Seed and vascular expression of a high-affinity transporter for cationic amino acids in Arabidopsis

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.

Amino acid permease RcAAP1 increases the uptake and phloem translocation of an L-valine-phenazine-1-carboxylic acid conjugate

Amino acid conjugates of pesticides can promote the phloem translocation of parent ingredients, allowing for the reduction of usage, and decreased environmental pollution. Plant transporters play important roles in the uptake and phloem translocation of such amino acid-pesticide conjugates such as L-Val-PCA (L-valine-phenazine-1-carboxylic acid conjugate). However, the effects of an amino acid permease, RcAAP1, on the uptake and phloem mobility of L-Val-PCA are still unclear. Here, the relative expression levels of RcAAP1 were found to be up-regulated 2.7-fold and 2.2-fold by the qRT-PCR after L-Val-PCA treatments of Ricinus cotyledons for 1 h and 3 h, respectively. Subsequently, expression of RcAAP1 in yeast cells increased the L-Val-PCA uptake (0.36 μmol/10⁷ cells), which was 2.1-fold higher than the control (0.17 μmol/10⁷ cells). Pfam analysis suggested RcAAP1 with its 11 transmembrane domains belongs to the amino acid transporter family. Phylogenetic analysis found RcAAP1 to be strongly similar to AAP3 in nine other species. Subcellular localization showed that fusion RcAAP1-eGFP proteins were observed in the plasma membrane of mesophyll cells and phloem cells. Furthermore, overexpression of RcAAP1 for 72 h significantly increased the phloem mobility of L-Val-PCA in Ricinus seedlings, and phloem sap concentration of the conjugate was 1.8-fold higher than the control. Our study suggested that RcAAP1 as carrier was involved in the uptake and phloem translocation of L-Val-PCA, which could lay foundation for the utilization of amino acids and further development of vectorized agrochemicals.

Comprehensive genomic characterization of cotton cationic amino acid transporter genes reveals that GhCAT10D regulates salt tolerance

BACKGROUND

The cationic amino acid transporters (CAT) play indispensable roles in maintaining metabolic functions, such as synthesis of proteins and nitric oxide (NO), biosynthesis of polyamine, and flow of amino acids, by mediating the bidirectional transport of cationic amino acids in plant cells.

RESULTS

In this study, we performed a genome-wide and comprehensive study of 79 CAT genes in four species of cotton. Localization of genes revealed that CAT genes reside on the plasma membrane. Seventy-nine CAT genes were grouped into 7 subfamilies by phylogenetic analysis. Structure analysis of genes showed that CAT genes from the same subgroup have similar genetic structure and exon number. RNA-seq and real-time PCR indicated that the expression of most GhCAT genes were induced by salt, drought, cold and heat stresses. Cis-elements analysis of GhCAT promoters showed that the GhCAT genes promoters mainly contained plant hormones responsive elements and abiotic stress elements, which indicated that GhCAT genes may play key roles in response to abiotic stress. Moreover, we also conducted gene interaction network of the GhCAT proteins. Silencing GhCAT10D expression decreased the resistance of cotton to salt stress because of a decrease in the accumulation of NO and proline.

CONCLUSION

Our results indicated that CAT genes might be related with salt tolerance in cotton and lay a foundation for further study on the regulation mechanism of CAT genes in cationic amino acids transporting and distribution responsing to abiotic stress.

Integrated Transcriptome and Targeted Metabolite Analysis Reveal miRNA-mRNA Networks in Low-Light-Induced Lotus Flower Bud Abortion

