Overexpression of GmAAP6a enhances tolerance to low nitrogen and improves seed nitrogen status by optimizing amino acid partitioning in soybean

TaNAM-6A is essential for nitrogen remobilisation and regulates grain protein content in wheat (Triticum aestivum L.)

Grain protein content (GPC) is a crucial quality trait in bread wheat, which is influenced by the key transcription factor TaNAM. However, the regulatory mechanisms of TaNAM have remained largely elusive. In this study, a new role of TaNAM was unveiled in regulating nitrogen remobilisation which impacts GPC. The TaNAM knockout mutants generated by clustered regularly interspaced short palindromic repeats/Cas9 exhibited significantly delayed senescence and lower GPC, while overexpression of TaNAM-6A resulted in premature senility and much higher GPC. Further analysis revealed that TaNAM directly activates the genes TaNRT1.1 and TaNPF5.5s, which are involved in nitrogen remobilisation. This activity aids in the transfer of nitrogen from leaves to grains for protein synthesis. In addition, an elite allele of TaNAM-6A, associated with high GPC, was identified as a candidate gene for breeding high-quality wheat. Overall, our work not only elucidates the potential mechanism of TaNAM-6A affecting bread wheat GPC, but also highlights the significance of nitrogen remobilisation from senescent leaves to grains for protein accumulation. Moreover, our research provides a new target and approach for improving the quality traits of wheat, particularly the GPC.

High Overexpression of SiAAP9 Leads to Growth Inhibition and Protein Ectopic Localization in Transgenic Arabidopsis

Amino acid permeases (AAPs) transporters are crucial for the long-distance transport of amino acids in plants, from source to sink. While Arabidopsis and rice have been extensively studied, research on foxtail millet is limited. This study identified two transcripts of SiAAP9, both of which were induced by NO3- and showed similar expression patterns. The overexpression of SiAAP9L and SiAAP9S in Arabidopsis inhibited plant growth and seed size, although SiAAP9 was found to transport more amino acids into seeds. Furthermore, SiAAP9-OX transgenic Arabidopsis showed increased tolerance to high concentrations of glutamate (Glu) and histidine (His). The high overexpression level of SiAAP9 suggested its protein was not only located on the plasma membrane but potentially on other organelles, as well. Interestingly, sequence deletion reduced SiAAP9's sensitivity to Brefeldin A (BFA), and SiAAP9 had ectopic localization on the endoplasmic reticulum (ER). Protoplast amino acid uptake experiments indicated that SiAAP9 enhanced Glu transport into foxtail millet cells. Overall, the two transcripts of SiAAP9 have similar functions, but SiAAP9L shows a higher colocalization with BFA compartments compared to SiAAP9S. Our research identifies a potential candidate gene for enhancing the nutritional quality of foxtail millet through breeding.

Hormonal Status of Transgenic Birch with a Pine Glutamine Synthetase Gene during Rooting In Vitro and Budburst Outdoors

Improving nitrogen use efficiency (NUE) is one of the main ways of increasing plant productivity through genetic engineering. The modification of nitrogen (N) metabolism can affect the hormonal content, but in transgenic plants, this aspect has not been sufficiently studied. Transgenic birch (Betula pubescens) plants with the pine glutamine synthetase gene GS1 were evaluated for hormone levels during rooting in vitro and budburst under outdoor conditions. In the shoots of the transgenic lines, the content of indoleacetic acid (IAA) was 1.5-3 times higher than in the wild type. The addition of phosphinothricin (PPT), a glutamine synthetase (GS) inhibitor, to the medium reduced the IAA content in transgenic plants, but it did not change in the control. In the roots of birch plants, PPT had the opposite effect. PPT decreased the content of free amino acids in the leaves of nontransgenic birch, but their content increased in GS-overexpressing plants. A three-year pot experiment with different N availability showed that the productivity of the transgenic birch line was significantly higher than in the control under N deficiency, but not excess, conditions. Nitrogen availability did not affect budburst in the spring of the fourth year; however, bud breaking in transgenic plants was delayed compared to the control. The IAA and abscisic acid (ABA) contents in the buds of birch plants at dormancy and budburst depended both on N availability and the transgenic status. These results enable a better understanding of the interaction between phytohormones and nutrients in woody plants.

