two with pollutant and bacteria without plant with control basin free from pollutant, the temperature between (20.6-36.2)°C, dissolved oxygen between (2.5-7.4) mg / L. The rate of kerosene removal from the water contaminated in the ponds of the plant without bacteria and plant with bacteria, bacteria only and control (76.5%), (81.9)%, (74.1%), (57.4) respectively.
... Root exudates refer to various substances secreted or released into the environment by different parts of the root system, and have been extensively examined in the underground root systems. The exudates from the underground roots mainly include sugars, amino acids, fatty acids, nucleotides, proteins and enzymes (Badri and Vivanco 2009;Baetz and Martinoia 2014;Bais et al. 2006;Czarnes et al. 2000a;Narasimhan et al. 2003;Read et al. 2003;Read and Gregory 1997), which can potentially attract beneficial microorganisms and affect the enrichment of the rhizosphere microbiota, thereby enhancing the ability of plants to adapt to the environment (Bulgarelli et al. 2013;Sigida et al. 2013;Zhalnina et al. 2018). Root exudates can also maintain the soil moisture in the rhizosphere and stabilise the structure of rhizosphere soil affected by the adsorption (Carminati 2012;Carminati et al. 2016;Czarnes et al. 2000b). ...
Aerial root mucilage can enhance nitrogen fixation by providing sugar and low oxygen environment to the rhizosphere microbiome in Sierra Mixe maize. Aerial root mucilage has long been documented in sorghum (Sorghum bicolor), but little is known about the biological significance, genotypic variation, and genetic regulation of this biological process. In the present study, we found that a large variation of mucilage secretion capacity existed in a sorghum panel consisting of 146 accessions. Mucilage secretion occurred primarily in young aerial roots under adequately humid conditions but decreased or stopped in mature long aerial roots or under dry conditions. The main components of the mucilage-soluble were glucose and fructose, as revealed by sugar profiling of cultivated and wild sorghum. The mucilage secretion capacity of landrace grain sorghum was significantly higher than that of wild sorghum. Transcriptome analysis revealed that 1844 genes were upregulated and 2617 genes were downregulated in mucilage secreting roots. Amongst these 4461 differentially expressed genes, 82 genes belonged to glycosyltransferases and glucuronidation pathways. Sobic.010G120200, encoding a UDP-glycosyltransferase, was identified by both GWAS and transcriptome analysis as a candidate gene, which may be involved in the regulation of mucilage secretion in sorghum through a negative regulatory mechanism.
... reference tars do not contain amounts of fatty acids similar to the artefacts). The most parsimonious explanation of fatty acids and alcohols in the two Königsaue artefacts is therefore that they result from soil contamination (Jambrina-Enríquez et al. 2019;Read et al. 2003) and cannot be used to make statements on the production technique (soil contamination is also supported by the presence of fatty alcohols C 28 and C 30 that are frequently derived from plant roots (Li et al. 2007) , Fig. 6a). The chromatogram of reference tar made with the condensation method contains polycyclic aromatic hydrocarbons of different families (including di, tri-and tetra-aromatics). ...
Unlabelled:
Birch tar is the oldest synthetic substance made by early humans. The earliest such artefacts are associated with Neanderthals. According to traditional interpretations, their study allows understanding Neanderthal tool behaviours, skills and cultural evolution. However, recent work has found that birch tar can also be produced with simple processes, or even result from fortuitous accidents. Even though these findings suggest that birch tar per se is not a proxy for cognition, they do not shed light on the process by which Neanderthals produced it, and, therefore, cannot evaluate the implications of that behaviour. Here, we address the question of how tar was made by Neanderthals. Through a comparative chemical analysis of the two exceptional birch tar pieces from Königsaue (Germany) and a large reference birch tar collection made with Stone Age techniques, we found that Neanderthals did not use the simplest method to make tar. Rather, they distilled tar in an intentionally created underground environment that restricted oxygen flow and remained invisible during the process. This degree of complexity is unlikely to have been invented spontaneously. Our results suggest that Neanderthals invented or developed this process based on previous simpler methods and constitute one of the clearest indicators of cumulative cultural evolution in the European Middle Palaeolithic.
Supplementary information:
The online version contains supplementary material available at 10.1007/s12520-023-01789-2.
... This would break the continuous water film in the soil pores, and thus reducing evaporative water loss from soil under drought conditions (Rabbi et al., 2021). On the other hand, the highly viscous mucilage can serve as a surfactant to reduce water surface tension by more than 30% (Read et al., 2003), which promotes water uptake by roots from the surrounding soil. It can also serve as a hydrogel to resist water drainage and improve water retention in rhizosheath soils . ...
Interactions between plant roots and the surrounding soils are crucial for plant growth and soil health. The rhizosheath, which is the sheath-like soil that is tightly attached to the plant root surface, is increasingly acknowledged to exhibit distinct features and critical environmental functions for the soil, plants, microbes, and ecosystems. However, not all features and functions have reached a consensus and the differentiation between the terms “rhizosheath” and “rhizosphere soils” remain ambiguous across literature. Here, we summarize the terminology, methodology, formation mechanisms, and findings of the signatures and functions of rhizosheaths via an integrated and quantitative approach. First, we propose a terminology framework of rhizosphere compartments, which allows for consistent definitions of different rhizosphere soils, and thus enhances comparability across studies. Further, we summarize the current knowledge of the mechanisms underlying rhizosheath formation. Moreover, based on quantitative analysis of previously reported results, we highlight the distinct features, such as high water, nutrient, and labile carbon contents, microbial biomass, and enzymatic activity of rhizosheath soils compared to other rhizosphere counterparts. We also discuss how these distinct features lead to the environmental and biogeochemical functions in sand fixation, soil aggregation, water uptake and retention, nutrient capture, carbon dynamics, and microbial colonization, and highlight their significance from micro- to ecosystem-scale. Finally, we propose future research perspectives to better understand and harness rhizosheath environmental benefits.
... the production of phytohormones and other molecules (Mawarda et al., 2020). The expansion of rapidly growing roots into unoccupied areas result in soil compression and alterations in soil pore size distribution (Guidi et al., 1985), creating new surfaces for microbial colonization and modifying the physicochemical soil habitat for microbes (Morel et al., 1991;Read et al., 2003). In addition, microbial inoculants can elevate soil nutrient contents in rhizosphere, creating conditions of high resource availability and environmental favorability. ...
Toward the restoration of the increasing numbers of abandoned mines across China, external-soil spray seeding technologies have become more extensively utilized. However, considerable challenges remain that seriously hamper the effectiveness of these technologies, such as inadequate nutrient availability for plants. Previous studies have shown that mineral-solubilizing microbial inoculants can increase the nodules of legumes. However, their effects on symbiotic nitrogen fixation (SNF), asymbiotic nitrogen fixation (ANF), and diazotrophic communities remain unknown. Further, research into the application of functional microorganisms for the restoration of abandoned mines has been conducted either in greenhouses, or their application in the field has been too brief. Thus, we established a four-year field experiment in an abandoned mine and quantified the SNF, ANF, and diazotrophic communities. To the best of our knowledge, this study is the first to describe the long-term application of specific functional microorganisms for the remediation of abandoned mine sites in the field. We revealed that mineral-solubilizing microbial inoculants significantly increased the soil ANF rate and SNF content. There was no significant correlation between the diazotrophic alpha diversity and soil ANF rate; however, there were strong positive associations between the relative abundance and biodiversity of keystone phylotype (module #5) within ecological clusters and the ANF rate. Molecular ecological networks indicated that microbial inoculants increased network complexity and stability. Moreover, the inoculants significantly enhanced the deterministic ratio of diazotrophic communities. Furthermore, homogeneous selection predominantly mediated the assembly of soil diazotrophic communities. It was concluded that mineral-solubilizing microorganisms played a critical role in maintaining and enhancing nitrogen, which offers a new solution with great potential for the restoration of ecosystems at abandoned mine sites.
