Genome-wide survey and expression analysis of the amino acid transporter superfamily in potato (Solanum tuberosum L.)

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.

Review: Nitrogen acquisition, assimilation, and seasonal cycling in perennial grasses

Perennial grasses seasonal nitrogen (N) cycle extends the residence and reuse time of N within the plant system, thereby enhancing N use efficiency. Currently, the mechanism of N metabolism has been extensively examined in model plants and annual grasses, and although perennial grasses exhibit similarities, they also possess distinct characteristics. Apart from assimilating and utilizing N throughout the growing season, perennial grasses also translocate N from aerial parts to perennial tissues, such as rhizomes, after autumn senescence. Subsequently, they remobilize the N from these perennial tissues to support new growth in the subsequent year, thereby ensuring their persistence. Previous studies indicate that the seasonal storage and remobilization of N in perennial grasses are not significantly associated with winter survival despite some amino acids and proteins associated with low temperature tolerance accumulating, but primarily with regrowth during the subsequent spring green-up stage. Further investigation can be conducted in perennial grasses to explore the correlation between stored N and dormant bud outgrowth in perennial tissues, such as rhizomes, during the spring green-up stage, building upon previous research on the relationship between N and axillary bud outgrowth in annual grasses. This exploration on seasonal N cycling in perennial grasses can offer valuable theoretical insights for new perennial grasses varieties with high N use efficiency through the application of gene editing and other advanced technologies.

Genome-wide identification and comparative analysis of the Amino Acid Transporter (AAT) gene family and their roles during Phaseolus vulgaris symbioses

Amino acid transporters (AATs) are essential integral membrane proteins that serve multiple roles, such as facilitating the transport of amino acids across cell membranes. They play a crucial role in the growth and development of plants. Phaseolus vulgaris, a significant legume crop, serves as a valuable model for studying root symbiosis. In this study, we have conducted an exploration of the AAT gene family in P. vulgaris. In this research, we identified 84 AAT genes within the P. vulgaris genome sequence and categorized them into 12 subfamilies based on their similarity and phylogenetic relationships with AATs found in Arabidopsis and rice. Interestingly, these AAT genes were not evenly distributed across the chromosomes of P. vulgaris . Instead, there was an unusual concentration of these genes located toward the outer edges of chromosomal arms. Upon conducting motif analysis and gene structural analysis, we observed a consistent presence of similar motifs and an intron-exon distribution pattern among the subfamilies. When we analyzed the expression profiles of PvAAT genes, we noted tissue-specific expression patterns. Furthermore, our investigation into AAT gene expression under rhizobial and mycorrhizal symbiotic conditions revealed that certain genes exhibited high levels of expression. Specifically, ATLa5 and LHT2 was notably upregulated under both symbiotic conditions. These findings point towards a potential role of AATs in the context of rhizobial and mycorrhizal symbiosis in P. vulgaris, in addition to their well-established regulatory functions.

Types of Membrane Transporters and the Mechanisms of Interaction between Them and Reactive Oxygen Species in Plants

Membrane transporters are proteins that mediate the entry and exit of substances through the plasma membrane and organellar membranes and are capable of recognizing and binding to specific substances, thereby facilitating substance transport. Membrane transporters are divided into different types, e.g., ion transporters, sugar transporters, amino acid transporters, and aquaporins, based on the substances they transport. These membrane transporters inhibit reactive oxygen species (ROS) generation through ion regulation, sugar and amino acid transport, hormone induction, and other mechanisms. They can also promote enzymatic and nonenzymatic reactions in plants, activate antioxidant enzyme activity, and promote ROS scavenging. Moreover, membrane transporters can transport plant growth regulators, solute proteins, redox potential regulators, and other substances involved in ROS metabolism through corresponding metabolic pathways, ultimately achieving ROS homeostasis in plants. In turn, ROS, as signaling molecules, can affect the activity of membrane transporters under abiotic stress through collaboration with ions and involvement in hormone metabolic pathways. The research described in this review provides a theoretical basis for improving plant stress resistance, promoting plant growth and development, and breeding high-quality plant varieties.

Genome-wide identification of AAAP gene family and expression analysis in response to saline-alkali stress in foxtail millet (Setaria italica L.)

Amino acid/auxin permease (AAAP) genes encode a large family of protein transporters that play important roles in various aspects of plant growth and development. Here, we performed genome-wide identification of members in the foxtail millet (Setaria italica L.) AAAP family (SiAAAP) and their saline-alkali stress-induced expression patterns, resulting in the identification of 65 SiAAAP genes, which could be divided into eight subfamilies. Except for SiAAAP65, the remaining 64 genes were located on nine chromosomes of foxtail millet. Gene structure and conserved motif analyses indicated that the members in the same subfamily are highly conserved. Gene duplication event analysis suggested that tandem duplication may be the main factor driving the expansion of this gene family, and Ka/Ks analysis indicated that all the duplicated genes have undergone purifying selection. Transcriptome analysis showed differential expression of SiAAAPs in roots, stems, leaves, and tassel inflorescence. Analysis of cis-acting elements in the promoter indicated that SiAAAPs contain stress-responsive cis-acting elements. Under saline-alkali stress, qRT-PCR analysis showed that SiAAP3, SiLHT2, and SiAAP16 were differentially expressed between salt-alkali tolerant millet variety JK3 and salt-alkali sensitive millet variety B175. These results suggest that these genes may be involved in or regulate the response to saline-alkali stress, providing a theoretical basis for further studying the function of SiAAAPs.

