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  • About
    Research items
    Research Experience
    Oct 2007
    Physiology, Genetics, and Molecular Research on Nutrient Use Efficiency in Plants
    • Physiology, Genetics, and Molecular Research on Nutrient Use Efficiency in Plants
    Apr 2002 - Sep 2007
    Ammonium Transporters in Plants
    • Ammonium Transporters in Plants
    Mar 2001 - Feb 2002
    QTL Analysis of Disease Resistance Gene in Maize
    • QTL Analysis of Disease Resistance Gene in Maize
    Apr 2002 - May 2007
    Hohenheim University
    Field of study
    • Plant Nutrition
    Sep 1996 - Jun 1999
    Chinese Academy of Agricultural Sciences
    Field of study
    • Biochemistry and Molecular Biology
    Sep 1992 - Jun 1996
    Beijing Normal University
    Field of study
    • Biology
    Current institution
    China Agricultural University | CAU
    Current position
    • Professor (Full)
    Top co-authors
    Research Items (75)
    Ammonium acquisition by plant roots is mediated by AMMONIUM TRANSPORTERs (AMTs), ubiquitous membrane proteins with essential roles in nitrogen nutrition in all organisms. In microbial and plant cells, ammonium transport activity is controlled by ammonium-triggered feedback inhibition to prevent cellular ammonium toxicity. Data from heterologous expression in yeast indicate that oligomerization of plant AMTs is critical for allosteric regulation of transport activity, in which the conserved cytosolic C terminus functions as a trans-activator. Employing the coexpressed transporters AMT1;1 and AMT1;3 from Arabidopsis thaliana as a model, we show here that these two isoforms form functional homo- and heterotrimers in yeast and plant roots and that AMT1;3 carrying a phosphomimic residue in its C terminus regulates both homo- and heterotrimers in a dominant-negative fashion in vivo. 15NH4+ influx studies further indicate that allosteric inhibition represses ammonium transport activity in roots of transgenic Arabidopsis expressing a phosphomimic mutant together with functional AMT1;3 or AMT1;1. Our study demonstrates in planta a regulatory role in transport activity of heterooligomerization of transporter isoforms, which may enhance their versatility for signal exchange in response to environmental triggers.
    High-affinity ammonium uptake in plant roots is mainly mediated by AMT1-type ammonium transporters, and their regulation varies depending on the plant species. In this study we aimed at characterizing AMT-mediated ammonium transport in maize, for which ammonium-based fertilizer is an important nitrogen (N) source. Two ammonium transporter genes, ZmAMT1;1a and ZmAMT1;3, were isolated from a maize root-specific cDNA library by functional complementation of an ammonium uptake-defective yeast mutant. Ectopic expression of both genes in an ammonium uptake-defective Arabidopsis mutant conferred high-affinity ammonium uptake capacities in roots with substrate affinities of 48 and 33 μM for ZmAMT1;1a and ZmAMT1;3, respectively. In situ hybridization revealed co-localization of both ZmAMT genes on the rhizodermis, suggesting an involvement in capturing ammonium from the rhizosphere. In N-deficient maize roots, influx increased significantly while ZmAMT expression did not. Ammonium resupply to N-deficient or nitrate-pre-cultured roots, however, rapidly enhanced both influx and ZmAMT transcript levels, revealing a substrate-inducible regulation of ammonium uptake. In conclusion, the two rhizodermis-localized transporters ZmAMT1;1a and ZmAMT1;3 are most probably the major components in the high-affinity transport system in maize roots. A particular regulatory feature is their persistent induction by ammonium rather than an up-regulation under N deficiency.
    That root system architecture (RSA) has an essential role in nitrogen acquisition is expected in maize, but the genetic relationship between RSA and nitrogen use efficiency (NUE) traits remains to be elucidated. Here, the genetic basis of RSA and NUE traits was investigated in maize using a recombination inbred line population that was derived from two lines contrasted for both traits. Under high-nitrogen and low-nitrogen conditions, 10 NUE- and 9 RSA-related traits were evaluated in four field environments and three hydroponic experiments, respectively. In contrast to nitrogen utilization efficiency (NutE), nitrogen uptake efficiency (NupE) had significant phenotypic correlations with RSA, particularly the traits of seminal roots (r = 0.15-0.31) and crown roots (r = 0.15-0.18). A total of 331 quantitative trait loci (QTLs) were detected, including 184 and 147 QTLs for NUE- and RSA-related traits, respectively. These QTLs were assigned into 64 distinct QTL clusters, and ~70% of QTLs for nitrogen-efficiency (NUE, NupE, and NutE) coincided in clusters with those for RSA. Five important QTLs clusters at the chromosomal regions bin1.04, 2.04, 3.04, 3.05/3.06, and 6.07/6.08 were found in which QTLs for both traits had favourable effects from alleles coming from the large-rooted and high-NupE parent. Introgression of these QTL clusters in the advanced backcross-derived lines conferred mean increases in grain yield of ~14.8% for the line per se and ~15.9% in the testcross. These results reveal a significant genetic relationship between RSA and NUE traits, and uncover the most promising genomic regions for marker-assisted selection of RSA to improve NUE in maize.
    Root system architecture (RSA) plays an important role in phosphorus (P) acquisition, but enhancing P use efficiency (PUE) in maize via genetic manipulation of RSA has not yet been reported. Here, using a maize recombinant inbred line (RIL) population, we investigate the genetic relationships between PUE and RSA, and develop P-efficient lines by selection of quantitative trait loci (QTLs) that coincide for both traits. In low-P (LP) fields, P uptake efficiency (PupE) is more closely correlated with PUE (r = 0.48–0.54), and RSA in hydroponics is significantly related to PupE (r = 0.25–0.30) but not to P utilization efficiency (PutE). QTL analysis detected a chromosome region where two QTLs for PUE, three for PupE and three for RSA were assigned into two QTL clusters, Cl-bin3.04a and Cl-bin3.04b. These QTLs have favorable effects from alleles coming from the large-rooted and high-PupE parent. Marker-assisted selection (MAS) identified nine advanced backcross-derived lines carrying Cl-bin3.04a or Cl-bin3.04b that displayed mean increases of 22%–26% in PUE in LP fields. Furthermore, a line L224 pyramiding Cl-bin3.04a and Cl-bin3.04b shows enhanced PupE, relying mainly on changes in root morphology, rather than root physiology, under both hydroponic and field conditions. These results highlight the physiological and genetic contributions of RSA to maize PupE, and provide a successful study case of developing P-efficient crops through QTL-based selection.
    Root system architecture (RSA) is seldom considered as a selection criterion to improve yield in maize breeding, mainly because of the practical difficulties with their evaluation under field conditions. In the present study, phenotypic profiling of 187 advanced-backcross BC(4)F(3) maize lines (Ye478 × Wu312) was conducted at different developmental stages under field conditions at two locations (Dongbeiwang in 2007 and Shangzhuang in 2008) for five quantitative root traits. The aims were to (1) understand the genetic basis of root growth in the field; (2) investigate the contribution of root traits to grain yield (GY); and (3) detect QTLs controlling root traits at the seedling (I), silking (II) and maturation (III) stages. Axial root (AR)-related traits showed higher heritability than lateral root (LR)-related traits, which indicated stronger environmental effects on LR growth. Among the three developmental stages, root establishment at stage I showed the closest relationship with GY (r = 0.33-0.43, P < 0.001). Thirty QTLs for RSA were detected in the BC(4)F(3) population and only 13.3 % of the QTLs were detected at stage III. Most important QTLs for root traits were located on chromosome 6 near the locus umc1257 (bin 6.02-6.04) at stage I, and chromosome 10 near the locus umc2003 (bin 10.04) for number of AR across all three developmental stages. The regions of chromosome 7 near the locus bnlg339 (bin 7.03) and chromosome 1 near the locus bnlg1556 (bin 1.07) harbored QTLs for both GY- and LR-related traits at stages I and II, respectively. These results help to understand the genetic basis of root development under field conditions and their contribution to grain yield.
    Although nitrate represents an important nitrogen (N) source for maize, a major crop of dryland areas, the molecular mechanisms of nitrate uptake and assimilation remain poorly understood. Here, we identified nine maize NIN-like protein (ZmNLP) genes and analyzed the function of one member, ZmNLP3.1, in nitrate nutrition and signaling. The NLP family genes were clustered into three clades in a phylogenic tree. Comparative genomic analysis showed that most ZmNLP genes had collinear relationships to the corresponding NLPs in rice, and that the expansion of the ZmNLP family resulted from segmental duplications in the maize genome. Quantitative PCR analysis revealed the expression of ZmNLP2.1, ZmNLP2.2, ZmNLP3.1, ZmNLP3.2, ZmNLP3.3, and ZmNLP3.4 was induced by nitrate in maize roots. The function of ZmNLP3.1 was investigated by overexpressing it in the Arabidopsis nlp7-1 mutant, which is defective in the AtNLP7 gene for nitrate signaling and assimilation. Ectopic expression of ZmNLP3.1 restored the N-deficient phenotypes of nlp7-1 under nitrate-replete conditions in terms of shoot biomass, root morphology and nitrate assimilation. Furthermore, the nitrate induction of NRT2.1, NIA1, and NiR1 gene expression was recovered in the 35S::ZmNLP3.1/nlp7-1 transgenic lines, indicating that ZmNLP3.1 plays essential roles in nitrate signaling. Taken together, these results suggest that ZmNLP3.1 plays an essential role in regulating nitrate signaling and assimilation processes, and represents a valuable candidate for developing transgenic maize cultivars with high N-use efficiency.
    High-affinity ammonium uptake in maize roots is mainly mediated by AMT1-type ammonium transporters ZmAMT1;1a and ZmAMT1;3, but whether the increased expression of ZmAMTs genes is able to enhance ammonium uptake capacity and subsequently improves overall nitrogen use efficiency remains to be elucidated. In this work, ZmAMT1;1a-overexpression transgenic maize plants were generated with the elevated levels of transcripts and proteins, and phenotypically analyzed together with wild-type plants grown in nutrient solution under two regimes of ammonium supply. Under low ammonium nutrition (0.04 mM), in relative to wild-type plants, the maize transgenic lines showed an approximately 17% increases in the high-affinity ammonium uptake capacity of roots as revealed by ¹⁵N-labeled ammonium influx assay and further contributed to about 7% increases in the total nitrogen uptake at the whole plant level. By contrast, when ammonium was supplied in high amounts (1 mM), wild-type plants expressed higher levels of ZmAMT1;1a, but exhibited a lower ammonium uptake capacity in roots. Furthermore, the transgenic maize line accumulated more amounts of ZmAMT1;1a protein, but did not translate into an enhanced ammonium acquisition, suggesting a possible post-translational down-regulation of ZmAMT1;1a by high ammonium. This study proved the possibility to enhance ammonium acquisition by elevating ZmAMTs expression in maize roots and provided an effective transgenic approach on developing high nitrogen use efficient maize cultivars.