... When there is a drought, both plants and microorganisms depend on mucilage, a polymeric material secreted from plant cells like root cap cells that are mostly composed of polysaccharides and contain only a minor number of phospholipids and proteins (Read et al., 2003). High viscosity, low surface tension, and the ability to absorb significant amounts of water because of the polysaccharide's hydroxyl groups are all thought to be vital for the activities of the rhizosphere since they mitigate the severe environmental shift (Carminati et al., 2017;Zarebanadkouki et al., 2019). ...
Abiotic stress poses a severe danger to agriculture since it negatively impacts cellular homeostasis and eventually stunts plant growth and development. Abiotic stressors like drought and excessive heat are expected to occur more frequently in the future due to climate change, which would reduce the yields of important crops like maize, wheat, and rice which may jeopardize the food security of human populations. The plant microbiomes are a varied and taxonomically organized microbial community that is connected to plants. By supplying nutrients and water to plants, and regulating their physiology and metabolism, plant microbiota frequently helps plants develop and tolerate abiotic stresses, which can boost crop yield under abiotic stresses. In this present study, with emphasis on temperature, salt, and drought stress, we describe current findings on how abiotic stresses impact the plants, microbiomes, microbe-microbe interactions, and plant-microbe interactions as the way microorganisms affect the metabolism and physiology of the plant. We also explore crucial measures that must be taken in applying plant microbiomes in agriculture practices faced with abiotic stresses.
... First, lipids (mainly phosphatidylcholines and saturated lipids) in root mucilage reduce the capillary force needed to mobilize H 2 PO 4 − that is adsorbed onto soil particles through intermolecular cohesion and adhesion forces. This phenomenon can increase the phosphate ion concentration in soil solution by 10% (Read et al., 2003). Second, the mucilage provides a moist layer around the roots that favors ion diffusion, increasing the likelihood that phosphate ions will enter the rhizosphere rather than remain in the bulk soil. ...
Root mucilage is a sticky polymer that is secreted through root border cells. Primarily composed of polysaccharides, with some protein and lipid components, mucilage causes soil particles to adhere to the roots, creating the characteristic rhizosheath that is visible around many plant root systems. The mucilage layer retains water, which is important for plants growing in water-limited environments. Plant signal molecules are transmitted through the mucilage to symbiotic and free-living soil microorganisms that promote plant growth. Furthermore, mucilage contains defensive compounds that can prevent infection by root pathogens. This chapter discusses multiple functions of root mucilage for plant health and resilience.
... En effet via des composés libérés par les racines vivantes, les plantes impactent la structure des communautés microbiennes, pouvant influencer en retour la dynamique des nutriments ou la capacité des plantes à résister aux maladies (Hartmann et al., 2009;Kuzyakov et Blagodatskaya, 2015;Carof et al., 2018). En plus d'interagir avec les organismes, les plantes peuvent intervenir dans de nombreux processus physico-chimiques du sol (Jastrow et al., 1998;Read et al., 2003;. Au coeur des interactions trophiques, l'effet des plantes sur les organismes du sol diffère en fonction de la quantité et de la composition de leur litière, de leurs traits fonctionnels et des composés libérés par leurs racines (Freschet et al., 2018). ...
L’utilisation des Fabacées dans les systèmes agricoles présente de nombreux avantages pour la préservation de l’environnement et le renforcement de la sécurité alimentaire. Parmi les nombreux services écosystémiques qu’ils fournissent, les Fabacées enrichissent le sol en carbone (C) et en azote (N) grâce à leur capacité à utiliser à la fois le C et l’N atmosphérique à travers la symbiose qu’elles forment avec les rhizobiacées et soutiennent ainsi les activités microbiennes rhizosphériques. Les travaux de cette thèse se sont intéressés à la fourniture de carbone et d’azote dans les sols à travers l’étude du processus de la rhizodéposition et de son impact sur la structure et l’activité des communautés microbiennes rhizosphériques. Ils ont porté sur deux Fabacées protéagineuses (pois et féverole) et deux Fabacées fourragères (trèfle blanc et trèfle incarnat) alors que le blé a été utilisé comme espèce non fixatrice d’N atmosphérique. Les objectifs de ce travail de thèse consistaient à évaluer (i) le flux de C et d’N de l'atmosphère vers le sol liés à la rhizodéposition, (ii) les conséquences sur la dynamique du C, de l’N et du phosphore (P) du sol par la mesure des activités enzymatiques et (iii) l’effet sur la structure des communautés microbiennes actives dans le sol. Les plantes ont été cultivées en conditions contrôlées. Une approche combinant le marquage au 13CO2 et la méthode de dilution au 15N du sol par l’N atmospherique a été mise en oeuvre pour mesurer la rhizodéposition des plantes tout au long de leur croissance en relation avec les traits fonctionnels. Un panel d’activités enzymatiques clefs en lien avec le cycle du C, de l’N et du P ont été mesurées et la technique de sondage isotopique 13C de l’ADN « DNA SIP » a été utilisée pour étudier le lien entre la rhizodéposition et la structure des communauté bactériennes et fongiques actives dans la rhizosphère. Nos résultats montrent que les rhizodéposition du C et de l’N sont fortement contrôlées par la photosynthèse et la fixation biologique de l’azote atmosphérique et en conséquence par la vitesse de croissance des plantes. Ils montrent également que la féverole et les deux trèfles possèdent de bonnes capacités de rhizodéposition du C et de l’N. D'autres traits liés à l'architecture des racines sont apparus comme de bons prédicteurs de la rhizodéposition des plantes. Nous avons également constaté que les espèces ayant présenté les quantités les plus importantes de rhizodépots présentent les niveaux d’activités enzymatiques les plus élevés dans le sol. Aussi, cet apport en nutriments N et C contribue aux besoins nutritionnels et à la croissance des microorganismes dans le sol. L’analyse de la structure des commuanutés actives montre que la féverole est associée à un microbiome rhizosphérique diversifié et enrichi en bactéries et champignons bénéfiques. Le determinisme des communautés actives est expliqué par la production de biomasse et la teneur en matière sèche des racines. Par ailleurs, l’analyse qualitative des exsudats devrait permettre de mieux comprendre l’impact de la rhizodéposition sur les microbiotes rhizosphèriques. Les comparaisons d’espèces observées ici en conditions contrôlées devront être validées par des approches complémentaires en plein champ.
... The interconnected network formed by EPS retains large quantities of water (Flemming and Wingender, 2001;McCully and Boyer, 1997;Read et al., 1999;Roberson and Firestone, 1992;Segura-Campos et al., 2014) and reduces soil hydraulic conductivity Kroener et al., 2018;Or et al., 2007;Rosenzweig et al., 2013;Zheng et al., 2018). Dissolved polymers also increase the viscosity of the soil solution Wingender, 2001, 2010;Naveed et al., 2017; and reduce the surface tension of liquids (Cooper et al., 1981;Raaijmakers et al., 2010;Read et al., 2003). All these features decelerate soil drying and reduce fluctuations in soil transport properties. ...