Most Nelumbo nucifera (lotus) flower buds were aborted during the growing season, notably in low-light environments. How lotus produces so many aborted flower buds is largely unknown. An integrated transcriptome and targeted metabolite analysis was performed to reveal the genetic regulatory networks underlying lotus flower bud abortion. A total of 233 miRNAs and 25,351 genes were identified in lotus flower buds, including 68 novel miRNAs and 1108 novel genes. Further enrichment analysis indicated that sugar signaling plays a potential central role in regulating lotus flower bud abortion. Targeted metabolite analysis showed that trehalose levels declined the most in the aborting flower buds. A potential regulatory network centered on miR156 governs lotus flower bud abortion, involving multiple miRNA-mRNA pairs related to cell integrity, cell proliferation and expansion, and DNA repair. Genetic analysis showed that miRNA156-5p-overexpressing lotus showed aggravated flower bud abortion phenotypes. Trehalose-6-P synthase 1 (TPS1), which is required for trehalose synthase, had a negative regulatory effect on miR156 expression. TPS1-overexpression lotus showed significantly decreased flower bud abortion rates both in normal-light and low-light environments. Our study establishes a possible genetic basis for how lotus produces so many aborted flower buds, facilitating genetic improvement of lotus' shade tolerance.

MAMP-elicited changes in amino acid transport activity contribute to restricting bacterial growth

Plants live under the constant challenge of microbes that probe the environment in search of potential hosts. Plant cells perceive microbe-associated molecular patterns (MAMPs) from incoming microbes and activate defense responses that suppress attempted infections. Despite the substantial progress made in understanding MAMP-triggered signaling pathways, the downstream mechanisms that suppress bacterial growth and disease remain poorly understood. Here, we uncover how MAMP perception in Arabidopsis (Arabidopsis thaliana) elicits dynamic changes in extracellular concentrations of free L-amino acids (AA). Within the first 3 h of MAMP perception, a fast and transient inhibition of AA uptake produces a transient increase in extracellular AA concentrations. Within 4 and 12 h of MAMP perception, a sustained enhanced uptake activity decreases the extracellular concentrations of AA. Gene expression analysis showed that salicylic acid-mediated signaling contributes to inducing the expression of AA/H+ symporters responsible for the MAMP-induced enhanced uptake. A screening of loss-of-function mutants identified the AA/H+ symporter lysin/histidine transporter-1 as an important contributor to MAMP-induced enhanced uptake of AA. Infection assays in lht1-1 seedlings revealed that high concentrations of extracellular AA promote bacterial growth in the absence of induced defense elicitation but contribute to suppressing bacterial growth upon MAMP perception. Overall, the data presented in this study reveal a mechanistic connection between MAMP-induced plant defense and suppression of bacterial growth through the modulation of AA transport activity.

Promoter profiling of Arabidopsis amino acid transporters: clues for improving crops

The review describes the importance of amino acid transporters in plant growth, development, stress tolerance, and productivity. The promoter analysis provides valuable insights into their functionality leading to agricultural benefits. Arabidopsis thaliana genome is speculated to possess more than 100 amino acid transporter genes. This large number suggests the functional significance of amino acid transporters in plant growth and development. The current article summarizes the substrate specificity, cellular localization, tissue-specific expression, and expression of the amino acid transporter genes in response to environmental cues. However, till date functionality of a majority of amino acid transporter genes in plant development and stress tolerance is unexplored. Considering, that gene expression is mainly regulated by the regulatory motifs localized in their promoter regions at the transcriptional levels. The promoter regions ( ~ 1-kbp) of these amino acid transporter genes were analysed for the presence of cis-regulatory motifs responsive to developmental and external cues. This analysis can help predict the functionality of known and unexplored amino acid transporters in different tissues, organs, and various growth and development stages and responses to external stimuli. Furthermore, based on the promoter analysis and utilizing the microarray expression data we have attempted to identify plausible candidates (listed below) that might be targeted for agricultural benefits.

Low-molecular-weight ligands in plants: role in metal homeostasis and hyperaccumulation