Genome-Wide Analysis of the Amino Acid Permeases Gene Family in Wheat and TaAAP1 Enhanced Salt Tolerance by Accumulating Ethylene

Amino acid permeases (AAPs) are proteins of the integral membrane that play important roles in plant growth, development, and responses to various stresses. The molecular functions of several AAPs were characterized in Arabidopsis and rice, but there is still limited information on wheat. Here, we identified 51 AAP genes (TaAAPs) in the wheat genome, classified into six groups based on phylogenetic and protein structures. The chromosome location and gene duplication analysis showed that gene duplication events played a crucial role in the expansion of the TaAAPs gene family. Collinearity relationship analysis revealed several orthologous AAPs between wheat and other species. Moreover, cis-element analysis of promoter regions and transcriptome data suggested that the TaAAPs can respond to salt stress. A TaAAP1 gene was selected and transformed in wheat. Overexpressing TaAAP1 enhanced salt tolerance by increasing the expression of ethylene synthesis genes (TaACS6/TaACS7/TaACS8) and accumulating more ethylene. The present study provides an overview of the AAP family in the wheat genome as well as information on systematics, phylogenetics, and gene duplication, and shows that overexpressing TaAAP1 enhances salt tolerance by regulating ethylene production. These results serve as a theoretical foundation for further functional studies on TaAAPs in the future.

Disruption of the amino acid transporter CsAAP2 inhibits auxin-mediated root development in cucumber

Amino acid transporters are the principal mediators of organic nitrogen distribution within plants and are essential for plant growth and development. Despite this importance, relatively few amino acid transporter genes have been explored and elucidated in cucumber (Cucumis sativus). Here, a total of 86 amino acid transporter genes were identified in the cucumber genome. We further identified Amino Acid Permease (AAP) subfamily members that exhibited distinct expression patterns in different tissues. We found that the CsAAP2 as a candidate gene encoding a functional amino acid transporter is highly expressed in cucumber root vascular cells. CsAAP2 knockout lines exhibited arrested development of root meristem, which then caused the delayed initiation of lateral root and the inhibition of root elongation. What is more, the shoot growth of aap2 mutants was strongly retarded due to defects in cucumber root development. Moreover, aap2 mutants exhibited higher concentrations of amino acids and lignin in roots. We found that the mutant roots had a stronger ability to acidize medium. Furthermore, in the aap2 mutants, polar auxin transport was disrupted in the root tip, leading to high auxin levels in roots. Interestingly, slightly alkaline media rescued their severely reduced root growth by stimulating auxin pathway.

The LHT Gene Family in Rice: Molecular Characterization, Transport Functions and Expression Analysis

Amino acid transporters (AATs) are integral membrane proteins and play important roles in plant growth and development as well as environmental responses. In contrast to the amino acid permease (AAP) subfamily, functional studies of the lysine and histidine transporter (LHT) subfamily have not been made in rice. In the current study, six LHT genes were found in the rice genome. To further investigate the functions of these genes, analyses were performed regarding gene and protein structures, chromosomal locations, evolutionary relationships, cis-acting elements of promoters, gene expression, and yeast complementation. We found that the six OsLHT genes are distributed on 4 out of the 12 chromosomes and that the six OsLHT genes were grouped into two clusters based on the phylogenetic analysis. Protein structure analyses showed that each OsLHT protein has 11 helical transmembrane domains. Yeast complementation assays showed that these OsLHT genes have conserved transport substrates within each cluster. The four members from cluster 1 showed broad amino acid selectivity, while OsLHT5 and OsLHT6 may transport other substrates besides amino acids. Additionally, quantitative real-time PCR analysis of the six OsLHT genes revealed that they have different expression patterns at different developmental stages and in different tissues. It also revealed that some OsLHT genes were responsive to PEG, NaCl and cold treatments, indicating their critical roles in abiotic stress response. Our results will be useful for further characterizing the crucial biological functions of rice LHT genes.

Impact of Foliar Application of Amino Acids on Essential Oil Content, Odor Profile, and Flavonoid Content of Different Mint Varieties in Field Conditions

Mint is an industrial plant that is a good source of essential oil and many phenolic compounds that have several positive benefits to human health and can be used to prevent the development of many diseases. The aim of this research was to investigate the possibility of increasing essential oil and flavonoid content, changing the chemical composition of these compounds in different mint cultivars under foliar application with precursors (phenylalanine, tryptophan, and tyrosine) at two concentrations, 100 and 200 mg L^-1, to enable the possibilities for wider use of these plants when they are grown in field conditions. Spraying with phenylalanine at 100 mg L^-1 concentration increased essential oil content in Mentha piperita 'Granada' plants by 0.53 percentage units. Foliar application with tyrosine solutions at 100 mg L^-1 concentration most effectively influenced the essential oil odor profile Mentha spicata 'Crispa'. The highest number of total flavonoids was in Mentha piperita 'Swiss' sprayed with tyrosine at 100 mg L^-1 concentration. The flavonoid content depended on the mint cultivar, amino acids, and their concentration. The results showed that the effect of amino acid solutions on different secondary metabolites' quantitative and qualitative composition differed depending on the mint cultivar; therefore, amino acids and their concentrations must be selected based on the cultivar they are targeting.