Research Progress on the Effect of Nitrogen on Rapeseed between Seed Yield and Oil Content and Its Regulation Mechanism

Rapeseed (Brassica napus L.) is one of the most important oil crops in China. Improving the oil production of rapeseed is an important way to ensure the safety of edible oil in China. Oil production is an important index that reflects the quality of rapeseed and is determined by the oil content and yield. Applying nitrogen is an important way to ensure a strong and stable yield. However, the seed oil content has been shown to be reduced in most rapeseed varieties after nitrogen application. Thus, it is critical to screen elite germplasm resources with stable or improved oil content under high levels of nitrogen, and to investigate the molecular mechanisms of the regulation by nitrogen of oil accumulation. However, few studies on these aspects have been published. In this review, we analyze the effect of nitrogen on the growth and development of rapeseed, including photosynthetic assimilation, substance distribution, and the synthesis of lipids and proteins. In this process, the expression levels of genes related to nitrogen absorption, assimilation, and transport changed after nitrogen application, which enhanced the ability of carbon and nitrogen assimilation and increased biomass, thus leading to a higher yield. After a crop enters the reproductive growth phase, photosynthates in the body are transported to the developing seed for protein and lipid synthesis. However, protein synthesis precedes lipid synthesis, and a large number of photosynthates are consumed during protein synthesis, which weakens lipid synthesis. Moreover, we suggest several research directions, especially for exploring genes involved in lipid and protein accumulation under nitrogen regulation. In this study, we summarize the effects of nitrogen at both the physiological and molecular levels, aiming to reveal the mechanisms of nitrogen regulation in oil accumulation and, thereby, provide a theoretical basis for breeding varieties with a high oil content.

Optimized leaf storage and photosynthetic nitrogen trade-off promote synergistic increases in photosynthetic rate and photosynthetic nitrogen use efficiency

A coordinated increase in the photosynthetic rate (A) and photosynthetic nitrogen use efficiency (PNUE) is an effective strategy for improving crop yield and nitrogen (N) utilization efficiency. PNUE tends to decrease with increasing N levels, but there are natural variations. Consequently, leaf functional N partitioning in Brassica napus genotypes under different N rates was measured to explore the optimized N allocation model for synchronously increasing A and PNUE values. The results showed that genotypes whose PNUE increased with increasing N supply (PNUE-I) produced an approximate A value with a relatively low leaf N content, owing to reduced storage N (Nstore ) and close photosynthetic N (Npsn ) content. Partial least squares path modeling showed that A was dominated by the Npsn content, and PNUE was directly influenced by A and Nstore . The A value increased with the Npsn content until the Npsn content exceeded the threshold value. The boundary line of PNUE varied with the Npsn and Nstore proportions, indicating that the optimum Npsn and Nstore proportions were 51.6% and 40.3%, respectively. The Nstore proportion of PNUE-I was closer to the thresholds and benefited from lower increments in Rubisco content and nonprotein form storage N content with improved N supply. Optimized Nstore and Npsn trade-off by regulating increments in Nstore content with increased N supply, thereby promoting coordinated increases in A and PNUE.

Genetic mapping and functional genomics of soybean seed protein

Soybean is an utterly important crop for high-quality meal protein and vegetative oil. Soybean seed protein content has become a key factor in nutrients for livestock feed as well as human dietary consumption. Genetic improvement of soybean seed protein is highly desired to meet the demands of rapidly growing world population. Molecular mapping and genomic analysis in soybean have identified many quantitative trait loci (QTL) underlying seed protein content control. Exploring the mechanisms of seed storage protein regulation will be helpful to achieve the improvement of protein content. However, the practice of breeding higher protein soybean is challenging because soybean seed protein is negatively correlated with seed oil content and yield. To overcome the limitation of such inverse relationship, deeper insights into the property and genetic control of seed protein are required. Recent advances of soybean genomics have strongly enhanced the understandings for molecular mechanisms of soybean with better seed quality. Here, we review the research progress in the genetic characteristics of soybean storage protein, and up-to-date advances of molecular mappings and genomics of soybean protein. The key factors underlying the mechanisms of the negative correlation between protein and oil in soybean seeds are elaborated. We also briefly discuss the future prospects of breaking the bottleneck of the negative correlation to develop high protein soybean without penalty of oil and yield.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01373-5.