When soils dry, water flow and nutrient diffusion cease as the hydraulic microenvironments vital for soil life become fragmented. To delay soil drying locally and related adverse effects, bacteria and plants modify their surroundings by releasing extracellular polymeric substances (EPS). As a result, the physical properties of hotspots like biological soil crusts or the rhizosphere differ from those of the surrounding bulk soil. Specifically, the presence of EPS delays evaporative soil drying. Despite the evidence of reduced evaporation from EPS-amended soils, the mechanisms controlling soil water content dynamics remain elusive. Thus, our study aimed to elucidate the potential of bacteria to modify their local environment when exposed to oscillations in soil water content induced by evaporative drying. We incubated sand microcosms with two contrasting strains of Bacillus subtilis for one week in a flow cabinet. At the end of the incubation period, local water loss was quantified and spatially resolved using time-series neutron radiography. Strain NCIB 3610, a complex biofilm producer steadily modified soil evaporation dynamics during the incubation period resulting in a substantial delay in soil drying due to hydraulic decoupling of the evaporation front from the soil surface. Evaporation dynamics remained largely unaltered in the microcosms inoculated with the domesticated strain 168 trp⁺ compared to the control treatment. The mechanism of hydraulic decoupling induced by NCIB 3610 was verified by estimates of diffusive fluxes and the position of the evaporation plane in the microcosm. Additionally, the role of polymeric substances in hydraulic decoupling was confirmed by an evaporation experiment using xanthan as an EPS analogue.
... The portion of water-soluble PAH fractions and organic acid-soluble PAH fractions could be used to characterize bioavailability (Calace 2001;Mayer et al. 2007). Celery root exudates are rich in liposoluble substances, such as linolenic acid, linoleic acid, palmitic acid, etc. (Ji and Meng 2014;Meng 2013), and they improve surface activity and increase bioavailability of hydrophobic organic compounds (Read et al. 2003). A similar phenomenon on solubilization of celery root exudates has been reported in some studies (Ji and Meng 2014;Wei et al. 2017). ...
Polycyclic aromatic hydrocarbon (PAH) contaminated agricultural soil is widespread globally. To develop a cost-effective remediation technique to effectively degrade PAH contaminated agricultural soil without disturbing crop production, an in-situ co-remediation of PAH contaminated agricultural soil using blood meal and celery was assessed in an agricultural greenhouse. Results showed PAH dissipation rates increased significantly in agricultural soil when co-remediated by blood meal and celery. Planting celery and addition of 5 g•kg−1 blood meal removed 53.13% of PAHs within three months. Residual PAH concentrations in plant tissue were much lower than China Standard food limits. Results indicated the ability of celery to enhance bioavailability of PAHs, and create favorable conditions for microbial, rather than direct plant uptake, played a vital role in degradation of PAHs. Addition of blood meal significantly enhanced soil enzyme activity and PAH degradation, and increased PAH remediation rate with optimal addition of blood meal (5 g•kg−1). Celery enhanced bioavailability of PAHs. Blood meal increased soil enzyme activity and increased PAH degrading bacterial activity, which acted as the main remediation pathway to co-remediate PAHs, where celery was safe to eat. Therefore, a novel in-situ agricultural soil PAH remedial method, which did not disturb normal agricultural production during remediation is presented in this study.
... What has often been disregarded is the fact that growing roots also change other physical-chemical aspects of the environment in the vicinity that impact the organisms, including microorganisms, that inhabit this space. Roots not only produce organic substrates, but also alter the pH, and the concentration of inorganic compounds, including nitrogen (N) and phosphorus (P) compounds, as well as the physical and moisture conditions, thereby creating new voids (or barriers) (Read et al., 2003;Yu et al., 2013;Perez-Jaramillo et al., 2017;Reid et al., 2021). Although changes induced in the soil by roots have been extensively studied by soil physicists and chemists, as well as by plant physiologists, rarely have they involved the dynamics of the community of microbes present in the rhizosphere, the rhizomicrobiome. ...
The rhizosphere-associated microbiome impacts plant performance and tolerance to abiotic and biotic stresses. Despite increasing recognition of the enormous functional role of the rhizomicrobiome on the survival of wild plant species growing under harsh environmental conditions, such as nutrient, water, temperature, and pathogen stresses, the utilization of the rhizosphere microbial community in domesticated rice production systems has been limited. Better insight into how this role of the rhizomicrobiome for the performance and survival of wild plants has been changed during domestication and development of present domesticated crops, may help to assess the potential of the rhizomicrobial community to improve the sustainable production of these crops. Here, we review the current knowledge of the effect of domestication on the microbial rhizosphere community of rice and other crops by comparing its diversity, structure, and function in wild versus domesticated species. We also examine the existing information on the impact of the plant on their physico-chemical environment. We propose that a holobiont approach should be explored in future studies by combining detailed analysis of the dynamics of the physicochemical microenvironment surrounding roots to systematically investigate the microenvironment-plant-rhizomicrobe interactions during rice domestication, and suggest focusing on the use of beneficial microbes (arbuscular mycorrhizal fungi and Nitrogen fixers), denitrifiers and methane consumers to improve the sustainable production of rice.
... Mucilage. Mucilage is a polymeric substance exudated from plant cells such as root cap cells, which mainly consists of polysaccharides as well as a small fraction of proteins and phospholipids and plays an important role for plants and microbes during drought (Bacic et al. 1986;Read et al. 2003). The biophysical properties such as capacity for absorbing large amount of water due to the hydroxyl groups of polysaccharide (Sealey et al. 1995), low surface tension, and high viscosity (Read and Gregory 1997) are considered to be important for rhizosphere functions by attenuating the drastic environmental change (Ahmed et al. 2014;Carminati et al. 2010Carminati et al. , 2017Zarebanadkouki et al. 2019). ...
Abiotic stress adversely affects cellular homeostasis and ultimately impairs plant growth, posing a serious threat to agriculture. Climate change modeling predicts increasing occurrences of abiotic stresses such as drought and extreme temperature, resulting in decreasing the yields of major crops such as rice, wheat, and maize, which endangers food security for human populations. Plants are associated with diverse and taxonomically structured microbial communities that are called the plant microbiota. Plant microbiota often assist plant growth and abiotic stress tolerance by providing water and nutrients to plants and modulating plant metabolism and physiology and, thus, offer the potential to increase crop production under abiotic stress. In this review, we summarize recent progress on how abiotic stress affects plants, microbiota, plant-microbe interactions, and microbe-microbe interactions, and how microbes affect plant metabolism and physiology under abiotic stress conditions, with a focus on drought, salt, and temperature stress. We also discuss important steps to utilize plant microbiota in agriculture under abiotic stress.
[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .
... Lipids play a part in the hydrophobicity of EPS and help microorganisms adhere to waxy, plastic (e.g., Teflon), and pyrite surfaces (Neu and Poralla, 1988;Neu et al., 1992). Similar functions of lipids were described for mucilage, e.g., they control mucilage hydrophobicity and thus the interaction of mucilage with soil solids, water, and transported ions (Read et al., 2003;Chen and Arye, 2017;Nazari, 2021). Mucilage turns hydrophobic upon drying , a process that causes water repellency in the rhizosphere and prevents hydraulic failure in the rhizosphere under drought (Carminati, 2013;Zickenrott et al., 2016). ...
Mucilage is a gelatinous high-molecular-weight substance produced by almost all plants, serving numerous functions for plant and soil. To date, research has mainly focused on hydraulic and physical functions of mucilage in the rhizosphere. Studies on the relevance of mucilage as a microbial habitat are scarce. Extracellular polymeric substances (EPS) are similarly gelatinous high-molecular-weight substances produced by microorganisms. EPS support the establishment of microbial assemblages in soils, mainly through providing a moist environment, a protective barrier, and serving as carbon and nutrient sources. We propose that mucilage shares physical and chemical properties with EPS, functioning similarly as a biofilm matrix covering a large extent of the rhizosphere. Our analyses found no evidence of consistent differences in viscosity and surface tension between EPS and mucilage, these being important physical properties. With regard to chemical composition, polysaccharide, protein, neutral monosaccharide, and uronic acid composition also showed no consistent differences between these biogels. Our analyses and literature review suggest that all major functions known for EPS and required for biofilm formation are also provided by mucilage, offering a protected habitat optimized for nutrient mobilization. Mucilage enables high rhizo-microbial abundance and activity by functioning as carbon and nutrient source. We suggest that the role of mucilage as a biofilm matrix has been underestimated, and should be considered in conceptual models of the rhizosphere.