Mineral nutrition is one of the key factors determining plant productivity. In plants, metal homeostasis is achieved through the functioning of a complex system governing metal uptake, translocation, distribution, and sequestration, leading to the maintenance of a regulated delivery of micronutrients to metal-requiring processes as well as detoxification of excess or non-essential metals. Low-molecular-weight ligands, such as nicotianamine, histidine, phytochelatins, phytosiderophores, and organic acids, play an important role in metal transport and detoxification in plants. Nicotianamine and histidine are also involved in metal hyperaccumulation, which determines the ability of some plant species to accumulate a large amount of metals in their shoots. In this review we extensively summarize and discuss the current knowledge of the main pathways for the biosynthesis of these ligands, their involvement in metal uptake, radial and long-distance transport, as well as metal influx, isolation and sequestration in plant tissues and cell compartments. It is analyzed how diverse endogenous ligand levels in plants can determine their different tolerance to metal toxic effects. This review focuses on recent advances in understanding the physiological role of these compounds in metal homeostasis, which is an essential task of modern ionomics and plant physiology. It is of key importance in studying the influence of metal deficiency or excess on various physiological processes, which is a prerequisite to the improvement of micronutrient uptake efficiency and crop productivity and to the development of a variety of applications in phytoremediation, phytomining, biofortification, and nutritional crop safety.

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.

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.

The soybean Rhg1 amino acid transporter gene alters glutamate homeostasis and jasmonic acid-induced resistance to soybean cyst nematode

Rhg1 (resistance to Heterodera glycines 1) is an important locus that contributes to resistance against soybean cyst nematode (SCN; Heterodera glycines Ichinohe), which is the most economically damaging disease of soybean worldwide. Simultaneous overexpression of three genes encoding a predicted amino acid transporter, an α-soluble N-ethylmaleimide-sensitive factor attachment protein (α-SNAP) and a predicted wound-induced protein resulted in resistance to SCN provided by this locus. However, the roles of two of these genes (excluding α-SNAP) remain unknown. Here, we report the functional characterization of Glyma.18G022400, a gene at the Rhg1 locus that encodes the predicted amino acid transporter Rhg1-GmAAT. Although the direct role of Rhg1-GmAAT in glutamate transport was not demonstrated, multiple lines of evidence showed that Rhg1-GmAAT impacts glutamic acid tolerance and glutamate transportation in soybean. Transcriptomic and metabolite profiling indicated that overexpression of Rhg1-GmAAT activated the jasmonic acid (JA) pathway. Treatment with a JA biosynthesis inhibitor reduced the resistance provided by the Rhg1-containing PI88788 to SCN, which suggested that the JA pathway might play a role in Rhg1-mediated resistance to SCN. Our results could be helpful for the clarification of the mechanism of resistance to SCN provided by Rhg1 in soybean.

Seasonal nitrogen cycling in temperate trees: Transport and regulatory mechanisms are key missing links

Nutrient accumulation, one of the major ecosystem services provided by forests, is largely due to the accumulation and retention of nutrients in trees. This review focuses on seasonal cycling of nitrogen (N), often the most limiting nutrient in terrestrial ecosystems. When leaves are shed during autumn, much of the N may be resorbed and stored in the stem over winter, and then used for new stem and leaf growth in spring. A framework exists for understanding the metabolism and transport of N in leaves and stems during winter dormancy, but many of the underlying genes remain to be identified and/or verified. Transport of N during seasonal N cycling is a particularly weak link, since the physical pathways for loading and unloading of amino N to and from the phloem are poorly understood. Short-day photoperiod followed by decreasing temperatures are the environmental cues that stimulate dormancy induction, and nutrient remobilization and storage. However, beyond the involvement of phytochrome, very little is known about the signal transduction mechanisms that link environmental cues to nutrient remobilization and storage. We propose a model whereby nutrient transport and sensing plays a major role in source-sink transitions of leaves and stems during seasonal N cycling.

Bacterial production and reconstitution in proteoliposomes of Solanum lycopersicum CAT2: a transporter of basic amino acids and organic cations