Natural variations of OsAUX5, a target gene of OsWRKY78, control the neutral essential amino acid content in rice grains

Grain essential amino acid (EAA) levels contribute to rice nutritional quality. However, the molecular mechanisms underlying EAA accumulation and natural variation in rice grains remain unclear. Here we report the identification of a previously unrecognized auxin influx carrier subfamily gene, OsAUX5, which encodes an amino acid transporter that functions in uptake of multiple amino acids. We identified an elite haplotype of Pro::OsAUX5^Hap2 that enhances grain EAA accumulation without an apparent negative effect on agronomic traits. Natural variations of OsAUX5 occur in the cis elements of its promoter, which are differentially activated because of the different binding affinity between OsWRKY78 and the W-box, contributing to grain EAA variation among rice varieties. The two distinct haplotypes were shown to have originated from different Oryza rufipogon progenitors, which contributed to the divergence between japonica and indica. Introduction of the indica-type Pro::OsAUX5^Hap2 genotype into japonica could significantly increase EAA levels, indicating that indica-type Pro::OsAUX5^Hap2 can be utilized to increase grain EAAs of japonica varieties. Collectively, our study uncovers an WRKY78-OsAUX5-based regulatory mechanism controlling grain EAA accumulation and provides a potential target for breeding EAA-rich rice.

Understandings and future challenges in soybean functional genomics and molecular breeding

Soybean (Glycine max) is a major source of plant protein and oil. Soybean breeding has benefited from advances in functional genomics. In particular, the release of soybean reference genomes has advanced our understanding of soybean adaptation to soil nutrient deficiencies, the molecular mechanism of symbiotic nitrogen (N) fixation, biotic and abiotic stress tolerance, and the roles of flowering time in regional adaptation, plant architecture, and seed yield and quality. Nevertheless, many challenges remain for soybean functional genomics and molecular breeding, mainly related to improving grain yield through high-density planting, maize-soybean intercropping, taking advantage of wild resources, utilization of heterosis, genomic prediction and selection breeding, and precise breeding through genome editing. This review summarizes the current progress in soybean functional genomics and directs future challenges for molecular breeding of soybean.

Carbon dots promoted soybean photosynthesis and amino acid biosynthesis under drought stress: Reactive oxygen species scavenging and nitrogen metabolism

With global warming and water scarcity, improving the drought tolerance and quality of crops is critical for food security and human health. Here, foliar application of carbon dots (CDs, 5 mg·L^-1) could scavenge reactive oxygen species accumulation in soybean leaves under drought stress, thereby enhancing photosynthesis and carbohydrate transport. Moreover, CDs stimulated root secretion (e.g., amino acids, organic acids, and auxins) and recruited beneficial microorganisms (e.g., Actinobacteria, Ascomycota, Acidobacteria and Glomeromycota), which facilitate nitrogen (N) activation in the soil. Meanwhile, the expression of GmNRT, GmAMT, and GmAQP genes were up-regulated, indicating enhanced N and water uptake. The results demonstrated that CDs could promote nitrogen metabolism and enhance amino acid biosynthesis. Particularly, the N content in soybean shoots and roots increased significantly by 13.2 % and 30.5 %, respectively. The amino acids content in soybean shoots and roots increased by 257.5 % and 57.5 %, respectively. Consequently, soybean yields increased significantly by 21.5 %, and the protein content in soybean kernels improved by 3.7 %. Therefore, foliar application of CDs can support sustainable nano-enabled agriculture to combat climate change.

CsAAP7.2 is involved in the uptake of amino acids from soil and the long-distance transport of theanine in tea plants (Camellia sinensis L.)

Tea plant roots can uptake both inorganic nitrogen (NH4+ and NO3-) and organic nitrogen (amino acids) from the soil. These amino acids are subsequently assimilated into theanine and transported to young shoots through the xylem. Our previous study showed that CsLHT1 and CsLHT6 transporters take up amino acids from the soil, and CsAAPs participate in the transport of theanine. However, whether other amino acid transporters are involved in this process remains unknown. In this study, we identified two new CsAAPs homologous to CsAAP7, named CsAAP7.1 and CsAAP7.2. Heterologous expression of CsAAP7.1 and CsAAP7.2 in the yeast mutant 22Δ10α showed that CsAAP7.2 had the capacity to transport theanine and other amino acids, whereas CsAAP7.1 had no transport activity. Transient expression of the CsAAP7.2-GFP fusion protein in tobacco leaf epidermal cells confirmed its localization to the endoplasmic reticulum. Tissue-specific analysis showed that CsAAP7.2 was highly expressed in roots and stems. In addition, CsAAP7.2 overexpression lines were more sensitive to high concentrations of theanine due to the high accumulation of theanine in seedlings. Taken together, these findings suggested that CsAAP7.2 plays an important role in the uptake of amino acids from soil and the long-distance transport of theanine. These results provide valuable tools for nitrogen nutrition studies and enrich our understanding of theanine transport in tea plants.