... The metabolites (i.e., rhizodeposits) that are released in soil either by the microbes or by the roots, can be present in soil water, readily utilized by microbes or adsorbed on the soil matrix (Fischer et al., 2010;Keiluweit et al., 2015). While dissolved in water the metabolites, such as phospholipids and betaine, can reduce the surface tension of water (Rabbi et al., 2021;Read et al., 2003). This lowering of surface tension may become highly beneficial for plants growing in drought conditions. ...
The development of crops better adapted to endure weather extremes requires knowledge of the belowground traits that have potential to improve plant water uptake. Recent evidence has indicated that root induced modification of soil pore geometry of the rhizosheath is linked with drought tolerance of crops. Here we sought to understand whether the regulation of plant water uptake by rhizosheath is mediated through the rhizodeposits present at the soil-root interface. We compared eight wheat cultivars and demonstrated that cultivars with longer root hairs and greater rhizosheath mass and cover transpired more water. The wheat cultivars with less rhizosheath had the highest concentration of trehalose in the rhizosheath. Trehalose may increase the surface tension of soil water, which could further modify the transpiration of cultivars with less rhizosheath. Moreover, rhizosheath mass and cover had significant negative association with the bacterial abundance, indicating a link between metabolite concentration and bacterial abundance in the rhizosheath. We propose that the complex interactions of root, microbes and rhizodeposits in the rhizosheath have the potential to regulate water uptake by crops. The root and rhizosheath traits highlighted here can be targeted to develop crops with better ability to uptake water.
... G. Bengough et al., 1997). In addition, mucilage or rhizodeposits may alter the mechanical properties around the root tip including hydraulic properties of the rhizosphere (Carminati et al., 2010;Read et al., 2003;Whalley et al., 2005). Roots produce more border cells and mucilage in mechanically impeded soils (Barber & Gunn, 1974;Iijima et al., 2000). ...
Mechanical impedance constrains root growth in most soils. Crop cultivation changed the impedance characteristics of native soils, through topsoil erosion, loss of organic matter, disruption of soil structure, and loss of biopores. Increasing adoption of Conservation Agriculture in high-input agroecosystems is returning cultivated soils to the soil impedance characteristics of native soils, but in the low-input agroecosystems characteristic of developing nations, ongoing soil degradation is generating more challenging environments for root growth. We propose that root phenotypes have evolved to adapt to the altered impedance characteristics of cultivated soil during crop domestication. The diverging trajectories of soils under Conservation Agriculture and low-input agroecosystems has implications for strategies to develop crops to meet global needs under climate change. We present several root ideotypes as breeding targets under the impedance regimes of both high-input and low-input agroecosystems, as well as a set of root phenotypes that should be useful in both scenarios. We argue that a ‘whole plant in whole soil’ perspective will be useful in guiding the development of future crops for future soils.
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... All rights reserved. dynamics at the root soil interface as constituents of mucilage (McCully, 1999;Read et al., 2003). Other depleted fatty acids, such as octadecenoic acids, can be found in root hairs of Glycine max (a relative of lupin within Fabaceae), where they are thought to be involved in interactions with symbiotic rhizobacteria (Brechenmacher et al., 2010). ...
Soil contamination with toxic metalloids, such as arsenic, can represent a substantial human health and environmental risk. Some plants are thought to tolerate soil toxicity using root exudation, however, the nature of this response to arsenic remains largely unknown. Here, white lupin plants were exposed to arsenic in a semi-hydroponic system and their exudates were profiled using untargeted liquid chromatography tandem mass spectrometry. Arsenic concentrations up to 1 ppm were tolerated and led to accumulation of 12.9 μg As g⁻¹ dry weight (DW) and 411 μg As g⁻¹ DW in aboveground and belowground tissues, respectively. From 193 exuded metabolites, 34 were significantly differentially abundant due to 1 ppm arsenic, including depletion of glutathione disulphide and enrichment of phytochelatins and coumarins. Significant enrichment of phytochelatins in exudates of arsenic-treated plants was further confirmed using exudate sampling with strict root exclusion. The chemical tolerance toolkit in white lupin included nutrient acquisition metabolites as well as phytochelatins, the major intracellular metal-binding detoxification oligopeptides which have not been previously reported as having an extracellular role. These findings highlight the value of untargeted metabolite profiling approaches to reveal the unexpected and inform strategies to mitigate anthropogenic pollution in soils around the world.
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Introduction
Strawberry ( Fragaria × ananassa Duch.) holds a preeminent position among small fruits globally due to its delectable fruits and significant economic value. However, strawberry cultivation is hampered by various plant diseases, hindering the sustainable development of the strawberry industry. The occurrence of plant diseases is closely linked to imbalance in rhizosphere microbial community structure.
Methods
In the present study, a systematic analysis of the differences and correlations among non-culturable microorganisms, cultivable microbial communities, and soil nutrients in rhizosphere soil, root surface soil, and non-rhizosphere soil of healthy and diseased strawberry plants affected by root rot was conducted. The goal was to explore the relationship between strawberry root rot occurrence and rhizosphere microbial community structure.
Results
According to the results, strawberry root rot altered microbial community diversity, influenced fungal community composition in strawberry roots, reduced microbial interaction network stability, and enriched more endophytic-phytopathogenic bacteria and saprophytic bacteria. In addition, the number of bacteria isolated from the root surface soil of diseased plants was significantly higher than that of healthy plants.
Discussion
In summary, the diseased strawberry plants changed microbial community diversity, fungal species composition, and enriched functional microorganisms significantly, in addition to reshaping the microbial co-occurrence network. The results provide a theoretical basis for revealing the microecological mechanism of strawberry root rot and the ecological prevention and control of strawberry root rot from a microbial ecology perspective.
Background
Phosphorus (P) is an essential nutrient for plant growth, taking part in primary cellular metabolic processes as a structural component of key biomolecules. Soil processes as adsorption, precipitation, and coprecipitation can affect P bioavailability, leading to limited plant growth and excessive use of P fertilizers, with adverse impacts on the environment and progressive depletion of P reserves. To cope with P stress, plants undergo several growth, development, and metabolic adjustments, aimed at increasing P-acquisition and -utilization efficiency. Recently, strigolactones (SLs) have emerged as newly defined hormones that mediate multiple levels of morphological, physiological and biochemical changes in plants as part of the P acclimation strategies to optimize growth. Therefore, understanding the soil processes affecting P availability and P acquisition strategies by plants can contribute to improved agronomical practices, resources optimization and environmental protection, and the development of plants with high P use efficiency for enhanced agricultural productivity.
Scope
In this review, we discuss the range of abiotic processes that control P retention in soil and how different concentrations or degrees of P bioavailability can trigger various responses in plants, while critically highlighting the inconsistent conditions under which experiments evaluating aspects of P nutrition in plants have been conducted. We also present recent advances in elucidating the role of SLs in the complex P signalling pathway, with a special focus on what has been discovered so far in the model plant tomato (Solanum lycopersicum L.).