Comprehensive analysis of the LHT gene family in tobacco and functional characterization of NtLHT22 involvement in amino acids homeostasis

Amino acids are vital nitrogen (N) sources for plant growth, development, and yield. The uptake and translocation of amino acids are mediated by amino acid transporters (AATs). The AATs family including lysine-histidine transporters (LHTs), amino acid permeases (AAPs), and proline transporters (ProTs) subfamilies have been identified in various plants. However, little is known about these genes in tobacco. In this study, we identified 23 LHT genes, the important members of AATs, in the tobacco genome. The gene structure, phylogenetic tree, transmembrane helices, chromosomal distribution, cis-regulatory elements, and expression profiles of NtLHT genes were systematically analyzed. Phylogenetic analysis divided the 23 NtLHT genes into two conserved subgroups. Expression profiles confirmed that the NtLHT genes were differentially expressed in various tissues, indicating their potential roles in tobacco growth and development. Cis-elements analysis of promoters and expression patterns after stress treatments suggested that NtLHT genes probable participate in abiotic stress responses of tobacco. In addition, Knock out and overexpression of NtLHT22 changed the amino acids homeostasis in the transgenic plants, the contents of amino acids were significantly decreased in NtLHT22 overexpression plants than wild-type. The results from this study provide important information for further studies on the molecular functions of the NtLHT genes.

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.

Amino acid transporter (AAT) gene family in Tartary buckwheat (Fagopyrum tataricum L. Gaertn.): Characterization, expression analysis and functional prediction

Tartary buckwheat (Fagopyrum tataricum L. Gaertn., TB) is an ancient minor crop and an important food source for humans to supplement nutrients such as flavonoids and essential amino acids. Amino acid transporters (AATs) play critical roles in plant growth and development through the transport of amino acids. In this study, 104 AATs were identified in TB genome and divided into 11 subfamilies by phylogenetic relationships. Tandem and segmental duplications promoted the expansion of FtAAT gene family, and the variations of gene sequence, protein structure and expression pattern were the main reasons for the functional differentiation of FtAATs. Based on RNA-seq and qRT-PCR, the expression patterns of FtAATs in different tissues and under different abiotic stresses were analyzed, and several candidate FtAATs that might affect grain development and response to abiotic stresses were identified, such as FtAAP12 and FtCAT7. Finally, combined with the previous studies, the expression patterns and phylogenetic relationships of AATs in multiple species, the functions of multiple high-confidence FtAAT genes were predicted, and the schematic diagram of FtAATs in TB was initially drawn. Overall, this work provided a framework for further functional analysis of FtAAT genes and important clues for the improvement of TB quality and stress resistance.

Genome-Wide Identification and Functional Analysis of Lysine Histidine Transporter (LHT) Gene Families in Maize

Amino acid transporters (AATs) are essential membrane proteins that transfer amino acids across cells. They are necessary for plant growth and development. The lysine histidine transporter (LHT) gene family in maize (Zea mays) has not yet been characterized. According to sequence composition and phylogenetic placement, this study found 15 LHT genes in the maize genome. The ZmLHT genes are scattered across the plasma membrane. The study also analyzed the evolutionary relationships, gene structures, conserved motifs, 3D protein structure, a transmembrane domain, and gene expression of the 15 LHT genes in maize. Comprehensive analyses of ZmLHT gene expression profiles revealed distinct expression patterns in maize LHT genes in various tissues. This study's extensive data will serve as a foundation for future ZmLHT gene family research. This study might make easier to understand how LHT genes work in maize and other crops.

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.

Theanine metabolism and transport in tea plants (Camellia sinensis L.): advances and perspectives

Theanine, a tea plant-specific non-proteinogenic amino acid, is the most abundant free amino acid in tea leaves. It is also one of the most important quality components of tea because it endows the "umami" taste, relaxation-promoting, and many other health benefits of tea infusion. Its content in tea leaves is directly correlated with the quality and price of green tea. Theanine biosynthesis primarily occurs in roots and is transported to new shoots in tea plants. Recently, great advances have been made in theanine metabolism and transport in tea plants. Along with the deciphering of the genomic sequences of tea plants, new genes in theanine metabolic pathway were discovered and functionally characterized. Theanine transporters were identified and were characterized on the affinity for: theanine, substrate specificity, spatiotemporal expression, and the role in theanine root-to-shoot transport. The mechanisms underlying the regulation of theanine accumulation by: cultivars, seasons, nutrients, and environmental factors are also being rapidly uncovered. Transcription factors were identified to be critical regulators of theanine biosynthesis. In this review, we summarize the progresses in theanine: biosynthesis, catabolism, and transport processes. We also discuss the future studies on theanine in tea plants, and application of the knowledge to crops to synthesize theanine to improve the health-promoting quality of non-tea crops.

Portuguese Common Bean Natural Variation Helps to Clarify the Genetic Architecture of the Legume's Nutritional Composition and Protein Quality

Common bean is a nutritious food legume widely appreciated by consumers worldwide. It is a staple food in Latin America, and a component of the Mediterranean diet, being an affordable source of protein with high potential as a gourmet food. Breeding for nutritional quality, including both macro and micronutrients, and meeting organoleptic consumers' preferences is a difficult task which is facilitated by uncovering the genetic basis of related traits. This study explored the diversity of 106 Portuguese common bean accessions, under two contrasting environments, to gain insight into the genetic basis of nutritional composition (ash, carbohydrates, fat, fiber, moisture, protein, and resistant starch contents) and protein quality (amino acid contents and trypsin inhibitor activity) traits through a genome-wide association study. Single-nucleotide polymorphism-trait associations were tested using linear mixed models accounting for the accessions' genetic relatedness. Mapping resolution to the gene level was achieved in 56% of the cases, with 102 candidate genes proposed for 136 genomic regions associated with trait variation. Only one marker-trait association was stable across environments, highlighting the associations' environment-specific nature and the importance of genotype × environment interaction for crops' local adaptation and quality. This study provides novel information to better understand the molecular mechanisms regulating the nutritional quality in common bean and promising molecular tools to aid future breeding efforts to answer consumers' concerns.