The vacuolar SlCAT2 was cloned, over-produced in E. coli and reconstituted in proteoliposomes. Arg, Ornithine and Lys were identified as substrates. Unexpectedly, also the organic cations Tetraethylammonium and Acetylcholine were transported indicating involvement of SlCAT2 in signaling. In land plants several transporters are involved in ion and metabolite flux across membranes of cells or intracellular organelles. The vacuolar amino acid transporter CAT2 from Solanum lycopersicum was investigated in this work. SlCAT2 was cloned from tomato flower cDNA, over-produced in Escherichia coli and purified by Nichel-chelating chromatography. For functional studies, the transporter was reconstituted in proteoliposomes. Competence of SlCAT2 for Arg transport was demonstrated measuring uptake of [³H]Arg in proteoliposomes which was trans-stimulated by internal Arg or ornithine. Uptake of [³H]Ornithine and [³H]Lys was also detected at lower efficiency with respect to [³H]Arg. Transport was activated by the presence of intraliposomal ATP suggesting regulation by the nucleotide. The prototype for organic cations tetraethylammonium (TEA) was also transported by SlCAT2. However, scarce reciprocal inhibition between TEA and Arg was found, while the biguanide metformin was able to strongly inhibit uptake of both substrates. These findings suggest that amino acids and organic cations may interact with the transporter through different functional groups some of which are common for the two types of substrates. Interestingly, reconstituted SlCAT2 showed competence for acetylcholine transport, which was also inhibited by metformin. Kinetics of Arg and Ach transport were performed from which Km values of 0.29 and 0.79 mM were derived, respectively.

Spatiotemporal expression patterns of wheat amino acid transporters reveal their putative roles in nitrogen transport and responses to abiotic stress

Amino acid transporters have roles in amino acid uptake from soil, long-distance transport, remobilization from vegetative tissues and accumulation in grain. Critically, the majority of wheat grain nitrogen is derived from amino acids remobilized from vegetative organs. However, no systematic analysis of wheat AAT genes has been reported to date. Here, 283 full length wheat AAT genes representing 100 distinct groups of homeologs were identified and curated by selectively consolidating IWGSC CSSv2 and TGACv1 Triticum aestivum genome assemblies and reassembling or mapping of IWGSC CSS chromosome sorted reads to fill any gaps. Gene expression profiling was performed using public RNA-seq data from root, leaf, stem, spike, grain and grain cells (transfer cell (TC), aleurone cell (AL), and starchy endosperm (SE)). AATs highly expressed in roots are good candidates for amino acid uptake from soil whilst AATs highly expressed in senescing leaves and stems may be involved in translocation to grain. AATs in TC (TaAAP2 and TaAAP19) and SE (TaAAP13) may play important roles in determining grain protein content and grain yield. The expression levels of AAT homeologs showed unequal contributions in response to abiotic stresses and development, which may aid wheat adaptation to a wide range of environments.

Spatiotemporal expression patterns of wheat amino acid transporters reveal their putative roles in nitrogen transport and responses to abiotic stress

Amino acid transporters have roles in amino acid uptake from soil, long-distance transport, remobilization from vegetative tissues and accumulation in grain. Critically, the majority of wheat grain nitrogen is derived from amino acids remobilized from vegetative organs. However, no systematic analysis of wheat AAT genes has been reported to date. Here, 283 full length wheat AAT genes representing 100 distinct groups of homeologs were identified and curated by selectively consolidating IWGSC CSSv2 and TGACv1 Triticum aestivum genome assemblies and reassembling or mapping of IWGSC CSS chromosome sorted reads to fill any gaps. Gene expression profiling was performed using public RNA-seq data from root, leaf, stem, spike, grain and grain cells (transfer cell (TC), aleurone cell (AL), and starchy endosperm (SE)). AATs highly expressed in roots are good candidates for amino acid uptake from soil whilst AATs highly expressed in senescing leaves and stems may be involved in translocation to grain. AATs in TC (TaAAP2 and TaAAP19) and SE (TaAAP13) may play important roles in determining grain protein content and grain yield. The expression levels of AAT homeologs showed unequal contributions in response to abiotic stresses and development, which may aid wheat adaptation to a wide range of environments.

Role of proline in cell wall synthesis and plant development and its implications in plant ontogeny

Proline is a proteogenic amino acid and accumulates both under stress and non-stress conditions as a beneficial solute in plants. Recent discoveries point out that proline plays an important role in plant growth and differentiation across life cycle. It is a key determinant of many cell wall proteins that plays important roles in plant development. The role of extensins, arabinogalactan proteins and hydroxyproline- and proline-rich proteins as important components of cell wall proteins that play pivotal roles in cell wall signal transduction cascades, plant development and stress tolerance is discussed in this review. Molecular insights are also provided here into the plausible roles of proline transporters modulating key events in plant development. In addition, the roles of proline during seed developmental transitions including storage protein synthesis are discussed.