Plant growth‐promoting rhizobacteria and other soil bacteria have the potential to improve soil hydro‐physical properties and processes through the production of extracellular polymeric substances (EPS). However, the mechanisms by which EPS mediates changes in soil properties and processes remain incompletely understood, partly due to variations in EPS composition produced under different environmental conditions. In this study, we investigated the influence of different bacterial traits on intrinsic soil properties and processes of evaporation and infiltration using sand treated with the wild‐type Bacillus subtilis variant (UD1022) and its two mutant variants, – and srf . The – mutant suppresses EPS production through alterations in the eps and tasA genes, while the srf mutant lacks the gene for surfactin production. Experimental results confirmed that the solution viscosity of the – mutant was the lowest and the solution surface tension of the srf mutant was the highest among the three tested bacteria strains. The distinct intrinsic properties of EPS produced by these bacterial strains resulted in varied hydro‐physical responses in the treated sand. Key influences included modifications in wettability, hydraulic decoupling (or mixed wettability), and aggregation, which collectively led to reduced evaporation rates and heterogeneous water distribution during infiltration in the bacteria‐treated sands. Our findings advance the understanding of the role bacterial EPS play in vadose zone hydrology and offer insights for the development of sustainable strategies for increasing water retention, supporting crop production in arid regions, and facilitating land restoration.
Mucilage, a polysaccharide-containing hydrogel, is hypothesized to play a key role in the rhizosphere as a self-organized system because it may vary its supramolecular structure with changes in the surrounding solution. However, there is currently limited research on how these changes are reflected in the physical properties of real mucilage. This study examines the role of solutes in maize root, wheat root, chia seed, and flax seed mucilage in relation to their physical properties. Two purification methods, dialysis and ethanol precipitation, were applied to determine the purification yield, cation content, pH, electrical conductivity, surface tension, viscosity, transverse ¹H relaxation time, and contact angle after drying of mucilage before and after purification. The two seed mucilage types contain more polar polymers that are connected to larger assemblies via multivalent cation crosslinks, resulting in a denser network. This is reflected in higher viscosity and water retention ability compared to root mucilage. Seed mucilage also contains fewer surfactants, making them better wettable after drying compared to the two root mucilage types. The root mucilage types, on the other hand, contain smaller polymers or polymer assemblies and become less wettable after drying. However, wettability not only depends on the amount of surfactants but also on their mobility, as well as the strength and mesh size of the network structure. The changes in physical properties and cation composition observed after ethanol precipitation and dialysis suggest that the polymer network of seed mucilage is more stable and specialized in protecting the seeds from unfavorable environmental conditions. In contrast, root mucilage is characterized by fewer cationic interactions and its network relies more on hydrophobic interactions. This allows root mucilage to be more flexible in responding to changing environmental conditions, facilitating nutrient and water exchange between root surfaces and the rhizosphere soil.
Graphical Abstract
Sulfonamides (SAs) in agricultural soils can be degraded in rhizosphere, but can also be taken up by vegetables, which thereby poses human health and ecological risks. A glasshouse experiment was conducted using multi-interlayer rhizoboxes to investigate the fate of three SAs in rape and hot pepper rhizosphere soil systems to examine the relationship between the accumulation and their physicochemical processes. SAs mainly entered pepper shoots in which the accumulation ranged from 0.40 to 30.64 mg kg-1, while SAs were found at high levels in rape roots ranged from 3.01 to 16.62 mg kg-1. The BCFpepper shoot exhibited a strong positive linear relationship with log Dow, while such relationship was not observed between other bioconcentration factors (BCFs) and log Dow. Other than lipophilicity, the dissociation of SAs may also influence the uptake and translocation process. Larger TF and positive correlation with log Dow indicate preferential translocation of pepper SAs. There was a significant (p < 0.05) dissipation gradient of SAs observed away from the vegetable roots. In addition, pepper could uptake more SAs under solo exposure, while rape accumulated more SAs under combined exposure. When SAs applied in mixture, competition between SAs might occur to influence the translocation and dissipation patterns of SAs.
Biogeoscience is a rapidly growing interdisciplinary field that aims to bring together biological and geophysical processes. This book builds an enhanced understanding of ecosystems by focusing on the integrative connections between ecological processes and the geosphere, hydrosphere and atmosphere. Each chapter provides studies by researchers who have contributed to the biogeoscience synthesis, presenting the latest research on the relationships between ecological processes, such as conservation laws and heat and transport processes, and geophysical processes, such as hillslope, fluvial and aeolian geomorphology, and hydrology. Highlighting the value of biogeoscience as an approach to understand ecosystems, this is an ideal resource for researchers and students in both ecology and the physical sciences.
Global industrialization has served as an era of the expedition of natural resources for experimenting with numerous advanced level technologies and has led to high-end commercial products like biosurfactants. The environmental issues such as yield of plants, productivity, heavy metal contamination in the soil or water, quality of food, etc. have posed a serious threat to mankind and thus there is an urgency to develop effective approaches to mitigate them. Biosurfactants are defined as amphiphilic molecules with a partition at liquid-liquid, liquid-gas, and liquid-solid interfaces, respectively. Challenges like agricultural stresses (biotic and abiotic) and issues of food security are associated with economic factors and need optimum reliability, effective exploitation of resources, least impact on the ecosystem during the production of food. There is a dire need to look into an eco-friendly approach which employs biological agent (i) to pacify or manage both biotic and abiotic stresses, (ii) to reinforce the natural defense mechanism of the plant via enhancing disease resistance. The production of biosurfactants is one of the significant technologies in the 21st century. Biosurfactants have an intensely positive effect on the environment issues prevailing all over the globe such as less toxicity, biodegradable, and it is considered important to commence sustainable industrial processes like exploitation of renewable resources or green products. The chapter renders an intense knowledge about biosurfactants piled up from the already documented reports and advancements made in this field for enhancing the bioavailability of nutrients for maintaining plant health.
Background and aim
Nature-based solutions to engineering challenges are essential to limit climate change impacts on the urban environment. Quantitative understanding of multiple “engineering functions” provided by soil-plant interactions of different species is needed for species selection and re-establishing natural processes affected by urbanisation.
Methods
Contrasting herbaceous species (legumes, grasses, and forbs) were selected and grown as monoculture or species mix in soil columns for a five-month growing season. Saturated hydraulic conductivity was initially tested for each column, and then the columns were monitored for three-weeks of evapotranspiration. Water loss, matric suction, and penetrometer resistance were measured. Finally, soil was tested for aggregate stability and water retention.
Results
Saturated hydraulic conductivity of vegetated soil was generally larger than that of fallow soil (6.9e⁻⁶ ± 1.4e⁻⁶ m/s in fallow soil). Saturated hydraulic conductivity was significantly different between species (e.g., from 9.9e⁻⁶ ± 1.3e⁻⁶ m/s in Festuca ovina to 3.9e⁻⁵ ± 1.2e⁻⁶ m/s in Lotus corniculatus) and was negatively correlated with specific root length. The water stored in the soil was efficiently removed by plant transpiration (> 60% of evapotranspiration). Large changes in soil structure were observed in vegetated soil, with significant increases in soil strength, aggregate stability, and alteration of water retention properties.
Conclusions
Multiple soil-plant interactions influence species selection for optimising nature-based solutions (e.g., bioretention barriers). Substantial scope exists to choose species mixes to manipulate soil hydro-mechanical properties. Enhanced biodiversity did not compromise the engineering services of nature-based solutions (e.g., water removal), and may have multiple benefits.
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.
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Understanding phosphorus (P) dynamics in the rhizosphere is crucial for sustainable crop production. P mobilization processes in the rhizosphere include the release of plant and microbially-derived protons and extracellular phosphatases. We investigated the effect of root hairs and soil texture on the spatial distribution and intensity of P mobilizing processes in the rhizosphere of Zea mays L. root-hair defective mutant (rth3) and wild-type (WT) grown in two substrates (loam, sand). We applied 2D-chemical imaging methods in custom-designed root windows installed in the field to visualize soil pH (optodes), acid phosphatase activity (zymography), and labile P and Mn fluxes (diffusive gradients in thin films, DGT).