Genome-Wide Analysis of AAT Genes and Their Expression Profiling during Fiber Development in Cotton

Amino acid transporters (AATs) are a kind of membrane proteins that mediate the transport of amino acids across cell membranes in higher plants. The AAT proteins are involved in regulating plant cell growth and various developmental processes. However, the biological function of this gene family in cotton fiber development is not clear. In this study, 190, 190, 101, and 94 full-length AAT genes were identified from Gossypiumhirsutum, G. barbadense, G. arboreum, and G. raimondii. A total of 575 AAT genes from the four cotton species were divided into two subfamilies and 12 clades based on phylogenetic analysis. The AAT genes in the four cotton species were distributed on all the chromosomes. All GhAAT genes contain multiple exons, and each GhAAT protein has multiple conserved motifs. Transcriptional profiling and RT qPCR analysis showed that four GhATT genes tend to express specifically at the fiber initiation stage. Eight genes tend to express specifically at the fiber elongation and maturity stage, and four genes tend to express specifically at the fiber initiation and elongation stages. Our results provide a solid basis for further elucidating the biological function of AAT genes related to cotton fiber development and offer valuable genetic resources for crop improvement in the future.

The tea plant CsLHT1 and CsLHT6 transporters take up amino acids, as a nitrogen source, from the soil of organic tea plantations

Organic tea is more popular than conventional tea that originates from fertilized plants. Amino acids inorganic soils constitute a substantial pool nitrogen (N) available for plants. However, the amino-acid contents in soils of tea plantations and how tea plants take up these amino acids remain largely unknown. In this study, we show that the amino-acid content in the soil of an organic tea plantation is significantly higher than that of a conventional tea plantation. Glutamate, alanine, valine, and leucine were the most abundant amino acids in the soil of this tea plantation. When ¹⁵N-glutamate was fed to tea plants, it was efficiently absorbed and significantly increased the contents of other amino acids in the roots. We cloned seven CsLHT genes encoding amino-acid transporters and found that the expression of CsLHT1, CsLHT2, and CsLHT6 in the roots significantly increased upon glutamate feeding. Moreover, the expression of CsLHT1 or CsLHT6 in a yeast amino-acid uptake-defective mutant, 22∆10α, enabled growth on media with amino acids constituting the sole N source. Amino-acid uptake assays indicated that CsLHT1 and CsLHT6 are H⁺-dependent high- and low-affinity amino-acid transporters, respectively. We further demonstrated that CsLHT1 and CsLHT6 are highly expressed in the roots and are localized to the plasma membrane. Moreover, overexpression of CsLHT1 and CsLHT6 in Arabidopsis significantly improved the uptake of exogenously supplied ¹⁵N-glutamate and ¹⁵N-glutamine. Taken together, our findings are consistent with the involvement of CsLHT1 and CsLHT6 in amino-acid uptake from the soil, which is particularly important for tea plants grown inorganic tea plantations.

Amino acid transporter (AAT) gene family in foxtail millet (Setaria italica L.): widespread family expansion, functional differentiation, roles in quality formation and response to abiotic stresses

Background

Amino acid transporters (AATs) plays an essential roles in growth and development of plants, including amino acids long-range transport, seed germination, quality formation, responsiveness to pathogenic bacteria and abiotic stress by modulating the transmembrane transfer of amino acids. In this study, we performed a genome-wide screening to analyze the AAT genes in foxtail millet (Setaria italica L.), especially those associated with quality formation and abiotic stresses response.

Results

A total number of 94 AAT genes were identified and divided into 12 subfamilies by their sequence characteristics and phylogenetic relationship. A large number (58/94, 62%) of AAT genes in foxtail millet were expanded via gene duplication, involving 13 tandem and 12 segmental duplication events. Tandemly duplicated genes had a significant impact on their functional differentiation via sequence variation, structural variation and expression variation. Further comparison in multiple species showed that in addition to paralogous genes, the expression variations of the orthologous AAT genes also contributed to their functional differentiation. The transcriptomic comparison of two millet cultivars verified the direct contribution of the AAT genes such as SiAAP1, SiAAP8, and SiAUX2 in the formation of grain quality. In addition, the qRT-PCR analysis suggested that several AAT genes continuously responded to diverse abiotic stresses, such as SiATLb1, SiANT1. Finally, combined with the previous studies and analysis on sequence characteristics and expression patterns of AAT genes, the possible functions of the foxtail millet AAT genes were predicted.

Conclusion

This study for the first time reported the evolutionary features, functional differentiation, roles in the quality formation and response to abiotic stresses of foxtail millet AAT gene family, thus providing a framework for further functional analysis of SiAAT genes, and also contributing to the applications of AAT genes in improving the quality and resistance to abiotic stresses of foxtail millet, and other cereal crops.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07779-9.

Identification, systematic evolution and expression analyses of the AAAP gene family in Capsicum annuum

BACKGROUND

The amino acid/auxin permease (AAAP) family represents a class of proteins that transport amino acids across cell membranes. Members of this family are widely distributed in different organisms and participate in processes such as growth and development and the stress response in plants. However, a systematic comprehensive analysis of AAAP genes of the pepper (Capsicum annuum) genome has not been reported.

RESULTS

In this study, we performed systematic bioinformatics analyses to identify AAAP family genes in the C. annuum 'Zunla-1' genome to determine gene number, distribution, structure, duplications and expression patterns in different tissues and stress. A total of 53 CaAAAP genes were identified in the 'Zunla-1' pepper genome and could be divided into eight subgroups. Significant differences in gene structure and protein conserved domains were observed among the subgroups. In addition to CaGAT1, CaATL4, and CaVAAT1, the remaining CaAAAP genes were unevenly distributed on 11 of 12 chromosomes. In total, 33.96% (18/53) of the CaAAAP genes were a result of duplication events, including three pairs of genes due to segmental duplication and 12 tandem duplication events. Analyses of evolutionary patterns showed that segmental duplication of AAAPs in pepper occurred before tandem duplication. The expression profiling of the CaAAAP by transcriptomic data analysis showed distinct expression patterns in various tissues and response to different stress treatment, which further suggest that the function of CaAAAP genes has been differentiated.