N7 -SSPP fusion gene improves salt stress tolerance in transgenic Arabidopsis and soybean through ROS scavenging

Considerable signal crosstalk exists in the regulatory network of senescence and stress response. Numerous senescence-associated genes are also involved in plant stress tolerance. However, the underlying mechanisms and application potential of these genes in stress-tolerant crop breeding remain poorly explored. We found that overexpression of SENESCENCE-SUPPRESSED PROTEIN PHOSPHATASE (SSPP), a negative regulator of leaf senescence, significantly improved plant salt tolerance by increasing reactive oxygen species (ROS) scavenging in both Arabidopsis and soybean. However, overexpression of SSPP severely suppressed normal plant growth, limiting its direct use in agriculture. We previously revealed that the N-terminal 1-14 residues of ACS7 (termed 'N7 ') negatively regulated its protein stability through the ubiquitin/proteasome pathway, and the N7 -mediated protein degradation was suppressed by environmental and senescence signals. To avoid the adverse effects of SSPP, the N7 element was fused to the N-terminus of SSPP. We demonstrated that N7 -SSPP fusion gene effectively rescued SSPP-induced growth suppression but maintained enhanced salt tolerance in Arabidopsis and soybean. Particularly, N7 -SSPP enhanced tolerance to long-term salt stress and increased seed yield in soybean. These results suggest that N7 -SSPP overcomes the disadvantages of SSPP on plant growth inhibition and effectively improves salt tolerance through enhanced ROS scavenging, providing an effective strategy of using posttranslational regulatory element for salt-tolerant crop breeding.

Amino acid transporter gene TaATLa1 from Triticum aestivum L. improves growth under nitrogen sufficiency and is down regulated under nitrogen deficiency

TaATLa1 was identified to respond to nitrogen deprivation through transcriptome analysis of wheat seedlings. TaATLa1 specifically transports Gln, Glu, and Asp, and affects the biomass of Arabidopsis and wheat. Nitrogen is an essential macronutrient and plays a crucial role in wheat production. Amino acids, the major form of organic nitrogen, are remobilized by amino acid transporters (AATs) in plants. AATs are commonly described as central components of essential developmental processes and yield formation via taking up and transporting amino acids in plants. However, few studies have reported the detailed biochemical properties and biological functions of these AATs in wheat. In this study, key genes encoding AATs were screened from transcriptome analysis of wheat seedlings treated with normal nitrogen (NN) and nitrogen deprivation (ND). Among them, 21 AATs were down-regulated and eight AATs were up-regulated under ND treatment. Among the homoeologs, TaATLa1.1-3A, TaATLa1.1-3B, and TaATLa1.1-3D (TaATLa1.1-3A, -3B, and -3D), belonging to amino acid transporter-like a (ATLa) subfamily, were significantly down-regulated in response to ND in wheat, and accordingly were selected for functional analyses. The results demonstrated that TaATLa1.1-3A, -3B, and -3D effectively transported glutamine (Gln), glutamate (Glu), and aspartate (Asp) in yeast. Overexpression of TaAILa1.1-3A, -3B, and -3D in Arabidopsis thaliana L. significantly increased amino acid content in leaves, storage protein content in seeds and the plant biomass under NN. Knockdown of TaATLa1.1-3A, -3B, and -3D in wheat seedlings resulted in a significant block of amino acid remobilization and growth inhibition. Taken together, TaATLa1.1-3A, -3B, and -3D contribute substantially to Arabidopsis and wheat growth. We propose that TaATLa1.1-3A, -3B, and -3D may participate in the source-sink translocation of amino acid, and they may have profound implications for wheat yield improvement.

Identification, Phylogenetic and Expression Analyses of the AAAP Gene Family in Liriodendron chinense Reveal Their Putative Functions in Response to Organ and Multiple Abiotic Stresses

In this study, 52 AAAP genes were identified in the L. chinense genome and divided into eight subgroups based on phylogenetic relationships, gene structure, and conserved motif. A total of 48 LcAAAP genes were located on the 14 chromosomes, and the remaining four genes were mapped in the contigs. Multispecies phylogenetic tree and codon usage bias analysis show that the LcAAAP gene family is closer to the AAAP of Amborella trichopoda, indicating that the LcAAAP gene family is relatively primitive in angiosperms. Gene duplication events revealed six pairs of segmental duplications and one pair of tandem duplications, in which many paralogous genes diverged in function before monocotyledonous and dicotyledonous plants differentiation and were strongly purification selected. Gene expression pattern analysis showed that the LcAAAP gene plays a certain role in the development of Liriodendron nectary and somatic embryogenesis. Low temperature, drought, and heat stresses may activate some WRKY/MYB transcription factors to positively regulate the expression of LcAAAP genes to achieve long-distance transport of amino acids in plants to resist the unfavorable external environment. In addition, the GAT and PorT subgroups could involve gamma-aminobutyric acid (GABA) transport under aluminum poisoning. These findings could lay a solid foundation for further study of the biological role of LcAAAP and improvement of the stress resistance of Liriodendron.