The average rhizosphere extent for phosphatase activity and pH was greater in sand than in loam, while the presence of root-hairs had no impact. Acidification was significantly stronger at young root tissue (<2 cm from root cap) than at older root segments (>4 cm from root cap) and stronger in WT than rth3. Accompanied with stronger acidification, higher P flux was observed mainly around young, actively growing root tissues for both genotypes. Our results indicate that acidification was linked to root growth and created a pH optimum for acid phosphatase activity, i.e., mineralization of organic P, especially at young root tissues which are major sites of P uptake. Both genotypes grew better in loam than in sand; however, the presence of root hairs generally resulted in higher shoot P concentrations and greater shoot biomass of WT compared to rth3. We conclude that soil substrate had a larger impact on the extent and intensity of P solubilization processes in the rhizosphere of maize than the presence of root hairs. For the first time, we combined 2D-imaging of soil pH, phosphatase activity, and nutrient gradients in the field and demonstrated a novel approach of stepwise data integration revealing the interplay of various P solubilizing processes in situ.
Plant roots and bacteria alter the soil physical properties by releasing polymeric blends into the soil pore space (e.g., extracellular polymeric substances and mucilage). The physical mechanisms by which these substances interact with the soil matrix and alter the spatial configuration of the liquid phase and the related hydraulic properties remain unclear. Here, we propose a theory to explain how polymer solutions form one‐dimensional filaments and two‐dimensional interconnected structures spanning across multiple pores. Unlike water, primarily shaped by surface tension, these polymeric structures remain connected during drying due to their high viscosity. The integrity of one‐dimensional structures is explained by the interplay of viscosity and surface tension forces (elegantly characterized by the Ohnesorge number), while the formation of two‐dimensional structures requires consideration of the interaction of the polymer solution with the solid surfaces and external drivers (e.g., drying rate). During drying, the viscosity of the liquid phase increases and at a critical point, when the friction between polymers and solid surfaces overcomes the water absorption of the polymers, the concentration of the polymer solution at the gas‐liquid interface increases asymptotically. At this critical point, polymers are deposited as two‐dimensional surfaces, such as hollow cylinders or interconnected surfaces. A model is introduced to predict the formation of such structures. Viscosity of the soil solution, specific soil surface, and drying rate are the key parameters determining the transition from one‐to two‐dimensional structures. Model results are in good agreement with observed structures formed in porous media during drying.
By altering the physical properties of soil through root activity, plants can act as important agents in affecting soil hydrothermal properties. However, we still know little about how plant roots regulate these properties in certain ecosystems, such as alpine meadows. Thus, we studied the influence of roots on soil hydrothermal properties in the Qinghai-Tibet Plateau (QTP). Root biomass as well as soil physicochemical and hydrothermal properties were examined at a depth of 0–30 cm at three study sites in the QTP. The relationship between root biomass and saturated soil hydraulic conductivity (Ks) was examined, as was the applicability of common soil hydrothermal properties models to the alpine meadow system. Results revealed that approximately 91.10%, 72.52%, and 76.84% of root biomass was located in the top 0–10 cm of soil at Maqu, Arou, and Naqu, respectively. Compared with the bulk soil, the water-holding capacity of rhizosphere soil was enhanced by 20 %–50%, while Ks was decreased by at least 2- to 3-fold. The thermal conductivity (λ) of rhizosphere soils was lower than that of the bulk soil by 0.23–0.82 W m⁻¹∙K⁻¹ on average. Lastly, soil hydrothermal properties models that do not explicitly consider root effects overestimated the Ks and λ in the rhizosphere soil of these systems. Overall, our results revealed distinctive differences in soil hydrothermal properties between the rhizosphere soil and the bulk soil in the QTP. This research has important implications for future modeling of soil hydrothermal processes of alpine meadow soils.
Human red cell lipids have been studied in a series of normal individuals. Cholesterol comprises 25% of total red cell lipid, and free fatty acid is present. Evidence is presented that the total amount of lipid phosphorus per average red cell is 1.40 × 10⁻¹¹ mg and that most extraction procedures fail to extract 8% or more (some as much as 40–50%) of red cell phospholipid.
The average percentage distribution of the individual phospholipids was: choline glycerophosphatides 30% sphingomyelin 24%, ethanolamine glycerophosphatides 26%, and serine glycerophosphatides 15%. Other minor phospholipid components were also identified. The normal range for total red cell plasmalogen was found to be 4.65–5.85 × 10⁻¹¹ μmole/cell. Evidence for the presence of a carbohydrate-containing lipid which was eluted from silicic acid columns with serine and inositol glycerophosphatides is also given.
The fatty acid distribution of each major phospholipid fraction is characteristic. Ethanolamine glycerophosphatides are high in 20:4, 22:5, and 22:6, while serine and inositol glycerophosphatides contain large amounts of 18:0 and 20:4. Lecithin is distinguished by 20–25 moles % 18:2 and sphingomyelin by high concentrations of 24:0 and 24:1.
The changes in fatty acid composition of maize leaf lipids caused by EPTC were generally similar to known effects of this herbicide in other plants: decreasing of linolenic acid content and increasing of its precursors, palmitic, stearic, oleic and linoleic acids. However, novel effects were detected in roots where the proportion of minor fatty acid palmitoleic acid was increased from 2.1 to 7.6 and 16.6% by EPTC and EPTC + dichlormid treatments, respectively. Simultaneously, the phospholipid content of root lipids was increased by both EPTC as well as EPTC + dichlormid treatments. The possible effects of EPTC and dichlormid on lipid biosynthesis of maize are discussed.
A new and relatively simple equation for the soil-water content-pressure head curve is described. The particular form of the equation enables one to derive closed-form analytical expressions for the relative hydraulic conductivity, when substituted in the predictive conductivity models of N. T. Burdine or Y. Mualem. The resulting expressions contain three independent parameters which may be obtained by fitting the proposed soil-water retention model to experimental data. Results obtained with the closed-form analytical expressions based on the Mualem theory are compared with observed hydraulic conductivity data for five soils with a wide range of hydraulic properties.
Surfactant solutions are being proposed for in situ flushing of organic contaminants from soils and aquifers. The feasibility of surfactant additives in remediation may depend in large part on how these chemicals affect the hydraulic conductivity of the porous media. While there is evidence in the literature of conductivity loss during surfactant flushing (Miller et al. 1975; Nash et al. 1987), there has been little research on quantifying the process for unconsolidated sediments. Surfactant-affected hydraulic conductivity reductions were measured in two soils (Teller loam and Daugherty sand). Testing was done with eight surfactants at a variety of concentrations (10-5 to 10-l mole/kg), surfactant mixtures, and added solution electrolytes. The Teller was also tested with its organic matter removed. Maximum hydraulic conductivity decreases were 47 percent for the sand and more than two orders of magnitude for the loam. Surfactant concentrations, surfactant mixtures, soil organic content, and added solution electrolytes all affected the degree of conductivity reduction. Results indicate that surfactant-affected hydraulic conductivity losses should be considered prior to in situ remediation and may preclude surfactant use in some fine grain soils.
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Dawes, Clinton J., and Edwin Bowler. (U. of California, Los Angeles.) Light and electron microscope studies of the cell wall structure of the root hairs of Raphanus sativus. Amer. Jour. Bot. 46(8): 561–565. Illus. 1959.—The structure and development of the cell wall of the root hair of Raphanus sativus were studied under the light and electron microscopes. The outer layer of the root hair consists of mucilage which covers the entire hair and forms a thick cap at the tip. Beneath the mucilage a thin cuticle covers the inner layers of the cell wall. These layers consist of cellulose microfibrils, varying in pattern, in a granular matrix, presumably pectic in nature. The microfibrils of the outer layer, apparently laid down at the tip, are reticulate in arrangement. In mature regions of the root hair, the wall is thickened by an inner layer of parallel and longitudinally orientated microfibrils. Pores in the cellulose wall are evident and increase in number and size near the base of the hair.