    Nitrogen is a major limiting factor for crop productivity. The relationship between photosynthesis and nitrogen nutrition has been widely studied. However, the molecular response of leaf photosynthesis to low nitrogen supply in crops is less clear. In this study, RNA sequencing technology (RNA-Seq) was used to investigate the gene expressions related to photosynthesis in maize in response to low nitrogen supply. It was found that low nitrogen supply down-regulated the expression of genes involved in photosystem I (PSI) and photosystem II (PSII). Thus, low nitrogen supply down-regulated the expression of genes related to the antenna system, reduced light absorption, light transport, and electron transport. Correspondingly, the parameters related to chlorophyll fluorescence were very sensitive to nitrogen deficiency. Under low nitrogen supply, leaf chlorophyll content, actual quantum yield of PSII photochemistry, photochemical quenching, and electron transport rate, were reduced. However, the thermal diffusion and chlorophyll fluorescence were increased. RNA-Seq was used to analyze the genes involved in the response of leaf photosynthesis to low nitrogen supply in maize. These results highlight the possibility of utilizing chlorophyll fluorescence parameters, and the related genes, as indicators for plant nitrogen nutrition. This could lead to the development of new tools to make precise nitrogen fertilizer recommendations and select nitrogen-efficient genotypes.
    Transporters involved in manganese (Mn) uptake and intracellular Mn homeostasis in Arabidopsis and rice are well characterized, while much less is known for barley, which is particularly prone to Mn deficiency. In this study we have investigated the role of the iron-regulated transporter 1 (IRT1) for Mn uptake and translocation in barley plants. We employed an RNAi approach to reduce HvIRT1 expression to 5% of the wild-type level. This enabled characterization of the functional role of HvIRT1 by use of advanced imaging and phenotyping techniques applied to plants growing in hydroponics or soils with different Mn availability. Our results highlight the importance of HvIRT1 for the transport of Mn across the root endodermis into the stele. In the hvirt1-RNAi lines, a chlorotic phenotype with reduced shoot Mn concentration and impaired photosynthetic functionality was observed, especially under conditions with low Mn availability. We also document that HvIRT1 controlled the Mn distribution within the barley grain. Surprisingly, unlike other IRT1 orthologues, HvIRT1 played no significant role in iron uptake. We conclude that the barley IRT1 orthologue has a novel function with respect to ensuring sufficient shoot Mn concentrations. The preference of IRT1 for Mn instead of Fe is discussed in an evolutionary context.
    Abstract Ammonium uptake in plant roots is mediated by AMT/MEP/Rh-type ammonium transporters. Out of five AMTs being expressed in Arabidopsis roots, four AMT1-type transporters contribute to ammonium uptake, whereas no physiological function has so far been assigned to the only homolog belonging to the MEP subfamily, AMT2;1. Based on the observation that under ammonium supply transcript levels of AMT2;1 increased and its promoter activity shifted preferentially to the pericycle, we assessed the contribution of AMT2;1 to xylem loading. When exposed to 15N-labeled ammonium, amt2;1 mutant lines translocated less tracer to the shoots and contained less ammonium in the xylem sap. Moreover, in an amt1;1 amt1;2 amt1;3 amt2;1 quadruple deletion line (qko), co-expression of AMT2;1 with either AMT1;2 or AMT1;3 significantly enhanced 15N translocation to shoots, indicating a cooperative action between AMT2;1 and AMT1 transporters. Under N deficiency proAMT2;1-GFP lines showed enhanced promoter activity predominantly in cortical root cells, which coincided with elevated ammonium influx conferred by AMT2;1 at millimolar substrate concentrations. We conclude that besides contributing moderately to root uptake in the low-affinity range, AMT2;1 functions mainly in root-to-shoot translocation of ammonium. These functions depend on its cell type-specific expression in response to the plant nutritional status and to local ammonium gradients.
    Root system architecture (RSA) plays an important role in the acquisition of both nitrogen (N) and phosphorus (P) from the environment. Currently RSA is rarely considered as criteria for selection to improve nutrient uptake efficiency in crop breeding. Under field conditions roots can be greatly influenced by uncontrolled environment factors. Therefore, it is necessary to develop fast selection methods for evaluating root traits of young seedlings in the lab which can then be related to high nutrient efficiency of adult plants in the field. Here, a maize recombination inbred line (RILs) population was used to compare the genetic relationship between RSA and nitrogen and phosphorous efficiency traits. The phenotypes of eight RSA-related traits were evaluated in young seedlings using three different growth systems (i.e., paper roll, hydroponics and vermiculite), and then subjected to correlation analysis with N efficiency and P efficiency related traits measured under field conditions. Quantitative trait loci (QTL) of RSA were determined and QTL co-localizations across different growth systems were further analyzed. Phenotypic associations were observed for most of RSA traits among all three culture systems. RSA-related traits in hydroponics and vermiculite weakly correlated with Nitrogen (NupE) uptake efficiency (r = 0.17–0.31) and Phosphorus (PupE) uptake efficiency (r = 0.22–0.34). This correlation was not found in the paper roll growth system. A total of 14 QTLs for RSA were identified in paper rolls, 18 in hydroponics, and 14 in vermiculite. Co-localization of QTLs for RSA traits were identified in six chromosome regions of bin 1.04/1.05, 1.06, 2.04/2.05, 3.04, 4.05, and 5.04/5.05. The results suggest the problem of using the phenotype from one growth system to predict those in another growth system. Assessing RSA traits at the seedling stage using either hydroponics or a vermiculite system appears better suited than the paper roll system as an important index to accelerate the selection of high N and P efficient genotypes for maize breeding programs.
    In order to understand the regulation of source-sink relationship on leaf senescence in maize, we investigated the effect of pollination prevention on leaf senescence, post-silking dry matter and nitrogen accumulation and remobilization in different vegetative organs in one landrace and ten representative maize hybrids released between 1973 and 2000 in China. When pollination was prevented by covering silk with paper bags at silking stage, leaf senescence of Nongda 60, Yedan 13, Shendan 7 and Zhengdan 958 was delayed as shown by the remaining green leaf area per plant at maturity. However, pollination-prevention did not have obvious effect on the leaf senescence of Baimaya, Danyu 6, Yedan 2 and Xianyu 335. By contrast, pollination-prevention accelerated the leaf senescence of maize hybrids Zhongdan 2, Danyu13 and Yedan 4. It was found that leaf senescence of the late-released (since 1985) Chinese hybrids tended to be delayed by pollination-prevention, except XY335. From silking to physiological maturity, nitrogen content increased in the stem (plus the sheath, cob, husk, and tassel) and root of the non-pollinated plants. However, there was still a net reduction in leaf nitrogen content. We also found that pollination-prevention reduced leaf nitrogen remobilization efficiency, with genotypic difference and variation between the two years of testing. The results suggested that the response of leaf senescence to pollination-prevention is at least partially due to the change of leaf nitrogen remobilization efficiency. Leaf senescence tended to be delayed if leaf nitrogen remobilization efficiency is highly reduced by pollination-prevention.
    To control gene expression directly by hormone concentrations, both animal and plant cells have exploited comparable mechanisms to sense small molecular hormones in nucleus. Whether nuclear entry of these hormones is actively transported or passively diffused, as conventionally postulated, through nuclear pore complex remains enigmatic. Here, we identified and characterized a jasmonate transporter in Arabidopsis thaliana, AtJAT1/ AtABCG16, which exhibits an unexpected dual localization in the nuclear envelope and plasma membrane. We show that AtJAT1/AtABCG16 controls the cytoplasmic and nuclear partition of jasmonate phytohormones by mediating both cellular efflux of jasmonic acid (JA) and nuclear influx of jasmonoyl-isoleucine (JA-Ile), and is essential for maintaining a critical nuclear JA-Ile concentration to activate JA signaling. These results illustrate that transporter-mediated nuclear entry of small hormone molecules is a new mechanism to regulate nuclear hormone signaling. Our findings provide an avenue to develop pharmaceutical agents targeting the nuclear entry of small molecules.
    The introduction of new hybrids and integrated crop-soil management has been causing maize grain yield to increase. However, less attention has been paid on the nutrient concentration of the grain; this aspect is of great importance to supplying calories and nutrients in the diets of both humans and animals worldwide. Increasing the retranslocation of nutrients from vegetative organs to grain can effectively increase the nutrient concentration of grain and general nutrient use efficiency. The present study involved monitoring the dynamic change of macro- and micronutrients in different organs of maize during the grain filling stage. In addition, the mobility of different elements and their contribution to grain nutrient content were evaluated in a 2-year experiment under low (LN, no N supplied) and high N (HN, 180 kg N ha⁻¹) supply. Under HN supply, the net remobilization efficiency (RE) of the vegetative organs as a whole (calculated as nutrient remobilization amount divided by nutrient content at silking) of N, P, K, Mn, and Zn were 44%, 60%, 13%, 15%, and 25%, respectively. The other nutrients (Mg, Ca, Fe, Cu, and B) showed a net accumulation in the vegetative organs as a whole during the grain filling stage. Among the different organs, N, P, and Zn were remobilized more from the leaves (RE of 44%, 51% and 43%, respectively) and the stalks (including leaf sheaths and tassels) (RE of 48%, 71% and 43%, respectively). K was mainly remobilized from the leaves with RE of 51%. Mg, Ca, Fe, Mn, and Cu were mostly remobilized from the stalks with the RE of 23%, 9%, 10%, 42%, and 28%, respectively. However, most of the remobilized Mg, Ca, Fe, Mn, Cu, and Zn were translocated to the husk and cob, which seemingly served as the buffer sink for these nutrients. The REs of all the nutrients except for P, K, and Zn were vulnerable to variations in conditions annually and were reduced when the grain yield and harvest index were lower in 2014 compared with 2013. Under LN stress, the RE was reduced in P and Zn in 2013, increased in Cu and unchanged in other nutrients. The concentration of these nutrients in the grain was either unchanged (P, K, Ca, Zn, and B) or decreased (N, Mg, Fe, Mn, and Cu). It is concluded that grain N, P, K, Mn, and Zn, but not Mg, Ca, Fe, Cu, and B concentration, can be improved by increasing their remobilization from vegetative organs. However, enhancing the senescence of maize plant via LN stress seems unable to increase grain mineral nutrient concentration. Genetic improvement aiming to increase nutrient remobilization should take into account the organ-specific remobilization pattern of the target nutrient.
    Although the remobilization of vegetative nitrogen (N) and post-silking N both contribute to grain N in maize (Zea mays L.), their regulation by grain sink strength is poorly understood. Here we use ¹⁵N labeling to analyze the dynamic behaviors of both pre- and post-silking N in relation to source and sink manipulation in maize plants. The results showed that the remobilization of pre-silking N started immediately after silking and the remobilized pre-silking N had a greater contribution to grain N during early grain filling, with post-silking N importance increasing during the later filling stage. The amount of post-silking N uptake was largely driven by post-silking dry matter accumulation in both grain as well as vegetative organs. Prevention of pollination during silking had less effect on post-silking N uptake, as a consequence of compensatory growth of stems, husk + cob and roots. Also, leaves continuously export N even though grain sink was removed. The remobilization efficiency of N in the leaf and stem increased with increasing grain yield (hence N requirement). It is suggested that the remobilization of N in the leaf is controlled by sink strength but not the leaf per se. Enhancing post-silking N uptake rather than N remobilization is more likely to increase grain N accumulation.
    Over the past five decades, Chinese grain production has increased 4-fold, from 110 Mt in 1961 to 557 Mt in 2014, with less than 9% of the world’s arable land feeding 22% of the world’s population, indicating a substantial contribution to global food security. However, compared with developed economies, such as the USA and the European Union, more than half of the increased crop production in China can be attributed to a rapid increase in the consumption of chemicals, particularly fertilizers. Excessive fertilization has caused low nutrient use efficiency and high environmental costs in grain production. We analysed the key requirements underpinning increased sustainability of crop production in China, as follows: (i) enhance nutrient use efficiency and reduce nutrient losses by fertilizing roots not soil to maximize root/rhizosphere efficiency with innovative root zone nutrient management; (ii) improve crop productivity and resource use efficiency by matching the best agronomic management practices with crop improvement; and (iii) promote technology transfer of the root zone nutrient management to achieve the target of high yields and high efficiency with low environmental risks on a broad scale. Coordinating grain production and environmental protection by increasing the sustainability of nutrient use will be a key step in achieving sustainable crop production in Chinese agriculture.