CONCLUSIONS

This study of CaAAAP genes provides a theoretical basis for exploring the roles of AAAP family members in C. annuum.

Nitrogen Uptake in Plants: The Plasma Membrane Root Transport Systems from a Physiological and Proteomic Perspective

Nitrogen nutrition in plants is a key determinant in crop productivity. The availability of nitrogen nutrients in the soil, both inorganic (nitrate and ammonium) and organic (urea and free amino acids), highly differs and influences plant physiology, growth, metabolism, and root morphology. Deciphering this multifaceted scenario is mandatory to improve the agricultural sustainability. In root cells, specific proteins located at the plasma membrane play key roles in the transport and sensing of nitrogen forms. This review outlines the current knowledge regarding the biochemical and physiological aspects behind the uptake of the individual nitrogen forms, their reciprocal interactions, the influences on root system architecture, and the relations with other proteins sustaining fundamental plasma membrane functionalities, such as aquaporins and H⁺-ATPase. This topic is explored starting from the information achieved in the model plant Arabidopsis and moving to crops in agricultural soils. Moreover, the main contributions provided by proteomics are described in order to highlight the goals and pitfalls of this approach and to get new hints for future studies.

Wheat amino acid transporters highly expressed in grain cells regulate amino acid accumulation in grain

Amino acids are delivered into developing wheat grains to support the accumulation of storage proteins in the starchy endosperm, and transporters play important roles in regulating this process. RNA-seq, RT-qPCR, and promoter-GUS assays showed that three amino acid transporters are differentially expressed in the endosperm transfer cells (TaAAP2), starchy endosperm cells (TaAAP13), and aleurone cells and embryo of the developing grain (TaAAP21), respectively. Yeast complementation revealed that all three transporters can transport a broad spectrum of amino acids. RNAi-mediated suppression of TaAAP13 expression in the starchy endosperm did not reduce the total nitrogen content of the whole grain, but significantly altered the composition and distribution of metabolites in the starchy endosperm, with increasing concentrations of some amino acids (notably glutamine and glycine) from the outer to inner starchy endosperm cells compared with wild type. Overexpression of TaAAP13 under the endosperm-specific HMW-GS (high molecular weight glutenin subunit) promoter significantly increased grain size, grain nitrogen concentration, and thousand grain weight, indicating that the sink strength for nitrogen transport was increased by manipulation of amino acid transporters. However, the total grain number was reduced, suggesting that source nitrogen remobilized from leaves is a limiting factor for productivity. Therefore, simultaneously increasing loading of amino acids into the phloem and delivery to the spike would be required to increase protein content while maintaining grain yield.

Genome Wide Analysis of Amino Acid Transporter Superfamily in Solanum lycopersicum

Amino acid transporters (AATs) are integral membrane proteins and have several functions, including transporting amino acids across cellular membranes. They are critical for plant growth and development. This study comprehensively identified AAT-encoding genes in tomato (Solanum lycopersicum), which is an important vegetable crop and serves as a model for fleshy fruit development. In this study, 88 genes were identified in the S. lycopersicum genome and grouped into 12 subfamilies, based on previously identified AATs in Arabidopsis, rice (Oryza sativa), and potato (Solanum tuberosum) plants. Chromosomal localization revealed that S. lycopersicum AAT (SlAAT) genes are distributed on the 12 S. lycopersicum chromosomes. Segmental duplication events contribute mainly to the expansion of SlAAT genes and about 32% (29 genes) of SlAAT genes were found to originate from this type of event. Expression profiles of SlAAT genes in various tissues of S. lycopersicum using RNA sequencing data from the Tomato Functional Genomics Database (http://ted.bti.cornell.edu/) showed that SlAAT genes exhibited tissue-specific expression patterns. Comprehensive data generated in this study will provide a platform for further studies on the SlAAT gene family and will facilitate the functional characterization of SlAAT genes.

Soil organic nitrogen: an overlooked but potentially significant contribution to crop nutrition

BACKGROUND

For more than a century, crop N nutrition research has primarily focused on inorganic N (IN) dynamics, building the traditional model that agricultural plants predominantly take up N in the form of NO₃ ^- and NH₄ ⁺. However, results reported in the ecological and agricultural literature suggest that the traditional model of plant N nutrition is oversimplified.

SCOPE

We examine the role of organic N (ON) in plant N nutrition, first by reviewing the historical discoveries by ecologists of plant ON uptake, then by discussing the advancements of key analytical techniques that have furthered the cause (stable isotope and microdialysis techniques). The current state of knowledge on soil ON dynamics is analyzed concurrently with recent developments that show ON uptake and assimilation by agricultural plant species. Lastly, we consider the relationship between ON uptake and nitrogen use efficiency (NUE) in an agricultural context.

CONCLUSIONS

We propose several mechanisms by which ON uptake and assimilation may increase crop NUE, such as by reducing N assimilation costs, promoting root biomass growth, shaping N cycling microbial communities, recapturing exuded N compounds, and aligning the root uptake capacity to the soil N supply in highly fertilized systems. These hypothetical mechanisms should direct future research on the topic. Although the quantitative role remains unknown, ON compounds should be considered as significant contributors to plant N nutrition.