This thoroughly updated edition continues to provide a concise overall coverage of colloid and surface chemistry, intermediate between the brief accounts in physical chemistry textbooks and the comprehensive coverage in specialized treatises. New information is included on the composition and structure of solid surfaces, dynamic light scattering, micro emulsions and colloid stability control. The book provides a sound, but easy to follow theoretical framework. It outlines relevant research techniques and considers technological applications. A basic knowledge of the principles of physical chemistry is assumed. It will appeal to a wide readership, both undergraduate and postgraduate students at universities and colleges of technology as well as scientists in industry who need a broad background in the subject.
In many plants, mucilage permeates the interface between root and soil, yet little is known about its physical properties or its influence on the physical properties of the rhizoasphere. Mucilage was collected from 3–4-d-old, axenically-grown maize (Zea mays L. cv. Freya) and lupin (Lupinus angustifolius L. cv. Merrit) seedlings. Surface tension and viscosity were measured over a range of mucilage hydration, and neutral sugar analyses of the hydrolysed mucilages were obtained by gas chromatography. Surface tension of both maize and lupin mucilage was reduced to ∼ 48 mN m−1 at total solute concentrations > 0.7 mg ml−1, indicating the presence of powerful surfactants. Mucilage viscosity increased with increasing solute concentration and decreasing temperature. At a total solute concentration of 0.7 mg ml−1, the viscosity of maize mucilage at 20 °C was 2.1 mPa s (approx. double that of pure water), increasing to 3.3 mPa s at 5 °C. Both maize and lupin mucilage showed viscoelastic behaviour. The major component of maize mucilage was found to be glucose, but in lupin it was fucose.
The surface tension and viscosity results support the idea that mucilage plays a major role in the maintenance of root-soil contact in drying soils. As surface tension decreases, the ability of the mucilage to wet the surrounding soil particles becomes greater. Also, as viscosity and elasticity increase, the resistance to movement of any soil particles in contact with the mucilage increases, a degree of stabilization of the rhizosphere structure is achieved and hydraulic continuity is maintained. It is unclear whether the surfactant is actively secreted by the root or is present simply as a result of leakage from root cells.
Ribulose 1,5-diphosphate carboxylase (Rubisco) comprises 35–40% of the protein in plant leaves. It is an ideal analogue for the nitrogenous part of leaf material. We have used 15N isotope dilution to study the mineralisation of nitrogen and carbon during Rubisco decomposition in a sandy soil in the presence or absence of sucrose. The results showed that gross N mineralisation was only slightly affected by the addition of sucrose but that gross N immobilisation increased markedly. They confirmed earlier work showing that the decomposition was effected by bacteria and that direct assimilation of peptides or amino acids occurred without prior mineralisation. One interpretation of these findings is that there are two functional groups of bacteria in this soil: one which uses protein for energy and C and N and another which uses carbohydrate as an energy and C source and NH4+ mineralised by the first group as an N source. They may provide a link between bacterial functionality in terms of nutritional requirements and the two main N transformations in soil-mineralisation and immobilisation. The implications of these conclusions in terms of current models of the soil N cycle are explored and a modified soil N cycle is proposed.
The neutral lipids of white clover leaves and stems have been separated into wax esters, free fatty acids, free fatty alcohols, free sterols, triglycerides and hydrocarbons. The wax esters were mainly of C18 di- and tri-unsaturated fatty acids and C30 fatty alcohol. Linolenic acid was the predominant free fatty acid and triacontanol was the principal free fatty alcohol. Of the hydrocarbons, C29 and C31 were present in the largest amounts.
It is shown how films of pure or mixed lipids can be investigated by means ofHe(I) photoelectron spectroscopy; the relationship between the results and those from pressure/area studies is discussed. Films of phosphatidylcholine (egg lecithin), cholesterol and mixtures of the two have been studied; a monolayer of lecithin facilitates the early stages of the surface adsorption of cholesterol but a monolayer of cholesterol has the reverse effect on lecithin.
Root mucilage of maize (Zea mays L.) was purified using Sephadex size-exclusion chromatography to allow subsequent analysis of the polysaccharides derived from this mucilage. Hydrolysis of the polysaccharides to their constituent monosaccharides and conversion of these monomers to volatile peracetate derivatives allowed analysis of these derivatives using gas chromatography. This permitted identification and broad quantification of the major components of the polysaccharides. Two methods have been developed: (1) a two-step hydrolysis/acetylation procedure and (2) a one-step acetolysis. Gas chromatograms obtained using the latter procedure are far simpler due to the formation of predominantly one anomer for each monomer component. In both cases, the major monosaccharide components of the polysaccharides were identified as fucose, arabinose, galactose and glucose. Analysis of the crude maize mucilage demonstrated that monosaccharides co-exist with polysaccharides. The monosaccharides were again converted to their peracetates and gas chromatography identified the major monosaccharide component as glucose.
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Broadly, the approach which researchers have adopted in this review has been to ask the following questions about mineral nutrients: What properties make them essential. How are they obtained. How effectively are they used. We shall not be considering two most important, but frequently reviewed, aspects of the subject, namely biological fixation of N/sub 2/ and its assimilation and mechanisms of membrane transport.
C and N dynamics were followed during decomposition of root mucilage, roots and glucose in soil, incubated for 6 months at 25°C. Each of the substrates, derived from maize plants, was added at two rates, which ranged from ca 100 to 450 mg C kg−1 soil. Carbon mineralization from each substrate was determined from 13C variations at natural abundance. Nitrogen fluxes were calculated by artificial 15N tracing of soil nitrate-N.The kinetics of C and N fluxes, expressed per unit of added C, were almost independent of the C rate for each of the three substrates. The calculated true mineralization rate of root mucilage-C was comparable to glucose mineralization; mineralization of roots-C was slower. A positive ‘priming effect’ was found and was proportional to the amount of substrate-C added. The priming increased throughout the incubation and was more pronounced for glucose than for mucilage or roots. In contrast, there was no priming effect for N during glucose decomposition. Gross immobilization of labelled nitrate-N reached a maximum of 72, 19 and 61 mg N g−1 added C for mucilage, roots and glucose, respectively. This immobilization was obtained when the true mineralization rates of carbon were 35, 38 and 40%. Gross N immobilization and true C mineralization were highly correlated during root decomposition. Total N assimilation, i.e. gross immobilization of NO3-N plus microbial assimilation as NH4-N or organic N, was estimated by applying the isotope dilution method to biomass-N. It reached 88, 66 and 61 mgNg−1 added C for mucilage, roots and glucose, respectively. The higher N assimilation for mucilage was probably due to exclusive bacterial decomposition, whereas fungi were involved for the two other substrates. The remineralization of N and the decline in biomass-N during the second part of the incubation were markedly faster and more complete for mucilage than for roots and glucose.
The glycolipids and phospholipids in fronds and rhizomes of Pteridium aquilinum were determined. The total quantity of polar lipid decreased towards the base of the frond, but increased in the storage rhizome. The monogalactosyl diglyceride/digalactosyl diglyceride ratio was 1.8 in the pinnae, 1.0 in the lower petiole and 0.3 in the storage rhizome.
This study was conducted to determine the influence of root mucilage (RM) from Zea mays, polygalacturonic acid (PGA), and galacturonic acid (GA), on the adsorption, desorption, and exchangeability of orthophosphate
(P) on a synthetic ferrihydrite in deionized water. The adsorption rate and affinity on the oxide decreased in the order P>GA>PGA≥RM.