    The quality of edible seeds for human and animal nutrition is crucially dependent on high zinc (Zn) and iron (Fe) seed concentrations. The micronutrient bioavailability is strongly reduced by seed phytate that forms complexes with seed cations. Superior genotypes with increased seed Zn concentrations had been identified, but low micronutrient seed levels often prevail when the plants are grown in Zn-deficient soils, which are globally widespread and correlate with human Zn-deficiency. Here, seed Zn concentrations of Arabidopsis accessions grown in Zn-deficient and Zn-amended conditions were measured together with seed Fe and manganese (Mn), in a panel of 108 accessions. By applying genome-wide association, de novo candidate genes potentially involved in the seed micronutrient accumulation were identified. However, a candidate inositol 1,3,4-trisphosphate 5/6-kinase 3 gene (ITPK3), located close to a significant nucleotide polymorphism associated with relative Zn seed concentrations, was dispensable for seed micronutrients accumulation in Col-0. Loss of this gene in itpk3-1 did neither affect phytate seed levels, nor seed Zn, Fe, and Mn. It is concluded that large natural variance of micronutrient seed levels is identified in the population and several accessions maintain high seed Zn despite growth in Zn-deficient conditions.
    The importance of root system in supporting shoot growth has been extensively studied and discussed under nutrient and/or water deficit conditions, but much less in the context of intensive cropping system in which plant density is high and nutrients can be supplemented through fertilizer application to match the requirement of plant growth and grain yield formation. Taking maize as a model crop, the ideotype root system architecture (RSA) under intensive production conditions was discussed in regard to high yield and resource use efficiency. An ideotype maize RSA should meet the requirements not only for efficient use of water and unevenly distributed nutrients, but also for greater root-lodging resistance. In addition, the construction of RSA should adapt to efficient utilization of carbon and/or nutrients within the plant. The embryonic root system and the postembryonic root system should be considered separately because they are controlled by different genetic mechanisms and function in different growth stages during which soil environment varies. Considering all the previously mentioned factors, we proposed two separate models for the ideotype maize RSAs at seedling stage and at adult plant stage, and the characteristics of axile root and lateral root traits for each model are described in detail.
    Nitrogen (N) plays a vital role in photosynthesis and crop productivity. Maize plants may be able to increase physiological N utilization efficiency (NUtE) under low-N stress by increasing photosynthetic rate (Pn) per unit leaf N, that is, photosynthetic N-use efficiency (PNUE). In this study, we analyzed the relationship between PNUE and N allocation in maize ear-leaves during the grain-filling stage under low N (no N application) and high N (180 kg N ha−1) in a 2-year field experiment. Under low N, grain yield decreased while NUtE increased. Low-N treatment reduced the specific N content of ear leaves by 38% without significant influencing Pn, thereby increasing PNUE by 54%. Under low-N stress, maize plants tended to invest relatively more N into bioenergetics to sustain electron transport. In contrast, N allocated to chlorophyll and light-harvesting proteins was reduced to control excess electron production. Soluble proteins were reduced to shrink the N storage reservoir. We conclude that optimization of N allocation within leaves is a key adaptive mechanism to maximize Pn and crop productivity when N is limited during the grain-filling stage in maize under low-N conditions.
    A plant's ability to maintain or improve its yield under limiting conditions, such as nutrient deficiency or drought, can be strongly influenced by root system architecture, the 3-dimensional distribution of the different root types in the soil. The ability to image, track and quantify these root system attributes in a dynamic fashion is a useful tool in assessing desirable genetic and physiological root traits. Recent advances in imaging technology and phenotyping software have resulted in substantive progress in describing and quantifying root system architecture. We have designed a hydroponic growth system which retains the 3-dimensional root system architecture (RSA) of the plant root system, while allowing for aeration, solution replenishment, and the imposition of nutrient treatments, as well as high quality imaging of the root system. The simplicity and flexibility of the system allows for modifications tailored the root system architecture of different crop species and improved throughput. This paper details the recent improvements and innovations in our root growth and imaging system which allows for greater image sensitivity (detection of fine roots and other root details), higher efficiency, and a broad array of growing conditions for plants that more closely mimic those found under field conditions.
    Maize (Zea mays L.) root morphology exhibits a high degree of phenotypic plasticity to nitrogen (N) deficiency, but the underlying genetic architecture remains to be investigated. Using an advanced BC4 F3 population, we investigated the root growth plasticity under two contrasted N levels and identified the QTLs with Q×E interaction effects. Principal components analysis (PCA) on changes of root traits to N deficiency (ΔLN-HN) showed that root length and biomass contributed for 45.8% in the same magnitude and direction on the first PC, while root traits scattered highly on PC2 and PC3. Hierarchical cluster analysis (HCA) on traits for ΔLN-HN further assigned the BC4 F3 lines into six groups, in which the special phenotypic responses to N deficiency was presented. These results revealed the complicated root plasticity of maize in response to N deficiency that can be caused by G×E interactions. Furthermore, Quantitative trait locus (QTL) mapping using a multi-environment analysis identified 35 QTLs for root traits. Nineof these QTLs exhibited significant QTL-environment (Q×E) interaction effects. Taken together, our findings contribute to understand the phenotypic and genotypic pattern of root plasticity to N deficiency, which will be useful for developing maize tolerance cultivars to N deficiency. This article is protected by copyright. All rights reserved.
    Key message Understanding the correlations of seven minerals for concentration, content and yield in maize grain, and exploring their genetic basis will help breeders to develop high grain quality maize. Abstract Biofortification by enhanced mineral accumulation in grain through genetic improvement is an efficient way to solve global nutrient malnutrition, in which one key step is to detect the underlying quantitative trait loci (QTL). Herein, a maize recombinant inbred population (RIL) was field grown to maturity across four environments (two locations × two years). Phenotypic data for grain mineral concentration, content and yield were determined for copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), magnesium (Mg), potassium (K) and phosphorus (P). Significant effects of genotype, location and year were observed for all investigated traits. The strongest location effects were found for Zn accumulation traits probably due to distinct soil Zn availabilities across locations. Heritability (H 2) of different traits varied with higher H 2 (72–85 %) for mineral concentration and content, and lower (48–63 %) for mineral yield. Significant positive correlations for grain concentration were revealed between several minerals. QTL analysis revealed 28, 25, and 12 QTL for mineral concentration, content and yield, respectively; and identified 8 stable QTL across at least two environments. All these QTL were assigned into 12 distinct QTL clusters. A cluster at chromosome Bin 6.07/6.08 contained 6 QTL for kernel weight, mineral concentration (Mg) and content (Zn, K, Mg, P). Another cluster at Bin 4.05/4.06 contained a stable QTL for Mn concentration, which were previously identified in other maize and rice RIL populations. These results highlighted the phenotypic and genetic performance of grain mineral accumulation, and revealed two promising chromosomal regions for genetic improvement of grain biofortification in maize.
    The size of roots and their physiological activity during the grain-filling stage affect water and nutrient uptake, and grain yield (GY) in maize (Zea mays L.). ­The aim of this study was to determine the effects of different N levels on postanthesis root senescence in Field-grown maize. ­Three N levels (0, 120, 240 kg N ha-1) was applied to -eld-grown maize, and the length and weight of roots in the 0-to 40-cm soil layer, nitrogenous compounds in xylem sap, N uptake, dry matter (DM), N accumulation, and GY were analyzed. Shoot N accumulation, but not grain yield, was higher in the N240 treatment than in the N120 treatment. At the silking stage, the root length was longest in the N0 treatment and shortest in the N240 treatment. ­ is was because of the high root/shoot (R/S) and root C/N ratios in N0. Total root length decreased throughout the grain-filling period, mainly because of the reduced length of early developed nodal roots. ­The root length decreased by 27% in the N0 treatment, to a smaller extent compared to that in N120 (52%) and N240 (51%). ­ e amounts of NO3--N, NH4+-N and amino acid-N in the xylem sap were highest in the N240 treatment during the entire grain-filling stage, suggesting that a high N supply increased root N-uptake activity. Based on these results, we conclude that high root N-uptake activity, not delayed root senescence, resulted in the large accumulation of N in the shoot under excess N. © 2015 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved.
    Root breeding has been proposed as a key factor in the “second green revolution” for increasing crop yield and the efficient use of nutrient and water resources. However, few studies have demonstrated that the genetic improvement of root characteristics directly contributes to enhancing nutrient-use-efficiency in crops. In this study, we evaluated the contribution of root growth improvement to efficient nitrogen (N) acquisition and grain yield under two different N-levels in a 3-year field experiment. We used two near-isogenic maize testcrosses, T-213 (large-root) and T-Wu312 (small-root), derived from a backcross of a BC4F3 population from two parents (Ye478 and Wu312) with contrasting root size. We found that the root length density, root surface area, and dry weight at the silking stage were 9.6–19.5% higher in T-213 compared with the control T-Wu312. The root distribution pattern in the soil profile showed no significant differences between the two genotypes. The overall increase in root growth in T-213 enhanced post-silking N uptake, which increased grain yield by 17.3%. Correspondingly, soil nitrate concentrations in the >30 cm soil layer were reduced in T-213 under the high N treatment. These positive effects occurred under both adequate and inadequate N-supply and different weather conditions. Our study provides a successful case that increasing root size via genetic manipulation contributes directly to efficient N-uptake and higher yield.
    A target in crop production is to simultaneously increase grain yield (GY) and grain nitrogen concentration (GNC). In maize, nitrogen (N) and genotype are two major factors affecting GY and GNC. Both N remobilization from vegetative tissues and post-silking N uptake contribute to grain N, but their relative contributions are genotype specific, and are affected by the N application rate. It is unclear whether the responses of GY and GNC to N application differ between genotypes with different post-silking N uptake and vegetative N remobilization characteristics. We investigated the effect of N application rate on post-silking N uptake, vegetative N remobilization, GY, and GNC of two high-yielding maize hybrids, ZD958 and XY335, which have contrasting N remobilization characteristics. We tested five N application rates (0, 60, 120, 180, 240 kg N ha−1) in a 4-year field study (from 2010 to 2013). There was a significant year × N × genotype interaction in the amount of vegetative N remobilization and N remobilization efficiency (NRE), and residual stalk N concentration at maturity. Compared with the low-NRE cultivar ZD958, XY335 showed the same GY but higher GNC because it had higher vegetative N remobilization, NRE but lower residual stalk N concentration under the favorable weather condition in 2010. The response of GNC to increasing N levels was the same between XY335 and ZD958 and was not affected by year conditions. The N level required to obtain the highest GY was the same in the two hybrids (156 ± 13 kg ha−1and 159 ± 19 kg ha−1), but that required to obtain the highest GNC was greater in XY335 (216 ± 30 kg ha−1) than in ZD958 (195 ± 23 kg ha−1). From these results, we conclude that precise N fertilizer management as well as the selection of high-yielding hybrids with high NRE can increase GNC without negatively affecting GY or leading to surplus N storage in vegetative organs.