Progress in Soybean Genetic Transformation Over the Last Decade

Soybean is one of the important food, feed, and biofuel crops in the world. Soybean genome modification by genetic transformation has been carried out for trait improvement for more than 4 decades. However, compared to other major crops such as rice, soybean is still recalcitrant to genetic transformation, and transgenic soybean production has been hampered by limitations such as low transformation efficiency and genotype specificity, and prolonged and tedious protocols. The primary goal in soybean transformation over the last decade is to achieve high efficiency and genotype flexibility. Soybean transformation has been improved by modifying tissue culture conditions such as selection of explant types, adjustment of culture medium components and choice of selection reagents, as well as better understanding the transformation mechanisms of specific approaches such as Agrobacterium infection. Transgenesis-based breeding of soybean varieties with new traits is now possible by development of improved protocols. In this review, we summarize the developments in soybean genetic transformation to date, especially focusing on the progress made using Agrobacterium-mediated methods and biolistic methods over the past decade. We also discuss current challenges and future directions.

The plant axis as the command centre for (re)distribution of sucrose and amino acids

Along with the increase in size required for optimal colonization of terrestrial niches, channels for bidirectional bulk transport of materials in land plants evolved during a period of about 100 million years. These transport systems are essentially still in operation - though perfected over the following 400 million years - and make use of hydrostatic differentials. Substances are accumulated or released at the loading and unloading ends, respectively, of the transport channels. The intermediate stretch between the channel termini is bifunctional and executes orchestrated release and retrieval of solutes. Analyses of anatomical and physiological data demonstrate that the release/retrieval zone extends deeper into sources and sinks than is commonly thought and covers usually much more than 99% of the translocation stretch. This review sketches the significance of events in the intermediate stretch for distribution of organic materials over the plant body. Net leakage from the channels does not only serve maintenance and growth of tissues along the pathway, but also diurnal, short-term or seasonal storage of reserve materials, and balanced distribution of organic C- and N-compounds over axial and terminal sinks. Release and retrieval are controlled by plasma-membrane transporters at the vessel/parenchyma interface in the contact pits along xylem vessels and by plasma-membrane transporters at the interface between companion cells and phloem parenchyma along sieve tubes. The xylem-to-phloem pathway vice versa is a bifacial, radially oriented system comprising a symplasmic pathway, of which entrance and exit are controlled at specific membrane checkpoints, and a parallel apoplasmic pathway. A broad range of specific sucrose and amino-acid transporters are deployed at the checkpoint plasma membranes. SUCs, SUTs, STPs, SWEETs, and AAPs, LTHs, CATs are localized to the plasma membranes in question, both in monocots and eudicots. Presence of Umamits in monocots is uncertain. There is some evidence for endo- and exocytosis at the vessel/parenchyma interface supplementary to the transporter-mediated uptake and release. Actions of transporters at the checkpoints are equally decisive for storage and distribution of amino acids and sucrose in monocots and eudicots, but storage and distribution patterns may differ between both taxa. While the majority of reserves is sequestered in vascular parenchyma cells in dicots, lack of space in monocot vasculature urges "outsourcing" of storage in ground parenchyma around the translocation path. In perennial dicots, specialized radial pathways (rays) include the sites for seasonal alternation of storage and mobilization. In dicots, apoplasmic phloem loading and a correlated low rate of release along the path would favour supply with photoassimilates of terminal sinks, while symplasmic phloem loading and a correlated higher rate of release along the path favours supply of axial sinks and transfer to the xylem. The balance between the resource acquisition by terminal and axial sinks is an important determinant of relative growth rate and, hence, for the fitness of plants in various habitats. Body enlargement as the evolutionary drive for emergence of vascular systems and mass transport propelled by hydrostatic differentials.