The results suggest that P and GA were adsorbed by ligand exchange on Fe-OH surface sites, while RM and PGA might in addition
also be adsorbed by hydrogen and van der Waals bonds to the oxide surface. Whereas P and GA could be adsorbed at sites located
inside the micropores of ferrihydrite aggregates, PGA and RM probably remained on the external surfaces because of their large
molecular weight. The preliminary adsorption of organic compounds decreased the subsequent P adsorption. The larger decrease
of P adsorption occurred when dry ferrihydrite powder was directly added to the RM/water suspension. This effect was due to
the flocculation of ferrihydrite aggregates, which limited the transport of P to the adsorption sites. Preliminary adsorption
of GA also decreased P adsorption, because both compounds competed for the same Fe-OH sites. The ability of the organic compounds
to desorb P from the ferrihydrite was not very pronounced and decreased in the order GA>PGA≥RM. The preliminary adsorption
of organic compounds on the oxide resulted in an increase in isotopically exchangeable P in 300 min compared to the treatment
without organic compounds. However, because of its higher affinity for the oxide surface, P exchangeability was not modified
when P was added before the organic compounds.
Mechanical impedance stimulates maize root exudation. The purpose of this work was to evaluate the direct effect of mechanical impedance on root exudation from the indirect effect involving root morphological modifications induced by mechanical impedance. Maize plants were grown in axenic hydroponic culture conditions for 4, 8, 12 and 16 days, and mechanical impedance was simulated by glass beads. At the end of the culture, exudation of plants in a nutrient solution was measured during 24 h. At harvest, plant growth and development parameters as well as carbon exudation were measured. The results demonstrated a major influence of mechanical impedance on root growth with a reduction in root elongation. Comparisons with previous studies in soil conditions have indicated that the glass-bead system realistically simulated mechanical impedance. The carbon exudation rate fluctuated from 0.2 to 1.2 mg C plant-1 day-1 and a fraction of this carbon (0.06 to 0.11 mg C plant-1 day-1) was recovered from glass beads in impeded conditions. The difference in exudation between both treatments for comparable plant morphologies lead to the conclusion that the mechanical impedance had a direct effect on exudation rate. Correlations between plant morphology and root exudation suggest that root morphology is probably involved in the modification of root exudation.
The use of surfactants for the removal of hydrophobic organics from soil by in situ flushing techniques shows great potential. Sodium dodecylsulfate (SDS), an anionic surfactant, is often chosen for soil flushing processes. The relationship between the aqueous equilibrium concentration and the loss of SDS from solution with and without the presence of an electrolyte (NaCl) was established using batch and column experiments. The results indicate that the maximum amount of SDS adsorption and precipitation was observed when the concentration of SDS was in the region of the critical micelle concentration (CMC), and this amount was even higher in the presence of the electrolyte. The interactions such as adsorption, cation exchange between soil and Na+ leading to the release of Ca2+ and subsequent precipitation of Ca(DS)2 are discussed. Column experiments were also conducted to investigate the change in hydraulic conductivity due to the interactions between soil and surfactant. Injection of surfactants to a soil matrix decreases the hydraulic conductivity, and this change is related to the clay content of the soil, the type and concentration of the surfactant, and the presence of electrolyte. The mechanisms which could be responsible for the change in hydraulic conductivity are clay expansion, sodium dispersion, fine particle mobilization and precipitation of calcium and magnesium dodecylsulfate. For the soil used in this experiment, the precipitation of divalent salts of surfactants appears to be the primary reason for the reduction of hydraulic conductivity.
Complete layers of some lipids are formed spontaneously on the surfaces of solutions of sufficient concentration in certain solvents, e.g. phosphatidylcholine (egg lecithin) and phosphatidylinositol (wheat germ) dissolved in hydroxypropionitrile. The He(I) photoelectron spectra of phosphatides in such layers contain: (1) a broad band, assigned to alkyl groups, with maximum intensity at about 11 eV, (2) another broad band at about 8 eV, assigned to the PO-4 group with about 1 % of the band area of (1).
This review examines the effect of tillage on microbial habitat space, and the roles of microbes in influencing N-transformation processes within a heterogeneous soil environment. Literature relating tillage to microbial processes is assessed critically focusing on (a) degrees of physical disruption and N-processes, (b) interactions between organisms and the soil pore network, and (c) the role of soil structure in mediating oxygen movement to sites of microbial activity in soil. Spatial heterogeneity is shown to be a key characteristic of soil structure and N-transformation processes, impacting on predator:prey relations, microbial habitable pore space, and the modelling of the soil system with respect to denitrification. The latter area is discussed with respect to the notion of how a functional appraisal of soil structure may be approached theoretically, at the aggregate and soil profile scale.
Surfactants are widely used for various purposes in industry, but for many years were mainly chemically synthesized. It has only been in the past few decades that biological surface-active compounds (biosurfactants) have been described. Biosurfactants are gaining prominence and have already taken over for a number of important industrial uses, due to their advantages of biodegradability, production on renewable resources and functionality under extreme conditions; particularly those pertaining during tertiary crude-oil recovery. Conflicting reports exist concerning their efficacy and the economics of both their production and application. At present, their uses are mainly in the oil and petroleum industries, where they are employed primarily for their emulsification capacity in both tertiary recovery and polluted-sites remediation. However, caution is frequently exercised with respect to their use because of possible subsequent microbial contamination of either underground oil reservoirs or products. The limited successes and applications for biosurfactants' production, recovery, use in oil pollution control, oil storage tank clean-up and enhanced oil-recovery are reviewed from the technological point of view.
Two types of experiments were carried out; either maize roots were incubated in L-[1-3H]fucose or membranes were prepared from root tips and these were incubated with GDP-L-[U-14C]fucose or UDP-D-[U-4C]glucose. The radioactively labelled lipids that were synthesized in vivo and in vitro were extracted and separated into polar and neutral components. The polar lipids had the characteristics of polyprenyl phosphate and diphosphate fucose or glucose derivatives, and the neutral lipids of sterol glycosides (fucose or glucose). A partial separation of the glycolipid synthetase reactions was achieved. Membranes were fractionated into material that sedimented at 20,000g and 100,000g. Most of the polar glycolipid synthetase activity (for the incorporation of both fucose and glucose) was located in the 100,000 g pellet, and this activity was probably located in the endoplasmic reticulum. The neutral lipid, which contained fucose, was synthesized mainly by membranes of the 20,000g pellet, and the activity was probably associated with the dictyosomes, whereas the neutral glucolipids were synthesized by all the membrane fractions. It is suggested that the polar (polyprenyl) lipids labelled with fucose could act as possible intermediates during the synthesis of the glycoproteins and slime in the root tip.
The root system of a plant is as complicated as the shoot in its diversity, in its reactions with the matrix of substances, and with the myriad organisms that surround it. Laboratory studies blind us to the complexity found by careful study of roots in soil. This complexity is illustrated in the much-studied corn root system, covering the changes along the framework roots: the surface tissues and their interactions with the soil, the water-conducting xylem, whose gradual elaboration dictates the water status of the root. A conspicuous manifestation of the changes is the rhizosheath, whose microflora differs from that on the mature bare zones. The multitude of fine roots is the most active part of the system in acquiring water and nutrients, with its own multitude of root tips, sites of intense chemical activity, that strongly modify the soil they contact, mobilize reluctant ions, immobilize toxic ions, coat the soil particles with mucilage, and select the microflora.
Advanced Level Practical Physics
- M Nelkon
- Jm Ogborn
Nelkon M, Ogborn JM. 1978. Advanced Level Practical Physics, 4th edn.
London, UK: Heinemann. Educational Books.