    During the grain-filling period in maize (Zea mays L.), increasing photosynthesis per unit plant N, namely photosynthetic N-use efficiency (PNUE) is a potential way to increase N-utilization efficiency (NUtE). This study aimed to understand whether the spatial distribution of PNUE within the canopy is optimized under low N conditions as a method of maximizing canopy productivity. Maize plants were grown in the field for 2 yr with three N levels (0, 180, 240 kg N ha−1). Changes in specific leaf N (SLN), net photosynthetic rate (PN), PNUE (calculated as PN/SLN), stomatal conductance (gs), green leaf area (GLA), and light interception of leaves at different positions were measured every 10 d after silking (DAS). Significant differences in SLN, PN, gs, and PNUE occurred among leaves at different positions: upper leaf ≥ ear leaf > lower leaf. A positive correlation was found between the spatial difference in gs and that in PNUE and in PN. Compared with the average of N180 and N240 across 2 yr, grain yield (GY) decreased by 56% but physiological NUtE increased by 22% without N (N0). Correspondingly, PNUE increased by 44, 25, and 35% in the lower, ear, and upper leaves, respectively. Light interception by the middle leaves was unchanged with N0. We concluded that the vertical distribution of PNUE was optimized with the light distribution pattern in the canopy. The greater whole-plant NUtE we found with N0 may be explained by the overall increase in PNUE in the leaves and the continued interception of light by middle leaves.
    Nitrogen (N), the most important mineral nutrient for plants, is critical to agricultural production systems. N deficiency could severely affect rice growth and decrease rice yields. However, the excessive use of N fertilizer has caused severe pollution to the agricultural and ecological environments. Breeding crops that require less input of N fertilizer has been recognized in the call for a ‘Second Green Revolution’. Here we identified a major quantitative trait locus on chromosome 12, Tolerance Of Nitrogen Deficiency 1 (TOND1), which confers tolerance to N deficiency of rice in the indica cultivar Teqing. Sequence verification of 150 rice cultivars (75 indica and 75 japonica cultivars) from 18 countries and regions demonstrates that only 27.3% cultivars (41 indica cultivars) contain TOND1 whereas 72.7% cultivars, including the remaining 34 indica cultivars and all 75 japonica cultivars, have not harbored the TOND1 allele. Over-expression of TOND1 has increased the tolerance to N deficiency in the TOND1-deficient rice cultivars. The identification of TOND1 provides a molecular basis for breeding rice varieties that improve grain yield with decreased N fertilizer input.This article is protected by copyright. All rights reserved.
    Maize plants respond to low-nitrogen stress by enhancing root elongation. The underlying physiological mechanism remains unknown. Seedlings of maize (Zea mays L., cv. Zhengdan 958) were grown in hydroponics with the control (4 mmol L−1) or low-nitrogen (40 μmol L−1) for 12 d, supplied as nitrate. Low nitrogen enhanced root elongation rate by 4.1-fold, accompanied by increases in cell production rate by 2.2-fold, maximal elemental elongation rate (by 2.5-fold), the length of elongation zone (by 1.5-fold), and final cell length by 1.8-fold. On low nitrogen, the higher cell production rate resulted from a higher cell division rate and in fact the number of dividing cells was reduced. Consequently, the residence time of a cell in the division zone tended to be shorter under low nitrogen. In addition, low nitrogen increased root diameter, an increase that occurred specifically in the cortex and was accompanied by an increase in cell number. It is concluded that roots elongates in response to low-nitrogen stress by accelerating cell production and expansion.
    Aldehyde dehydrogenases (ALDHs) represent a large protein family, which includes several members that catalyze the oxidation of an aldehyde to its corresponding carboxylic acid in plants. Genes encoding members of the ALDH7 subfamily have been suggested to play important roles in various stress adaptations in plants. In this study, quantitative RT-PCR analysis revealed that a maize ALDH7 subfamily member (ZmALDH7B6) was constitutively expressed in various organs, including roots, leaves, immature ears, tassels, and developing seeds. The abundance of ZmALDH7B6 mRNA transcripts in maize roots was increased by ammonium, NaCl, and mannitol treatments. To further analyze tissue-specific and stress-induced expression patterns, the 1.5-kb 5′-flanking ZmALDH7B6 promoter region was fused to the β-glucuronidase (GUS) reporter gene and introduced into maize plants. In roots of independent transgenic lines, there was significant induction of GUS activity in response to ammonium supply, confirming ammonium-dependent expression of ZmALDH7B6 at the transcript level. Histochemical staining showed that GUS activity driven by the ZmALDH7B6 promoter was mainly localized in the vascular tissues of maize roots. These results suggested that ZmALDH7B6 is induced by multiple environmental stresses in maize roots, and may play a role in detoxifying aldehydes, particularly in vascular tissue.
    The plasticity of root architecture is crucial for plants to acclimate unfavorable environments including low nitrogen (LN) stress. How maize roots coordinate the growth of axile roots and lateral roots (LRs), as well as longitudinal and radial cell behaviours in response to LN stress remains unclear. Maize plants were cultivated hydroponically under control (4 mM nitrate) and LN (40 μM) conditions. Temporal and spatial samples were taken to analyse changes in the morphology, anatomical structure, and carbon/nitrogen (C/N) ratio in the axile and lateral roots. LN stress increased axile root elongation, reduced the number of crown roots, and decreased LR density and length. LN extended cell elongation zones and increased the mature cell length in the roots. LN reduced the cell diameter and total area of vessels and increased the amount of aerenchyma, but the number of cell layers in the crown root cortex was unchanged. The C/N ratio was higher in axile roots than in LRs. Maize roots acclimate to LN by optimizing the anatomical structure and N allocation. As a result axile root elongation is favored to efficiently find available N in the soil.
    During the past several decades, maize (Zea mays L.) grain yield (GY) has increased dramatically, while grain nitrogen concentration (GNC) has declined in modern hybrids. Genetic improvement to increase both high GY and GNC is necessary to improve maize nutritional quality. In the present study, we characterized the plant traits in three maize cultivars (YD13, ZD958, XY335) with contrasting GY and GNC in a two-year field experiment in two soils (Fu-jia-jie with infertile sandy soil and Quan-yan-gou with fertile clay soil). The hybrid YD13 as a control had low yield and high GNC. In comparison to YD13, ZD958 had higher GY but lower GNC, whereas XY335 had higher GY and similar GNC. Both ZD958 and XY335 had higher total and post-silking dry matter (DM) accumulation and N uptake than YD13, and were also characterized by delayed leaf senescence and a sustained net photosynthetic rate after silking. In addition, XY335 also had higher N remobilization efficiency (NRE) and higher photosynthetic nitrogen use efficiency (PNUE) than ZD958. It is supposed that higher NRE together with higher N and DM accumulation are the target traits to improve the modern stay-green cultivars to increase GNC without penalty in GY in the area of northeast China. Increasing PNUE of the leaves may be a possible way to maintain whole-plant photosynthesis and DM accumulation under efficient N remobilization.
    Isopentenyltransferase (IPJ) catalyzed the synthesis of cytokinin and subsequently played important roles in cytokinin regulatory processes. IPT proteins were encoded by small multigene family in planta. Poaceae is the most important family for food crops (e. g. rice, maize and wheat). To understand the evolutionary patterns of IPT members in Poaceae crops, we surveyed IPT genes from the genomes of four fully sequenced Poaceae (Brachypodium, rice, sorghum and maize) in 2010. The results showed that Poaceae IPT genes could be divided into nine ortholog groups. The members from each ortholog group were located in the colinearity chromosome regions cross Poaceae species, and distinct from those of any paralog groups, indicating gene expansions within IPT family happened in the common cereal ancestor before the divergence of Poaceae plants. Duplication analysis revealed that the deletion and retention of segmental duplicates were shared by the orthologs cross Poaceae species. Organ-dependent expression patterns and protein properties of members among Poaceae orthologs were also revealed to be similar among the orthologs. These results suggested conserved evolution features of Poaceae IPT genes, and further reflected a conserved biological function among the orthologs.
    Application of organic nitrogen (N) has been proposed as a method to reduce potential environmental pollution due to N loss without sacrificing grain yield. The mechanism responsible for organic N regulation of maize (Zea mays L.) N metabolism is nonetheless largely unknown. In this study, we compared the expression of genes related to N assimilation and remobilization during the grain filling stage in maize plants grown under field conditions. We applied five different N treatments, which consisted of N supplied in organic and/or inorganic forms at the following rates: 0/45, 0/120, 0/240, 120/120, and 240/0 kg ha−1 organic/inorganic N. Yield was found to increase with increasing N input, but no significant difference was found in grain yield among 0/240, 240/0 and 120/120 treatments. Organic N application tended to decrease N accumulation and to increase N utilization efficiency. Genes related to N assimilation activity in leaves, such as ZmNR1 and ZmFd-GOGAT1, were unaffected by different N forms. In contrast, genes related to N remobilization activity in leaves, such as ZmGS1.1 and ZmGDH1, were up-regulated, especially in old leaves, by the pure organic N supply treatment (240/0). These data suggest that application of pure organic N likely induces a N-deficiency response in maize plants, with a consequent increase in physiological N utilization efficiency due to up-regulation of key genes involved in N remobilization processes.
    Background and aims Modern maize breeding has increased maize yields worldwide. The changes in above-ground traits accompanying yield improvement are well-known, but less information is available as to the effect of modern plant breeding on changes in maize root traits. Methods Root growth, nitrogen uptake, dry matter accumulation and yield formation of six maize hybrids released from 1973 to 2000 in China were compared. Experiments were conducted under low and high nitrogen supply in a black soil in Northeast China in 2010 and 2011. Results While nitrogen accumulation, dry matter production and yield formation have been increased, modern maize breeding in China since 1990 has reduced root length density in the topsoil without much effect on root growth in the deeper soil. The efficiency of roots in acquiring N has increased so as to match the requirement of N accumulation for plant growth and yield formation. The responses of root growth, nitrogen and dry matter accumulation, and grain yield to low-N stress were similar in the more modern hybrids as in the older ones. Conclusions Modern maize breeding has constitutively changed root and shoot growth and plant productivity without producing any specific enhancement in root responsiveness to soil N availability.
    Maize plants adapt to low phosphorus (P) stress by increasing root growth. It is of importance to know the extent to which genetic improvement of root growth can enhance P acquisiton. In the present study, the contribution of root growth improvement to efficient P acquisition was evaluated in two soils using T149 and T222, a pair of near isogenic maize testcrosses which were derived from a backcross BC4F3 population. T149 and T222 showed no difference in shoot biomass and leaf area under normal growth conditions, but differed greatly in root growth. T149 had longer lateral roots and a larger root surface area compared to T222. In calcareous soil, when P was insufficient, i.e., when P was either supplied as KH2PO4 at a concentration of 50 mg P kg−1 soil, or in the form of Phy-P, Ca3-P or Ca10-P, a 43% increase in root length in T149 compared to T222 resulted in an increase in P uptake by 53%, and shoot biomass by 48%. In acid soil, however, when P supply was insufficient, i.e., when P was supplied as KH2PO4 at a concentration of 100 mg P kg−1 soil, or in the form of Phy-P, Fe-P or Al-P, a 32% increase in root length in T149 compared to T222 resulted in an increase in P uptake by only 12%, and shoot biomass by 7%. No significant differences in the exudation of organic acids and APase activity were found between the two genotypes. It is concluded that genetic improvement of root growth can efficiently increase P acquisition in calcareous soils. In acid soils, however, improvements in the physiological traits of roots, in addition to their size, seem to be required for efficient P acquisition.