Strategies for engineering improved nitrogen use efficiency in crop plants via redistribution and recycling of organic nitrogen

Global use of nitrogen (N) fertilizers has increased sevenfold from 1960 to 1995 but much of the N applied is lost to the environment. Modifying the temporal and spatial distribution of organic N within the plant can lead to improved grain yield and/or grain protein content for the same or reduced N fertilizer inputs. Biotechnological approaches to modify whole plant distribution of amino acids and ureides has proven successful in several crop species. Manipulating selective autophagy pathways in crops has also improved N remobilization efficiency to sink tissues whilst the contribution of ribophagy, RNA and purine catabolism to N recycling in crops is still too early to foretell. Improved recycling and remobilization of N must exploit N-stress responsive transcriptional regulators, N-sensing or phloem-localized promotors and genetic variation for N-responsive traits.

Transcriptomic, proteomic, and physiological studies reveal key players in wheat nitrogen use efficiency under both high and low nitrogen supply

The effective use of available nitrogen (N) to improve crop grain yields provides an important strategy to reduce environmental N pollution and promote sustainable agriculture. However, little is known about the common genetic basis of N use efficiency (NUE) at varying N availability. Two wheat (Triticum aestivum L.) cultivars were grown in the field with high, moderate, and low N supply. Cultivar Zhoumai 27 outperformed Aikang 58 independent of the N supply and showed improved growth, canopy leaf area index, flag leaf surface area, grain number, and yield, and enhanced NUE due to both higher N uptake and utilization efficiency. Further, transcriptome and proteome analyses were performed using flag leaves that provide assimilates for grain growth. The results showed that many genes or proteins that are up- or down-regulated under all N regimes are associated with N and carbon metabolism and transport. This was reinforced by cultivar differences in photosynthesis, assimilate phloem transport, and grain protein/starch yield. Overall, our study establishes that improving NUE at both high and low N supply requires distinct adjustments in leaf metabolism and assimilate partitioning. Identified key genes/proteins may individually or concurrently regulate NUE and are promising targets for maximizing crop NUE irrespective of the N supply.

Targeting Nitrogen Metabolism and Transport Processes to Improve Plant Nitrogen Use Efficiency

In agricultural cropping systems, relatively large amounts of nitrogen (N) are applied for plant growth and development, and to achieve high yields. However, with increasing N application, plant N use efficiency generally decreases, which results in losses of N into the environment and subsequently detrimental consequences for both ecosystems and human health. A strategy for reducing N input and environmental losses while maintaining or increasing plant performance is the development of crops that effectively obtain, distribute, and utilize the available N. Generally, N is acquired from the soil in the inorganic forms of nitrate or ammonium and assimilated in roots or leaves as amino acids. The amino acids may be used within the source organs, but they are also the principal N compounds transported from source to sink in support of metabolism and growth. N uptake, synthesis of amino acids, and their partitioning within sources and toward sinks, as well as N utilization within sinks represent potential bottlenecks in the effective use of N for vegetative and reproductive growth. This review addresses recent discoveries in N metabolism and transport and their relevance for improving N use efficiency under high and low N conditions.

Amino Acid Transporters in Plant Cells: A Brief Review

Amino acids are not only a nitrogen source that can be directly absorbed by plants, but also the major transport form of organic nitrogen in plants. A large number of amino acid transporters have been identified in different plant species. Despite belonging to different families, these amino acid transporters usually exhibit some general features, such as broad expression pattern and substrate selectivity. This review mainly focuses on transporters involved in amino acid uptake, phloem loading and unloading, xylem-phloem transfer, import into seed and intracellular transport in plants. We summarize the other physiological roles mediated by amino acid transporters, including development regulation, abiotic stress tolerance and defense response. Finally, we discuss the potential applications of amino acid transporters for crop genetic improvement.

Manipulating Amino Acid Metabolism to Improve Crop Nitrogen Use Efficiency for a Sustainable Agriculture

In a context of a growing worldwide food demand coupled to the need to develop a sustainable agriculture, it is crucial to improve crop nitrogen use efficiency (NUE) while reducing field N inputs. Classical genetic approaches based on natural allelic variations existing within crops have led to the discovery of quantitative trait loci controlling NUE under low nitrogen conditions; however, the identification of candidate genes from mapping studies is still challenging. Amino acid metabolism is the cornerstone of plant N management, which involves N uptake, assimilation, and remobilization efficiencies, and it is finely regulated during acclimation to low N conditions and other abiotic stresses. Over the last two decades, biotechnological engineering of amino acid metabolism has led to promising results for the improvement of crop NUE, and more recently under low N conditions. This review summarizes current work carried out in crops and provides perspectives on the identification of new candidate genes and future strategies for crop improvement.