Genome-wide survey of the amino acid transporter gene family in wheat (Triticum aestivum L.): Identification, expression analysis and response to abiotic stress

Amino acid transporters (AATs), which transport amino acids across cell membranes, play important roles in alleviating plant damage under stresses and in plant growth. To data, little is known about the AAT genes in wheat because of its complex genome. In this study, a total of 296 AAT genes were identified from the latest wheat genome sequence (IWGSC v1.1) and classified into 12 distinct subfamilies based upon their sequence composition and phylogenetic relationship. The expansion of the wheat AAT family was mainly the results of whole-genome duplication (WGD) and tandem events. The unequal expansion of different subfamilies brought new features to TaAATs. TaAATs were highly expressed and exhibited distinct expression patterns in different tissues. On the basis of homology and expression pattern analysis, we identified several wheat AAT family members that may affect grain quality. In addition, TaAAP3, TaATLa2 and TaATLb13 exhibited sustained expression in response to drought and high-temperature stress. These genes are involved in the response of wheat to abiotic stress by regulating the transport and distribution of amino acids. Overall, our results help to understand the complexity of TaAATs and provide a theoretical basis for further identification and utilization of AATs in wheat and other crop species.

The amino acid transporter AAP1 mediates growth and grain yield by regulating neutral amino acid uptake and reallocation in Oryza sativa

Nitrogen (N) is a major element necessary for crop yield. In most plants, organic N is primarily transported in the form of amino acids. Here, we show that amino acid permease 1 (AAP1) functions as a positive regulator of growth and grain yield in rice. We found that the OsAAP1 gene is highly expressed in rice axillary buds, leaves, and young panicles, and that the OsAAP1 protein is localized to both the plasma membrane and the nuclear membrane. Compared with the wild-type ZH11, OsAAP1 overexpression (OE) lines exhibited increased filled grain numbers as a result of enhanced tillering, while RNAi and CRISPR (clustered regularly interspaced short palindromic repeat; Osaap1) knockout lines showed the opposite phenotype. In addition, OsAAP1-OE lines had higher concentrations of neutral and acidic amino acids, but lower concentrations of basic amino acids in the straw. An exogenous treatment with neutral amino acids promoted axillary bud outgrowth more strongly in the OE lines than in the WT, RNAi, or Osaap1 lines. Transcriptome analysis of Osaap1 further demonstrated that OsAAP1 may affect N transport and metabolism, and auxin, cytokinin, and strigolactone signaling in regulating rice tillering. Taken together, these results support that increasing neutral amino acid uptake and reallocation via OsAAP1 could improve growth and grain yield in rice.

Amino Acid Transporters in Plants: Identification and Function

Amino acid transporters are the main mediators of nitrogen distribution throughout the plant body, and are essential for sustaining growth and development. In this review, we summarize the current state of knowledge on the identity and biological functions of amino acid transporters in plants, and discuss the regulation of amino acid transporters in response to environmental stimuli. We focus on transporter function in amino acid assimilation and phloem loading and unloading, as well as on the molecular identity of amino acid exporters. Moreover, we discuss the effects of amino acid transport on carbon assimilation, as well as their cross-regulation, which is at the heart of sustainable agricultural production.

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.

Evolutionary significance of amino acid permease transporters in 17 plants from Chlorophyta to Angiospermae

Background

Nitrogen is an indispensable nutrient for plant growth. It is used and transported in the form of amino acids in living organisms. Transporting amino acids to various parts of plants requires relevant transport proteins, such as amino acid permeases (AAPs), which were our focus in this study.

Results

We found that 5 AAP genes were present in Chlorophyte species and more AAP genes were predicted in Bryophyta and Lycophytes. Two main groups were defined and group I comprised 5 clades. Our phylogenetic analysis indicated that the origin of clades 2, 3, and 4 is Gymnospermae and that these clades are closely related. The members of clade 1 included Chlorophyta to Gymnospermae. Group II, as a new branch consisting of non-seed plants, is first proposed in our research. Our results also indicated that the AAP family was already present in Chlorophyta and then expanded accompanying the development of vasculature. Concurrently, the AAP family experienced multiple duplication events that promoted the generation of new functions and differentiation of sub-functions.

Conclusions

Our findings suggest that the AAP gene originated in Chlorophyta, and some non-seed AAP genes clustered in one group. A second group, which contained plants of all evolutionary stages, indicated the evolution of AAPs. These new findings can be used to guide future research.

Genome-wide identification, characterization and expression analysis of the amino acid permease gene family in tea plants (Camellia sinensis)

Amino acid permeases (AAPs) are involved in transporting a broad spectrum of amino acids and regulating physiological processes in plants. In this study, 19 AAP genes were identified from the tea plants genome database and named CsAAP1-19. Based on phylogenetic analysis, the CsAAP genes were classified into three groups, having significantly different structures and conserved motifs. In addition, an expression analysis revealed that most of CsAAP genes were specifically expressed in different tissues, especially CsAAP19 was expressed only in root. These genes also were significantly expressed in the Baiye 1 and Huangjinya cultivars. Nitrogen treatments indicated that the CsAAPs were obviously expressed in root. CsAAP2, -6, -12, -13 and - 16 were significantly expressed at 6 d after the glutamate treatment, while the expression trend at 24 h after contained the ammonium. These results improve our understanding of the CsAAP genes and their functions in nitrogen utilization in tea plants.

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.

Seasonal Theanine Accumulation and Related Gene Expression in the Roots and Leaf Buds of Tea Plants (Camellia Sinensis L.)