    Approximately 35-55% of total nitrogen in maize plants is taken up by the root at the reproductive stage. Little is known about how the root of an adult plant responds to heterogeneous nutrient supply. In the present study, root morphological and physiological adaptations to nitrate-rich and nitrate-poor patches and corresponding gene expression of ZmNrt2.1 and ZmNrt2.2 of maize seedlings and adult plants were characterized. Local high nitrate supply (LoHN) increased both lateral root length and density of the treated nodal roots of adult maize plants, but only increased lateral root length of the treated primary roots of seedlings. LoHN also increased plant total N acquisition but not N influx rate of the treated roots, when expressed as per unit of root length. Furthermore, LoHN markedly increased specific root length (m g(-1) ) of the treated roots but significantly inhibited the growth of the lateral roots outside of the nitrate-rich patches, suggesting a systemic carbon saving strategy within a whole root system. Surprisingly, local low nitrate supply (LoLN) stimulated nodal root growth of adult plants although LoLN inhibited growth of primary roots of seedlings. LoLN inhibited the N influx rate of the treated roots, and did not change plant total N content. The gene expression of ZmNrt2.1 and ZmNrt2.2 of the treated roots of seedlings and adult plants was inhibited by LoHN but enhanced by LoLN. In conclusion, maize adult roots responded to nitrate-rich and nitrate-poor patches by adaptive morphological alterations and displayed carbon saving strategies in response to heterogeneous nitrate supply.
    The inhibitory effect of ammonium on primary root growth has been well documented; however the underlying physiological and molecular mechanisms are still controversial. To avoid ammonium toxicity to shoot growth, we used a vertical two-layer split plate system, in which the upper layer contained nitrate and the lower layer contained ammonium. In this way, nitrogen status was maintained and only the apical part of the root system was exposed to ammonium. Using a kinematic approach, we show here that 1 mM ammonium reduces primary root growth, decreasing both elemental expansion and cell production. Ammonium inhibits the length of elongation zone and the maximum elemental expansion rate. Ammonium also decreases the apparent length of the meristem as well as the number of dividing cells without affecting cell division rate. Moreover, ammonium reduces the number of root cap cells but appears to affect neither the status of root stem cell niche nor the distal auxin maximum at the quiescent center. Ammonium also inhibits root gravitropism and concomitantly down-regulates the expression of two pivotal auxin transporters, AUX1 and PIN2. Insofar as ammonium inhibits root growth rate in AUX1 and PIN2 loss-of-function mutants almost as strongly as in wild type, we conclude that ammonium inhibits root growth and gravitropism by largely distinct pathways.
    Breeding high-yielding and nutrient-efficient cultivars is one strategy to simultaneously resolve the problems of food security, resource shortage, and environmental pollution. However, the potential increased yield and reduction in fertilizer input achievable by using high-yielding and nutrient-efficient cultivars is unclear. In the present study, we evaluated the yield and nitrogen use efficiency (NUE) of 40 commercial maize hybrids at five locations in North and Northeast China in 2008 and 2009. The effect of interaction between genotype and nitrogen (N) input on maize yield was significant when the yield reduction under low-N treatment was 25%-60%. Based on the average yields achieved with high or low N application, the tested cultivars were classified into four types based on their NUE: efficient-efficient (EE) were efficient under both low and high N inputs, high-N efficient (HNE) under only high N input, low-N efficient (LNE) under only low N input, and nonefficient-nonefficient under neither low nor high N inputs. Under high N application, EE and HNE cultivars could potentially increase maize yield by 8%-10% and reduce N input by 16%-21%. Under low N application, LNE cultivars could potentially increase maize yield by 12%. We concluded that breeding for N-efficient cultivars is a feasible strategy to increase maize yield and/or reduce N input.
    The impact of global changes on food security is of serious concern. Breeding novel crop cultivars adaptable to climate change is one potential solution, but this approach requires an understanding of complex adaptive traits for climate-change conditions. In this study, plant growth, nitrogen (N) uptake, and yield in relation to climatic resource use efficiency of nine representative maize cultivars released between 1973 and 2000 in China were investigated in a 2-year field experiment under three N applications. The Hybrid-Maize model was used to simulate maize yield potential in the period from 1973 to 2011. During the past four decades, the total thermal time (growing degree days) increased whereas the total precipitation and sunshine hours decreased. This climate change led to a reduction of maize potential yield by an average of 12.9% across different hybrids. However, the potential yield of individual hybrids increased by 118.5 kg ha(-1) yr(-1) with increasing year of release. From 1973 to 2000, the use efficiency of sunshine hours, thermal time, and precipitation resources increased by 37%, 40%, and 41%, respectively. The late developed hybrids showed less reduction in yield potential in current climate conditions than old cultivars, indicating some adaptation to new conditions. Since the mid-1990s, however, the yield impact of climate change exhibited little change, and even a slight worsening for new cultivars. Modern breeding increased ear fertility and grain-filling rate, and delayed leaf senescence without modification in net photosynthetic rate. The trade-off associated with delayed leaf senescence was decreased grain N concentration rather than increased plant N uptake, therefore N agronomic efficiency increased simultaneously. It is concluded that modern maize hybrids tolerate the climatic changes mainly by constitutively optimizing plant productivity. Maize breeding programs in the future should pay more attention to cope with the limiting climate factors specifically.
    China's economy has made great strides since 1949, and especially after China has initiated economic reforms and the open-door policy in the 1980s. The growth in agricultural production has been one of the main national accomplishments. By 1996 China was feeding 22% of the global human population with only 7% of the world's arable land. The use of fertilizers and irrigation has played a crucial role. However, further increases in crop production will be more problematic than has been the case, with availability of water being a major limiting factor in China. Agricultural inputs must be reduced, especially nitrogen and phosphorus fertilizers, whose overuse led to environmental problems such as increased greenhouse gas emissions and severe water pollution in parts of China. We emphasize two pivotal components for increasing crop productivity with efficient use of resources (e.g. nutrients and water): (1) the development of integrated soil-crop systems management (ISSM), which will address key constraints using existing crop varieties, and (2) the production of new crop varieties that offer higher yields but use less water, fertilizer or other inputs.
    Root and rhizosphere research has been conducted for many decades, but the underlying strategy of root/rhizosphere processes and management in intensive cropping systems remain largely to be determined. Improved grain production to meet the food demand of an increasing population has been highly dependent on chemical fertilizer input based on the traditionally assumed notion of 'high input, high output', which results in overuse of fertilizers but ignores the biological potential of roots or rhizosphere for efficient mobilization and acquisition of soil nutrients. Root exploration in soil nutrient resources and root-induced rhizosphere processes plays an important role in controlling nutrient transformation, efficient nutrient acquisition and use, and thus crop productivity. The efficiency of root/rhizosphere in terms of improved nutrient mobilization, acquisition, and use can be fully exploited by: (1) manipulating root growth (i.e. root development and size, root system architecture, and distribution); (2) regulating rhizosphere processes (i.e. rhizosphere acidification, organic anion and acid phosphatase exudation, localized application of nutrients, rhizosphere interactions, and use of efficient crop genotypes); and (3) optimizing root zone management to synchronize root growth and soil nutrient supply with demand of nutrients in cropping systems. Experiments have shown that root/rhizosphere management is an effective approach to increase both nutrient use efficiency and crop productivity for sustainable crop production. The objectives of this paper are to summarize the principles of root/rhizosphere management and provide an overview of some successful case studies on how to exploit the biological potential of root system and rhizosphere processes to improve crop productivity and nutrient use efficiency.
    Interspecific root/rhizosphere interactions affect phosphorus (P) uptake and the productivity of maize/faba bean and maize/wheat intercropping systems. The aim of these experiments was to determine whether manipulation of maize root growth could improve the productivity of the two intercropping systems. Two near isogenic maize hybrids (the larger-rooted T149 and smaller-rooted T222) were intercropped with faba bean and wheat, under conditions of high- and low-P availability. The larger-rooted T149 showed greater competitive ability than the smaller-rooted T222 in both maize/faba bean and maize/wheat intercropping systems. The higher competitive ability of T149 improved the productivity of the maize/faba bean intercropping system in P-sufficient conditions. In maize/wheat intercropping systems, root growth, shoot biomass, and P uptake of maize were inhibited by wheat, regardless of the P-supply. Compared with T222, the larger-rooted T149 suffered less in the intercropping systems. The total biomass of the maize/wheat intercropping system was higher for wheat/T149 than for wheat/T222 under low-P conditions. These data suggested that genetic improvement of maize root size could enhance maize growth and its ability to compete for P resources in maize/faba bean and maize/wheat intercropping systems. In addition, depending on the P availability, larger maize roots could increase the productivity of intercropping systems.
    With 2 figures and 2 tables Plant height (PH), ear height (EH) and the PH/EH ratio have a great effect on plant lodging in maize (Zea mays L.) under intensive cropping systems. To understand the genetic mechanisms controlling PH, EH and the PH/EH ratio in response to nitrogen (N) or phosphorus (P) supply, a set of 218 recombinant inbred lines (RILs) was used to evaluate PH, EH, PH/EH ratio and grain yield (GY) and grain yield components under low N, low P and normal N and P supply (NP). A total of 100 QTLs (quantitative trait loci) were detected for the traits investigated. Several QTLs were found to be associated with PH (bin 1.06/1.07) only, both PH and EH (umc1692 at bin 5.01/5.03; umc2313-umc1006 at bin 6.02) and the PH/EH ratio (bnlg1484-bnlg1866 at bin 1.03; umc1164-um1757 at bin 4.01) which was also associated with grain yield. N- or P-inducible QTLs for PH, EH and the PH/EH ratio were found in both years and are sensitive to weather conditions.
    To investigate responses to nitrogen and phosphorus stress, 218 recombinant inbred maize (Zea mays L.) lines were grown under low nitrogen, low phosphorus, and control (i.e., nitrogen and phosphorus sufficient) conditions and evaluated at the silking stage for various traits, including leaf area, leaf chlorophyll content, flowering time, the interval between anthesis and silking, and grain yield. Among the 83 quantitative trait loci (QTL) detected, 29 were for controls, another 29 were for low nitrogen, and 25 were low phosphorus. These loci indicate that there were both common and specific genetic mechanisms underlying the investigated traits. Overlapping QTL for leaf size (area, length, and width) leaf chlorophyll level, flowering time, anthesis–silking interval, and grain yield were located at chromosome bin 2.03/2.04, bin 2.06/2.07/2.08, bin 4.01/4.02, bin 5.03/5.04, bin 6.07, bin 9.03, and bin 10.03/10.04. Many of these loci overlapped with previously reported loci controlling root growth as well as tolerance or response to nutrient deficiency. These QTL identify chromosome regions as targets for genetic improvement of low nitrogen and low phosphorus tolerance.
    Urea-based nitrogen fertilizer was widely utilized in maize production, but transporters involved in urea uptake, translocation and cellular homeostasis have not been identified. Here, we isolated three maize aquapoin genes, ZmNIP2;1, ZmNIP2;4 and ZmTIP4;4, from a cDNA library by heterogeneous complementation of a urea uptake-defective yeast. ZmNIP2;1 and ZmNIP2;4 belonged to the nodulin 26-like intrinsic proteins (NIPs) localized at plasma membrane, and ZmTIP4;4 belonged to the tonoplast intrinsic protein (TIPs) at vacuolar membrane. Quantitative RT-PCR revealed that ZmNIP2;1 was expressed constitutively in various organs while ZmNIP2;4 and ZmTIP4;4 transcripts were abundant in reproductive organs and roots. Expression of ZmTIP4;4 was significantly increased in roots and expanded leaves under nitrogen starvation, while those of ZmNIP2;1 and ZmNIP2;4 remained unaffected. Functions of maize aquapoin genes in urea transport together with their distinct expression manners suggested that they might play diverse roles on urea uptake and translocation, or equilibrating urea concentration across tonoplast.
    Background Crop production in China has been greatly improved by increasing phosphorus (P) fertilizer input, but overuse of P by farmers has caused low use efficiency, increasing environmental risk and accumulation of P in soil. From 1980 to 2007, average 242 kg P ha−1 accumulated in soil, resulting in average soil Olsen P increasing from 7.4 to 24.7 mg kg−1. China is facing huge challenges to improve P use efficiency through optimizing corresponding technology and policies. The problem is exacerbated because people have been shifting their diet from plant-based to animal-enriched foods. This results in higher P load in the food chain and lower P use efficiency. Scope A multidisciplinary approach has been used to improve P management at the field and national level in China. Management strategies based on the soil and on the plant rhizosphere have been developed to increase efficient use of P. A national soil testing and fertilizer recommendation program has been used since 2005 to control build-up and maintenance of P levels. Interactions between root growth and the rhizosphere have been manipulated in intercropping systems and plant genetic traits have been exploited. Phosphorus surplus is highly associated with animal concentrated feed. Conclusions The P-saving potential by the integrated P management strategies of P flow reaches 1.46 Mt P in 2050 compared to 2005.
    In recent years, agricultural growth in China has accelerated remarkably, but most of this growth has been driven by increased yield per unit area rather than by expansion of the cultivated area. Looking towards 2030, to meet the demand for grain and to feed a growing population on the available arable land, it is suggested that annual crop production should be increased to around 580 Mt and that yield should increase by at least 2% annually. Crop production will become more difficult with climate change, resource scarcity (e.g. land, water, energy, and nutrients) and environmental degradation (e.g. declining soil quality, increased greenhouse gas emissions, and surface water eutrophication). To pursue the fastest and most practical route to improved yield, the near-term strategy is application and extension of existing agricultural technologies. This would lead to substantial improvement in crop and soil management practices, which are currently suboptimal. Two pivotal components are required if we are to follow new trajectories. First, the disciplines of soil management and agronomy need to be given increased emphasis in research and teaching, as part of a grand food security challenge. Second, continued genetic improvement in crop varieties will be vital. However, our view is that the biggest gains from improved technology will come most immediately from combinations of improved crops and improved agronomical practices. The objectives of this paper are to summarize the historical trend of crop production in China and to examine the main constraints to the further increase of crop productivity. The paper provides a perspective on the challenge faced by science and technology in agriculture which must be met both in terms of increased crop productivity but also in increased resource use efficiency and the protection of environmental quality.
    Root growth has a fundamental role in nitrogen (N) use efficiency. Nevertheless, little is known about how modern breeding progress has affected root growth and its responses to N supply. The root and shoot growth of a core set of 11 representative Chinese maize (Zea mays L.) hybrids released between 1973 and 2009 were investigated under high N (4 mmol L(-1), HN) and low N (0.04 mmol L(-1), LN) levels in a solution culture system. Compared with LN, HN treatment decreased root dry weight (RDW), the root: shoot ratio (R/S), and the relative growth rate for root dry weight (RGR(root)), but increased the total root length (TRL) and the total lateral root length (LRL). The total axial root length (ARL) per plant was reduced under HN, mostly in hybrids released before the 1990s. The number of seminal roots (SRN) was largely unaffected by different N levels. More recently released hybrids showed higher relative growth rates in the shoot under both HN and LN. However, the roots only showed increased RGR under HN treatment. Correspondingly, there was a positive linear relationship with the year of hybrid release for TRL, LRL and ARL under HN treatment. Together, these results suggest that while shoot growth of maize has improved, its root growth has only improved under high N conditions over the last 36 years of selective breeding in China. Improving root growth under LN conditions may be necessary to increase the N use efficiency of maize.
    With increasing demand of agricultural production, phosphorus (P) is receiving more attention as a non-renewable resource. Applications of chemical P fertilizers and manure have improved soil P fertility and crop production, but caused environmental damage in the past decades. Maintaining a proper P-supplying level at the root zone can maximize the efficiency of plant roots to mobilize and acquire P from the rhizosphere by an integration of root morphological and physiological adaptive strategies. Furthermore, P uptake and utilization by plants plays a vital role in the determination of final crop yield. Better understanding of P dynamics from soil to plant is necessary for optimizing P management and improving P-use efficiency, aiming at reducing consumption of chemical P fertilizer, maximizing exploition of the intrinsic biological potential of root/rhizosphere processes for efficient mobilization and acquisition of soil P by crops as well as recycling P from manure and waste. This update focuses on the dynamic processes determining P availability in the soil and in the rhizosphere, P mobilization, uptake and utilization by plants. It highlights recent advances in the understanding of the P dynamics in the soil/rhizosphere-plant continuum.
    AMMONIUM TRANSPORTER (AMT) proteins are conserved in all domains of life and mediate the transport of ammonium or ammonia across cell membranes. AMTs form trimers and use intermolecular interaction between subunits to regulate activity. So far, binding forces that stabilize AMT protein complexes are not well characterized. High temperature or reducing agents released mono- and dimeric forms from trimeric complexes formed by AMT1;1 from Arabidopsis and tomato. However, in the paralogue LeAMT1;3, trimeric complexes were not detected. LeAMT1;3 differs from the other AMTs by an unusually short N-terminus, suggesting a role for the N-terminus in oligomer stability. Truncation of the N-terminus in LeAMT1;1 destabilized the trimer and led to loss of functionality when expressed in yeast. Swapping of the N-terminus between LeAMT1;1 and LeAMT1;3 showed that sequences in the N-terminus of LeAMT1;1 are necessary and sufficient for stabilization of the interaction among the subunits. Two N-terminal cysteine residues are highly conserved among AMT1 transporters in plants but are lacking in LeAMT1;3. C3S or C27S variants of LeAMT1;1 showed reduced complex stability, which coincided with lower transport capacity for the substrate analogue methylammonium. Both cysteine-substituted LeAMT1;1 variants showed weaker interactions with the wildtype as determined by a quantitative analysis of the complex stability using the mating-based split-ubiquitin assay. These data indicate that the binding affinity of AMT1 subunits is stabilized by cysteines in the N-terminus and suggest a role for disulphide bridge formation via apoplastic N-terminal cysteine residues.
    The primary objective of this study was to better understand how root morphological alteration stimulates N uptake in maize plants after root growth restriction, by investigating the changes in length and number of lateral roots, (15)NO(3)(-) influx, the expression level of the low-affinity Nitrate transporter ZmNrt1.1, and proteomic composition of primary roots. Maize seedlings were hydroponically cultured with three different types of root systems: an intact root system, embryonic roots only, or primary roots only. In spite of sufficient N supply, root growth restriction stimulated compensatory growth of remaining roots, as indicated by the increased lateral root number and root density. On the other hand, there was no significant difference in (15)NO(3)(-) influx between control and primary root plants; neither in ZmNrt1.1 expression levels in primary roots of different treatments. Our data suggested that increased N uptake by maize seedlings experiencing root growth restriction is attributed to root morphological adaptation, rather than explained by the variation in N uptake activity. Eight proteins were differentially accumulated in embryonic and primary root plants compared to control plants. These differentially accumulated proteins were closely related to signal transduction and increased root growth.
    Background Crop production in China has been greatly improved by increasing phosphorus (P) fertilizer input, but overuse of P by farmers has caused low use efficiency, increasing environmental risk and accumulation of P in soil. From 1980 to 2007, average 242 kg P ha−1 accumulated in soil, resulting in average soil Olsen P increasing from 7.4 to 24.7 mg kg−1. China is facing huge challenges to improve P use efficiency through optimizing corresponding technology and policies. The problem is exacerbated because people have been shifting their diet from plant-based to animal-enriched foods. This results in higher P load in the food chain and lower P use efficiency. Scope A multidisciplinary approach has been used to improve P management at the field and national level in China. Management strategies based on the soil and on the plant rhizosphere have been developed to increase efficient use of P. A national soil testing and fertilizer recommendation program has been used since 2005 to control build-up and maintenance of P levels. Interactions between root growth and the rhizosphere have been manipulated in intercropping systems and plant genetic traits have been exploited. Phosphorus surplus is highly associated with animal concentrated feed. Conclusions The P-saving potential by the integrated P management strategies of P flow reaches 1.46 Mt P in 2050 compared to 2005.
    Root traits are important in improving nutrient and water use efficiency. Vertical root pulling resistance (VRPR) has been shown to be closely related to root system characteristics in maize (Zea mays L.). In the present study, two genetic populations derived from the same parents, one containing 218 recombinant inbred lines (RILs) and the other containing 187 advanced backcross BC4F3 lines, were genotyped using 184 SSR markers and evaluated for VRPR, grain yield (GY), stover yield (SY), and nitrogen uptake (Nup) under field conditions over 2years. Our aims were (1) to locate QTLs associated with VRPR, SY, GY, and Nup, (2) to determine whether QTL detection is consistent between the BC4F3 and RIL populations, and (3) to identify backcross lines harboring favorable VRPR QTLs for use in future breeding programs. Using composite interval mapping (CIM), 12 and 17 QTLs were detected in BC4F3 and RIL populations, respectively. An important QTL region in bin 6.02 within the interval umc1006-umc1257 was found to control VRPR, SY, and Nup in both populations. These favorable alleles were contributed by the large-rooted parent Ye478. A significant positive correlation was detected between VRPR, SY, and Nup, but not between VRPR and GY. Backcross lines harboring VRPR QTLs could be useful germplasm for developing near isogenic lines (NILs) and for map-based cloning of genes controlling root growth. KeywordsMaize (Zea mays L.)–Vertical root pulling resistance (VRPR)–QTL mapping–N uptake
    Cytokinin dehydrogenase (CKX) degrades the cytokinin hormone in plants and plays an important role in cytokinin regulatory processes. CKX proteins are encoded by a multigene family with a varying number of members. In this study, 13 maize CKX sequences were collected in which ten transcripts were confirmed by RT-PCR. The tissue- and cytokinin-dependent expression studies indicated that ZmCKX genes exhibit a variety of expression patterns, suggesting diverse functions. Besides 13 maize CKXs, 7 Arabidopsis, 9 poplar, and 11 rice CKX proteins were further used to construct a phylogenetic tree. The CKX members were assigned to six groups, and the intron/exon structures, sequence motifs, and protein properties were conserved within groups. The genome distribution of CKXs supports that segmental duplication contributes to the expansion of the CKX gene family. By quantitative RT–PCR analysis of maize members and digital Northern analysis of Arabidopsis, poplar, and rice members for their tissue expression patterns, highly correlative expression profiles of CKX genes were found among some of the orthologs, whereas different expression manners were found between some of the paralogs. These results suggest functional conservation within each group of the CKX family and provide a clue for transfer of a gene function from one species to the other and further contribute to uncovering the role of CKX genes in planta. KeywordsCytokinin-Cytokinin dehydrogenase/oxidase (CKX)-Evolution-Gene expression-Maize
    The use of nitrogen (N) fertilizers has contributed to the production of a food supply sufficient for both animals and humans despite some negative environmental impact. Sustaining food production by increasing N use efficiency in intensive cropping systems has become a major concern for scientists, environmental groups, and agricultural policymakers worldwide. In high-yielding maize systems the major method of N loss is nitrate leaching. In this review paper, the characteristic of nitrate movement in the soil, N uptake by maize as well as the regulation of root growth by soil N availability are discussed. We suggest that an ideotype root architecture for efficient N acquisition in maize should include (i) deeper roots with high activity that are able to uptake nitrate before it moves downward into deep soil; (ii) vigorous lateral root growth under high N input conditions so as to increase spatial N availability in the soil; and (iii) strong response of lateral root growth to localized nitrogen supply so as to utilize unevenly distributed nitrate especially under limited N conditions.