Theanine, a unique and abundant non-proteinogenic amino acid in tea, confers to the tea infusion its umami taste and multiple health benefits. Its content in new tea shoots is dynamic in winter and spring. However, its seasonal accumulation pattern and the underlying regulation mechanism of tea plants remain largely unknown. In this study, we measured the theanine contents in the roots and leaf buds of 13 tea cultivars at four time points from winter to spring (Dec. 12, Mar. 1, Mar. 23, and Apr. 13). We found theanine accumulated significantly in the roots to as high as ∼6% dry weight. We found theanine content in the roots was constant or slightly decreased on Mar. 1 compared with Dec.12 but increased consistently on Mar. 23 and then decreased on Apr. 13 in all 13 cultivars. In the leaf buds, theanine content kept increasing from Mar. 1 to Mar. 23 and decreasing from Apr. 13 in most of the 13 cultivars, meaning it was probably both season- and developmental stage-dependent. The expression of theanine biosynthesis and amino acid transporter genes in the roots and buds at the four time points was then examined. The correlation analyses between the gene expression and theanine content suggested the expression of theanine-biosynthesis genes was generally and negatively correlated with theanine content; however, the expression of amino acid transporter genes including CsLHT was generally and positively correlated with theanine contents. Finally, we showed that CsLHT has theanine transport activity. Taken together, this study provided insight into the seasonal regulation of theanine biosynthesis and transport in tea plants during winter and spring.

Genome-wide in silico identification of LysM-RLK genes in potato (Solanum tuberosum L.)

The receptor like kinases (RLKs) belong to the RLK/Pelle superfamily, one of the largest gene families in plants. RLKs play an important role in plant development, as well as in response to biotic and abiotic stresses. The lysine motif receptor like kinases (LysM-RLKs) are a subfamily of RLKs containing at least one lysine motif (LysM) that are involved in the perception of elicitors or pathogen-associated molecular patterns (PAMPs). In the present study, 77 putative RLKs genes and three receptor like proteins were identified in potato (Solanum tuberosum) genome, following a genome-wide search. The 77 potato RLK proteins are classified into two major phylogenetic groups based on their kinase domain amino acid sequence similarities. Out of 77 RLKs, 10 proteins had at least one LysM. Among them three RLP proteins were found in potato genome with either 2 or three tandem LysM but these lacked a cytoplasmic kinase domain. Expression analyses of a potato LysM-RLKs (StLysM-RLK05) was carried out by a Real time RT-PCR, following inoculation of potato leaves and immature tubers with late blight and common scab pathogens, respectively. The expression was significantly higher in resistant than in susceptible following S. scabies inoculation. The StLysM-RLK05 sequence was verified and it was polymorphic in scab susceptible cultivar. The present study provides an overview of the StLysM-RLKs gene family in potato genome. This information is helpful for future functional analysis of such an important protein family, in Solanaceae species.

Genome-wide identification and expression analysis of the AAAP family in Medicago truncatula

The amino acid/auxin permease (AAAP) gene family plays an important role in the long-distance amino acid transport pathway and takes part in various stages of plant growth and development. However, little is known about the AAAP gene family in Medicago truncatula. Here, we identified 86 putative MtAAAP family members using genome sequence information. Based on phylogenetic analysis, these MtAAAP genes were categorized into eight distinct subfamilies. The MtAAAP genes were mapped on 8 chromosomes and duplication events appeared widely, with 19 and 21 pairs of MtAAAP genes showing segment and tandem duplication events, respectively. Ratio of Ka/Ks indicated that duplicated genes underwent purifying selection. Analysis of RNA-seq data showed that MtAAAP genes exhibited specific expression patterns among different tissues and abiotic stress, indicating that MtAAAP members were involved in plant developmental regulation and stress responses. Expression patterns of 16 MtAAAP genes under abiotic stress were verified by qRT-PCR. The present study provides a foundation for the functional analysis of MtAAAPs in developmental regulation and stress responses.

Blocking amino acid transporter OsAAP3 improves grain yield by promoting outgrowth buds and increasing tiller number in rice

Amino acid transporters (AATs) play indispensable roles in nutrient allocation during plant development. In this study, we demonstrated that inhibiting expression of the rice amino acid transporter OsAAP3 increased grain yield due to a formation of larger numbers of tillers as a result of increased bud outgrowth. Elevated expression of OsAAP3 in transgenic plants resulted in significantly higher amino acid concentrations of Lys, Arg, His, Asp, Ala, Gln, Gly, Thr and Tyr, and inhibited bud outgrowth and rice tillering. However, RNAi of OsAAP3 decreased significantly Arg, Lys, Asp and Thr concentrations to a small extent, and thus promoted bud outgrowth, increased significantly tiller numbers and effective panicle numbers per plant, and further enhanced significantly grain yield and nitrogen use efficiency (NUE). The promoter sequences of OsAAP3 showed some divergence between Japonica and Indica rice, and expression of the gene was higher in Japonica, which produced fewer tillers than Indica. We generated knockout lines of OsAAP3 on Japonica ZH11 and KY131 using CRISPR technology and found that grain yield could be increased significantly. These results suggest that manipulation of OsAAP3 expression could be used to increase grain yield in rice.

Understanding the genetic regulation of anthocyanin biosynthesis in plants - Tools for breeding purple varieties of fruits and vegetables

Anthocyanins are naturally occurring flavonoids derived from the phenylpropanoid pathway. There is increasing evidence of the preventative and protective roles of anthocyanins against a broad range of pathologies, including different cancer types and metabolic diseases. However, most of the fresh produce available to consumers typically contains only small amounts of anthocyanins, mostly limited to the epidermis of plant organs. Therefore, transgenic and non-transgenic approaches have been proposed to enhance the levels of this phytonutrient in vegetables, fruits, and cereals. Here, were review the current literature on the anthocyanin biosynthesis pathway in model and crop species, including the structural and regulatory genes involved in the differential pigmentation patterns of plant structures. Furthermore, we explore the genetic regulation of anthocyanin biosynthesis and the reasons why it is strongly repressed in specific cell types, in order to create more efficient breeding strategies to boost the biosynthesis and accumulation of anthocyanins in fresh fruits and vegetables.