    Nitrogen (N) over-application is a serious problem in intensive agricultural production areas with consequent large N losses and environmental pollution. In contrast to N, potassium (K) application has been neglected in many developing countries and this has resulted in soil K depletion in agricultural ecosystems and prevented increases in crop yields. Nitrogen-potassium interaction is currently a topic of interest in many studies and the focus of this review is K nutrition under varied N regimes. Nitrogen form and application rate and time influence soil K fixation and release, as well as K uptake, transport, cycling and reutilization within crops. High yielding quality crops can be obtained by optimal N: K nutritional ratios. High rates of applications of N and K do not necessarily lead to increased yield increments and may even reduce yield. Yield response to K uptake depends on N nutritional status and the interaction is usually positive when NO3−-N is supplied. Antagonism between NH 4+ and K+ in uptake was mostly attributed to simple competitive effects in the past while evidence showing mixed-noncompetitive interactions existed. Two components of membrane transport systems for K uptake by plants are a high-affinity K+ transport system which is inhibited by NH 4+ and a low-affinity K+ transport system which is relatively NH 4+ insensitive. Potassium is highly mobile within plants but its flow and partitioning can change depending on the forms of N supply. NH 4+ nutrition in comparison to NO 3−-supply results in more K translocation to leaves. A better understanding of the mechanism of N-K interaction can be a useful guide to best nutrient management in agricultural practice in order to achieve high yields with high nutrient use efficiency.
    background and aims: Roots typically respond to localized nitrate by enhancing lateral-root growth. Polar auxin transport has important roles in lateral-root formation and growth; however, it is a matter of debate whether or how auxin plays a role in the localized response of lateral roots to nitrate. Treating maize (Zea mays) in a split-root system, auxin levels were quantified directly and polar transport was assayed by the movement of [(3)H]IAA. The effects of exogenous auxin and polar auxin transport inhibitors were also examined. Auxin levels in roots decreased more in the nitrate-fed compartment than in the nitrate-free compartment and nitrate treatment appeared to inhibit shoot-to-root auxin transport. However, exogenous application of IAA only partially reduced the stimulatory effect of localized nitrate, and auxin level in the roots was similarly reduced by local applications of ammonium that did not stimulate lateral-root growth. It is concluded that local applications of nitrate reduced shoot-to-root auxin transport and decreased auxin concentration in roots to a level more suitable for lateral-root growth. However, alteration of root auxin level alone is not sufficient to stimulate lateral-root growth.
    Maize β-glucosidase (ZmGLU1) hydrolyzes cytokinin conjugates to release free cytokinins, and regulation of its expression is essential to control cytokinin homeostasis during plant growth and development. Although the promoter of ZmGLU1 gene was found to confer its organ-dependent expression, the underlying mechanism remained to be uncovered. In this study, by promoter deletion assay, a 40-bp A/T-rich sequence in ZmGLU1 promoter region ranging from −798 to −758 (T40) was identified as mediating the repression of promoter activity in seeds and shoot but not in root. Additional fusion of T40 upstream of the constitutive cauliflower mosaic virus 35S promoter (35S) suppressed 35S-driven gene expression in shoot and seeds. Electrophoretic mobility shift assay further revealed a specific interaction between T40 and an unknown binding protein in seeds and shoot but not in root. The negative correlation between ZmGLU1 gene expression/promoter activity and the intensity of T40–protein interaction suggests that T40 acts as a repressor cis-element involved in ZmGLU1 organ-dependent expression. Furthermore, this result provides a new A/T-rich sequence exhibiting transcriptional repression in higher plant. KeywordsA/T-rich cis-element–Promoter–Transcriptional repression– ZmGLU1
    The acquisition of nutrients requires tight regulation to ensure optimal supply while preventing accumulation to toxic levels. Ammonium transporter/methylamine permease/rhesus (AMT/Mep/Rh) transporters are responsible for ammonium acquisition in bacteria, fungi, and plants. The ammonium transporter AMT1;1 from Arabidopsis thaliana uses a novel regulatory mechanism requiring the productive interaction between a trimer of subunits for function. Allosteric regulation is mediated by a cytosolic C-terminal trans-activation domain, which carries a conserved Thr (T460) in a critical position in the hinge region of the C terminus. When expressed in yeast, mutation of T460 leads to inactivation of the trimeric complex. This study shows that phosphorylation of T460 is triggered by ammonium in a time- and concentration-dependent manner. Neither Gln nor l-methionine sulfoximine-induced ammonium accumulation were effective in inducing phosphorylation, suggesting that roots use either the ammonium transporter itself or another extracellular sensor to measure ammonium concentrations in the rhizosphere. Phosphorylation of T460 in response to an increase in external ammonium correlates with inhibition of ammonium uptake into Arabidopsis roots. Thus, phosphorylation appears to function in a feedback loop restricting ammonium uptake. This novel autoregulatory mechanism is capable of tuning uptake capacity over a wide range of supply levels using an extracellular sensory system, potentially mediated by a transceptor (i.e., transporter and receptor).
    Pollen represents an important nitrogen sink in flowers to ensure pollen viability. Since pollen cells are symplasmically isolated during maturation and germination, membrane transporters are required for nitrogen import across the pollen plasma membrane. This study describes the characterization of the ammonium transporter AtAMT1;4, a so far uncharacterized member of the Arabidopsis AMT1 family, which is suggested to be involved in transporting ammonium into pollen. The AtAMT1;4 gene encodes a functional ammonium transporter when heterologously expressed in yeast or when overexpressed in Arabidopsis roots. Concentration-dependent analysis of (15)N-labeled ammonium influx into roots of AtAMT1;4-transformed plants allowed characterization of AtAMT1;4 as a high-affinity transporter with a K(m) of 17 microM. RNA and protein gel blot analysis showed expression of AtAMT1;4 in flowers, and promoter-gene fusions to the green fluorescent protein (GFP) further defined its exclusive expression in pollen grains and pollen tubes. The AtAMT1;4 protein appeared to be localized to the plasma membrane as indicated by protein gel blot analysis of plasma membrane-enriched membrane fractions and by visualization of GFP-tagged AtAMT1;4 protein in pollen grains and pollen tubes. However, no phenotype related to pollen function could be observed in a transposon-tagged line, in which AtAMT1;4 expression is disrupted. These results suggest that AtAMT1;4 mediates ammonium uptake across the plasma membrane of pollen to contribute to nitrogen nutrition of pollen via ammonium uptake or retrieval.
    Despite the fact that urea is a ubiquitous nitrogen source in soils and the most widespread form of nitrogen fertilizer used in agricultural plant production, membrane transporters that might contribute to the uptake of urea in plant roots have so far been characterized only in heterologous systems. Two T-DNA insertion lines, atdur3-1 and atdur3-3, that showed impaired growth on urea as a sole nitrogen source were used to investigate a role of the H+/urea co-transporter AtDUR3 in nitrogen nutrition in Arabidopsis. In transgenic lines expressing AtDUR3-promoter:GFP constructs, promoter activity was upregulated under nitrogen deficiency and localized to the rhizodermis, including root hairs, as well as to the cortex in more basal root zones. Protein gel blot analysis of two-phase partitioned root membrane fractions and whole-mount immunolocalization in root hairs revealed the plasma membrane to be enriched in AtDUR3 protein. Expression of the AtDUR3 gene in nitrogen-deficient roots was repressed by ammonium and nitrate but induced after supply of urea. Higher accumulation of urea in roots of wild-type plants relative to atdur3-1 and atdur3-3 confirmed that urea was the substrate transported by AtDUR3. Influx of 15N-labeled urea in atdur3-1 and atdur3-3 showed a linear concentration dependency up to 200 microM external urea, whereas influx in wild-type roots followed saturation kinetics with an apparent Km of 4 microM. The results indicate that AtDUR3 is the major transporter for high-affinity urea uptake in Arabidopsis roots and suggest that the high substrate affinity of AtDUR3 reflects an adaptation to the low urea levels usually found in unfertilized soils.
    The AMMONIUM TRANSPORTER (AMT) family comprises six isoforms in Arabidopsis thaliana. Here, we describe the complete functional organization of root-expressed AMTs for high-affinity ammonium uptake. High-affinity influx of (15)N-labeled ammonium in two transposon-tagged amt1;2 lines was reduced by 18 to 26% compared with wild-type plants. Enrichment of the AMT1;2 protein in the plasma membrane and localization of AMT1;2 promoter activity in the endodermis and root cortex indicated that AMT1;2 mediates the uptake of ammonium entering the root via the apoplasmic transport route. An amt1;1 amt1;2 amt1;3 amt2;1 quadruple mutant (qko) showed severe growth depression under ammonium supply and maintained only 5 to 10% of wild-type high-affinity ammonium uptake capacity. Transcriptional upregulation of AMT1;5 in nitrogen-deficient rhizodermal and root hair cells and the ability of AMT1;5 to transport ammonium in yeast suggested that AMT1;5 accounts for the remaining uptake capacity in qko. Triple and quadruple amt insertion lines revealed in vivo ammonium substrate affinities of 50, 234, 61, and 4.5 muM for AMT1;1, AMT1;2, AMT1;3, and AMT1;5, respectively, but no ammonium influx activity for AMT2;1. These data suggest that two principle means of achieving effective ammonium uptake in Arabidopsis roots are the spatial arrangement of AMT1-type ammonium transporters and the distribution of their transport capacities at different substrate affinities.
    Ammonium transporter (AMT) proteins of the AMT family mediate the transport of ammonium across plasma membranes. To investigate whether AMTs are regulated at the posttranscriptional level, a gene construct consisting of the cauliflower mosaic virus 35S promoter driving the Arabidopsis (Arabidopsis thaliana) AMT1;1 gene was introduced into tobacco (Nicotiana tabacum). Ectopic expression of AtAMT1;1 in transgenic tobacco lines led to high transcript levels and protein levels at the plasma membrane and translated into an approximately 30% increase in root uptake capacity for 15N-labeled ammonium in hydroponically grown transgenic plants. When ammonium was supplied as the major nitrogen (N) form but at limiting amounts to soil-grown plants, transgenic lines overexpressing AtAMT1;1 did not show enhanced growth or N acquisition relative to wild-type plants. Surprisingly, steady-state transcript levels of AtAMT1;1 accumulated to higher levels in N-deficient roots and shoots of transgenic tobacco plants in spite of expression being controlled by the constitutive 35S promoter. Moreover, steady-state transcript levels were decreased after addition of ammonium or nitrate in N-deficient roots, suggesting a role for N availability in regulating AtAMT1;1 transcript abundance. Nitrogen deficiency-dependent accumulation of AtAMT1;1 mRNA was also observed in 35S:AtAMT1;1-transformed Arabidopsis shoots but not in roots. Evidence for a regulatory role of the 3'-untranslated region of AtAMT1;1 alone in N-dependent transcript accumulation was not found. However, transcript levels of AtAMT1;3 did not accumulate in a N-dependent manner, even though the same T-DNA insertion line atamt1;1-1 was used for 35S:AtAMT1;3 expression. These results show that the accumulation of AtAMT1;1 transcripts is regulated in a N- and organ-dependent manner and suggest mRNA turnover as an additional mechanism for the regulation of AtAMT1;1 in response to the N nutritional status of plants.