Transcriptome analysis and physiological responses of the potato plantlets in vitro under red, blue, and white light conditions

Light is an important factor for plant development and has serious effects on the growth, production and quality of potatoes. However, the physical and molecular mechanisms by which potato plantlets cope with different light qualities are not understood. In this study, the potato "Zhuanxinwu", which is a germplasm potato resource with a high anthocyanin content, was used for physiological and transcriptome profiling analyses to uncover the different mechanisms that occur in response to blue, red and white light conditions, with the white light condition serving as the control. Multiple growth indexes, protective enzyme activity and metabolite accumulation were measured. The results indicated that white light promoted a shift in biomass allocation away from tubers to leaves to enhance dry leaf matter and reduce tuber fresh/dry weight relative to the effects of blue or red light. The leaf area and anthocyanin content values were greater for plants grown in blue light than those grown in white or red light, suggesting that combinations of different spectra were more conducive to regulating potato growth. A total of 2220 differentially expressed genes (DEGs) were found among the three samples, and the DEGs in the three comparison sets were analyzed. A total of 1180 and 984 DEGs were identified in the red light (Red) and blue light (Blue) conditions compared to the control condition, respectively, and 359 DEGs overlapped between the two comparison sets (Blue_vs_White and Red_vs_White). Interestingly, the 24 most common overlapped DEGs were involved in photosynthesis, respiration, and reactive oxygen species (ROS) scavenging. Of these DEGs, four genes involved in photosynthesis and two genes involved in pigment synthesis were highly expressed, implying that some genes could be implemented to cope with different light spectra by regulating the expression of DEGs involved in the corresponding metabolic pathways. In conclusion, our study characterizes physiological responses of potato to different light qualities and identifies potential pathways and candidate genes involved in these responses, thus providing a basis for further research on artificial light regulation of potato plant growth.

Genome-Wide Identification and Characterization of the Potato bHLH Transcription Factor Family

Plant basic/helix–loop–helix (bHLH) transcription factors participate in a number of biological processes, such as growth, development and abiotic stress responses. The bHLH family has been identified in many plants, and several bHLH transcription factors have been functionally characterized in Arabidopsis. However, no systematic identification of bHLH family members has been reported in potato (Solanum tuberosum). Here, 124 StbHLH genes were identified and named according to their chromosomal locations. The intron numbers varied from zero to seven. Most StbHLH proteins had the highly conserved intron phase 0, which accounted for 86.2% of the introns. According to the Neighbor-joining phylogenetic tree, 259 bHLH proteins acquired from Arabidopsis and potato were divided into 15 groups. All of the StbHLH genes were randomly distributed on 12 chromosomes, and 20 tandem duplicated genes and four pairs of duplicated gene segments were detected in the StbHLH family. The gene ontology (GO) analysis revealed that StbHLH mainly function in protein and DNA binding. Through the RNA-seq and quantitative real time PCR (qRT-PCR) analyses, StbHLH were found to be expressed in various tissues and to respond to abiotic stresses, including salt, drought and heat. StbHLH1, 41 and 60 were highly expressed in flower tissues, and were predicted to be involved in flower development by GO annotation. StbHLH45 was highly expressed in salt, drought and heat stress, which suggested its important role in abiotic stress response. The results provide comprehensive information for further analyses of the molecular functions of the StbHLH gene family.

Genome-wide identification of the potato WRKY transcription factor family

WRKY transcription factors play pivotal roles in regulation of stress responses. This study identified 79 WRKY genes in potato (Solanum tuberosum). Based on multiple sequence alignment and phylogenetic relationships, WRKY genes were classified into three major groups. The majority of WRKY genes belonged to Group II (52 StWRKYs), Group III had 14 and Group I consisted of 13. The phylogenetic tree further classified Group II into five sub-groups. All StWRKY genes except StWRKY79 were mapped on potato chromosomes, with eight tandem duplication gene pairs and seven segmental duplication gene pairs found from StWRKY family genes. The expression analysis of 22 StWRKYs showed their differential expression levels under various stress conditions. Cis-element prediction showed that a large number of elements related to drought, heat and salicylic acid were present in the promotor regions of StWRKY genes. The expression analysis indicated that seven StWRKYs seemed to respond to stress (heat, drought and salinity) and salicylic acid treatment. These genes are candidates for abiotic stress signaling for further research.

Update on amino acid transporter functions and on possible amino acid sensing mechanisms in plants

Amino acids are essential components of plant metabolism, not only as constituents of proteins, but also as precursors of important secondary metabolites and as carriers of organic nitrogen between the organs of the plant. Transport across intracellular membranes and translocation of amino acids within the plant is mediated by membrane amino acid transporters. The past few years have seen the identification of a new family of amino acid transporters in Arabidopsis, the characterization of intracellular amino acid transporters, and the discovery of new roles for already known proteins. While amino acid metabolism needs to be tightly coordinated with amino acid transport activity and carbohydrate metabolism, no gene involved in amino acid sensing in plants has been unequivocally identified to date. This review aims at summarizing the recent data accumulated on the identity and function of amino acid transporters in plants, and discussing the possible identity of amino acid sensors based on data from other organisms.