    In Arabidopsis four root-expressed AMT genes encode functional ammonium transporters, which raises the question of their role in primary ammonium uptake. After pre-culturing under nitrogen-deficiency conditions, we quantified the influx of (15)N-labeled ammonium in T-DNA insertion lines and observed that the loss of either AMT1;1 or AMT1;3 led to a decrease in the high-affinity ammonium influx of approximately 30%. Under nitrogen-sufficient conditions the ammonium influx was lower in Columbia glabra compared with Wassilewskija (WS), and AMT1;1 did not contribute significantly to the ammonium influx in Col-gl. Ectopic expression of AMT1;3 under the control of a 35S promoter in either of the insertion lines amt1;3-1 or amt1;1-1 increased the ammonium influx above the level of their corresponding wild types. In transgenic lines carrying AMT-promoter-GFP constructs, the promoter activities of AMT1;1 and AMT1;3 were both upregulated under nitrogen-deficiency conditions and were localized to the rhizodermis, including root hairs. AMT gene-GFP fusions that were stably expressed under the control of their own promoters were localized to the plasma membrane. The double insertion line amt1;1-1amt1;3-1 showed a decreased sensitivity to the toxic ammonium analog methylammonium and a decrease in the ammonium influx of up to 70% relative to wild-type plants. These results suggest an additive contribution of AMT1;1 and AMT1;3 to the overall ammonium uptake capacity in Arabidopsis roots under nitrogen-deficiency conditions.
    Forty six maize inbred lines, including 22 CIMMYT QPM lines and 24 Chinese lines, were investigated with SSRs in the study. The objectives of this research were to: (1) investigate the genetic diversity among the 46 inbred lines, (2) assess the genetic distance between subtropical and Chinese Quality Protein Maize (QPM) inbred lines and (3) estimate the genetic relationships between subtropical QPM and Chinese normal inbred lines. In total , 64 simple-sequence repeats (SSRs) primers distributed uniformly throughout the maize genome were considered. The mean value (0.66) of the polymorphic index content (PIC) for the SSR loci provided sufficient discrimination ability for the assessment of genetic diversity among the inbred lines. The mean estimates of genetic similarity (GS) among the 18 Chinese normal maize lines (0.344) and among the six Chinese QPM lines (0.486) were significantly higher (P < 0.01) than the mean GS among 22 CIMMYT QPM lines (0.310). Chinese normal maize lines and QPM lines were considered to have a narrower genetic diversity than the CIMMYT QPM lines. The result of GS estimates among CIMMYT subtropical QPM lines indicated that these lines were derived from such a broad-based germplasm that some lines with a similar pedigree showed large genetic distances. The average linkage (UPGMA) cluster analysis classified the 46 inbred lines into two main clusters. Most Chinese normal maize lines were classified into a large cluster and most CIMMYT subtropical QPM lines were grouped in a different cluster. These results indicated that incorporating CIMMYT subtropical QPM lines into Chinese maize germplasm according to their genetic relationships would enhance the genetic basis of the latter.
    Boron (B) is essential for plants but toxic when present in excess. Arabidopsis thaliana BOR1 is a B exporter for xylem loading and is essential for efficient B translocation from roots to shoots under B limitation. B translocation to shoots was enhanced under B limitation in WT but not in bor1-1 mutant plants. The enhanced translocation was suppressed upon resupply of high levels of B within several hours. Unlike a number of transporters for essential mineral nutrients, BOR1 mRNA accumulation was not strongly affected by B conditions. However, accumulation of a constitutively expressed BOR1-GFP fusion protein was elevated under conditions of limited B supply. Upon resupply of high levels of B, BOR1-GFP was degraded within several hours. These findings demonstrate that posttranscriptional mechanisms play a major role in regulation of BOR1 accumulation. Confocal laser scanning microscopy of root tip cells showed that BOR1-GFP is localized to the plasma membrane under B limitation. Shortly after B application, the protein was observed in dot-like structures in the cytoplasm before degradation. Colocalization studies of the fusion protein with an endocytic tracer FM4-64 and an endosomal Rab-GTPase Ara7 fused to monomeric red fluorescent protein suggested that BOR1 is transferred from the plasma membrane via the endosomes to the vacuole for degradation. These results establish that endocytosis and degradation of BOR1 are regulated by B availability, to avoid accumulation of toxic levels of B in shoots under high-B supply, while protecting the shoot from B deficiency under B limitation. • endosome • FM4-64 • Ara7
    While membrane transporters mediating ammonium uptake across the plasma membrane have been well described at the molecular level, little is known about compartmentation and cellular export of ammonium. (The term ammonium is used to denote both NH3 and NH4+ and chemical symbols are used when specificity is required.) We therefore developed a yeast (Saccharomyces cerevisiae) complementation approach and isolated two Arabidopsis (Arabidopsis thaliana) genes that conferred tolerance to the toxic ammonium analog methylammonium in yeast. Both genes, AtTIP2;1 and AtTIP2;3, encode aquaporins of the tonoplast intrinsic protein subfamily and transported methylammonium or ammonium in yeast preferentially at high medium pH. AtTIP2;1 expression in Xenopus oocytes increased 14C-methylammonium accumulation with increasing pH. AtTIP2;1- and AtTIP2;3-mediated methylammonium detoxification in yeast depended on a functional vacuole, which was in agreement with the subcellular localization of green fluorescent protein-fusion proteins on the tonoplast in planta. Transcript levels of both AtTIPs were influenced by nitrogen supply but did not follow those of the nitrogen-derepressed ammonium transporter gene AtAMT1;1. Transgenic Arabidopsis plants overexpressing AtTIP2;1 did not show altered ammonium accumulation in roots after ammonium supply, although AtTIP2;1 mRNA levels in wild-type plants were up-regulated under these conditions. This study shows that AtTIP2;1 and AtTIP2;3 can mediate the extracytosolic transport of methyl-NH2 and NH3 across the tonoplast membrane and may thus participate in vacuolar ammonium compartmentation.
    In a previous study, two chromosome regions (Scmv1 and Scmv2), conferring sugarcane mosaic virus (SCMV) resistance in maize, were enriched with EcoRI/MseI AFLP (Eco-AFLP) markers (methylation insensitive) by targeted bulked segregant analysis (tBSA). The objec- tive of the present study was to further saturate these two regions with PstI/MseI AFLP (Pst-AFLP) markers (methylation sensitive) using the same tBSA approach, and to compare the genomic distribution of both Pst-AFLP and Eco-AFLP markers. Out of 470 PstI/MseI primer combinations screened, four Pst-AFLP markers were identified in the Scmv1 region (chromosome 6), and none in the Scmv2 region (chromosome 3). These Pst-AFLP markers were more closely linked to the Scmv1a gene than any of the Eco-AFLP markers, and could be useful for marker-assisted selection and even map-based cloning. In addition, Pst-AFLP and Eco-AFLP markers were dissimilarly distri- buted in both target regions. Pst-AFLP markers were equally distributed across both regions, while Eco-AFLPs were clustered in the Scmv2 region.
    Sugarcane mosaic virus (SCMV) is an important virus disease of maize (Zea mays L.) in Europe. In this study, we mapped and characterized two major quantitative trait locus (QTL) regions conferring resistance to SCMV in a maize population consisting of 121 F 3 lines from cross F7 (susceptible) x FAP1360 (resistant). This population was evaluated for SCMV resistance in replicated field trials across two environments under artificial inoculation. Two previously identified chromosome regions (chromosome 6S and 3L) containing QTL conferring resistance were mapped with 24 SSR markers by composite interval mapping. Both regions together explained between 33.0% and 71.1% of the phenotypic variance for SCMV resistance according to the stage of plant development. The Scmv1 region harbored two QTL rather than one QTL as identifed in previous studies. Clustering of QTL for SCMV resistance was confirmed for at least one chromosome region.
    In this report we identified QTLs conferring resistance to Sugarcane Mosaic Virus (SCMV) and their use in marker-assisted selection (MAS) was described. The genetic linkage map covering 1543 cM on ten chromosomes with an average interval of 17.3 cM was constructed with 89 SSR marker loci based on a maize population consisting of 184 F 2 individuals from the cross Huangzao4 (resistant)xYe107 (susceptible). The population of 184 F 2:3 families was phenotyped for resistance to SCMV under artificial inoculation at four developmental stages (seedling, elongation, anthesis, and grain-filling). With the method of composite interval mapping, three QTLs conferring resistance to SCMV were detected on chromosomes 3, 5, and 10 at seedling stage; four QTLs on chromosomes 3, 5, 6, 10 at elongation and anthesis stages; and five on chromosomes 1, 3, 5, 6, 10 at grain-filling stage. Both QTLs on chromosomes 3 and 10 showed significant additive gene effects, while the other three QTLs on chromosomes 1, 5 and 6 displayed dominant, over-dominant and partial dominant effects, respectively. Two major QTLs on chromosomes 3 R-scm1 and 10 R-scm3, explaining 7.2% to 26.9% and 15.3% to 15.8% of the phenotypic variance, were consistently detected at four stages. Based on this study, it was recommended that the MAS approach for SCMV resistance, combined with conventional selection method, could employ the QTLs on chromosomes 3, 5 and 10.
    Genetic similarities of 13 inbred lines of maize (Zea mays L.) were analyzed by restriction fragment length polymorphisms (RFLPs). The objectives of the study were to detect genetic similarities among 13 inbreds and to assign them to heterotic groups. By means of 24 probe-enzyme combinations (PECs) selected for locus specificity, clear patterns and reproducibility, 85 alleles were found with an average of 3.3 alleles per locus. The allelic frequency data were used to estimate genetic similarities among lines, and as a result the diversity index of 0.499 was obtained. Genetic similarities between the pairs of 13 lines ranged from 0.523 up to 0.802 with an average of 0.649. The UPGMA clustering algorithm analysis classified the 13 lines into five groups, which generally corresponded to known maize heterotic groups based on pedigree information. The authors concluded that RRLP-based markers could be used for investigating genetic relationships between maize inbred lines and assigning them to heterotic groups, but it seemed that a large number of PECs were needed to obtain reliable estimates of genetic similarity.
    RFLPs, SSRs, AFLPs and RAPDs were used to detect the genetic diversity among 15 maize inbred lines. A total of 56 probe enzyme combinations, 66 SSR primers, 20 RAPD primers and 9 AFLP primer combinations were identified with polymorphism among the entries, which produced 167, 201, 180 and 87 alleles respectively. SSR markers have the highest polymorphism information content (PIC, 0.47) and AFLP markers have the lowest value (0.36), while AFLP markers possess the highest assay efficiency index (Ai, 32.4). A comparison of genetic similarity matrices revealed that the estimates of correlation coefficients based on RFLPs, SSRs, AFLPs and RAPDs were significantly correlated, but the correlation of RAPD maker data with other markers was lower. These inbred lines were classified into five groups based on four molecular markers data, which are Tangsipingtou, Luda Red Cob, Lancaster, Reid, and PN group. They are consistent with the grouping based on the available pedigree data. Based on the results, we recommend RFLPs and SSRs for genetic diversity analysis among maize germplasm.
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