Accumulation, selection and covariation of amino acids in sieve tube sap of tansy (Tanacetum vulgare) and castor bean (Ricinus communis): evidence for the function of a basic amino acid transporter and the absence of a γ-amino butyric acid transporter

Sieve tube sap was obtained from Tanacetum by aphid stylectomy and from Ricinus after apical bud decapitation. The amino acids in sieve tube sap were analyzed and compared with those from leaves. Arginine and lysine accumulated in the sieve tube sap of Tanacetum more than 10-fold compared to the leaf extracts and they were, together with asparagine and serine, preferably selected into the sieve tube sap, whereas glycine, methionine/tryptophan and γ-amino butyric acid were partially or completely excluded. The two basic amino acids also showed a close covariation in sieve tube sap. The acidic amino acids also grouped together, but antagonistic to the other amino acids. The accumulation ratios between sieve tube sap and leaf extracts were smaller in Ricinus than in Tanacetum. Arginine, histidine, lysine and glutamine were enriched and preferentially loaded into the phloem, together with isoleucine and valine. In contrast, glycine and methionine/tryptophan were partially and γ-amino butyric acid almost completely excluded from sieve tube sap. The covariation analysis grouped arginine together with several neutral amino acids. The acidic amino acids were loaded under competition with neutral amino acids. It is concluded from comparison with the substrate specificities of already characterized plant amino acid transporters, that an AtCAT1-like transporter functions in phloem loading of basic amino acids, whereas a transporter like AtGAT1 is absent in phloem. Although Tanacetum and Ricinus have different minor vein architecture, their phloem loading specificities for amino acids are relatively similar.

Altered growth and improved resistance of Arabidopsis against Pseudomonas syringae by overexpression of the basic amino acid transporter AtCAT1

Amino acid transporters in plants are crucial for distributing amino acids between plant organs and cellular compartments. The H(+)-coupled plasma membrane transporter CAT1 (cationic amino acid transporter 1) facilitates the high-affinity uptake of basic amino acids. The uptake of lysine (Lys) via the roots was not altered in loss-of-function mutants, in accordance with the minor expression of CAT1 in roots, but plants ectopically overexpressing CAT1 incorporated Lys at higher rates. Exogenous Lys inhibited the primary root of Arabidopsis, whereas lateral roots were stimulated. These effects were augmented by the presence or absence of CAT1. Furthermore, the total biomass of soil-grown plants ectopically overexpressing CAT1 was reduced and the time to flowering was accelerated. These effects were accompanied by only minor changes in the overall amino acid profile. Interestingly, CAT1 belongs to a specific small cluster of nitrogen-containing metabolite transporter genes that are rapidly up-regulated upon infection with Pseudomonas syringae and that may participate in the systemic response of plants to pathogen attack. The overexpression of CAT1 indeed enhanced the resistance to the hemibiotrophic bacterial pathogen P. syringae via a constitutively activated salicylic acid (SA) pathway, which is consistent with the developmental defects and the resistance phenotype.

The AAP gene family for amino acid permeases contributes to development of the cyst nematode Heterodera schachtii in roots of Arabidopsis

The beet cyst nematode Heterodera schachtii is able to infect Arabidopsis plants and induce feeding sites in the root. These syncytia are the only source of nutrients for the nematodes throughout their life and are a nutrient sink for the host plant. We have studied here the role of amino acid transporters for nematode development. Arabidopsis contains a large number of different amino acid transporters in several gene families but those of the AAP family were found to be especially expressed in syncytia. Arabidopsis contains 8 AAP genes and they were all strongly expressed in syncytia with the exception of AAP5 and AAP7, which were slightly downregulated. We used promoter::GUS lines and in situ RT-PCR to confirm the expression of several AAP genes and LHT1, a lysine- and histidine-specific amino acid transporter, in syncytia. The strong expression of AAP genes in syncytia indicated that these transporters are important for the transport of amino acids into syncytia and we used T-DNA mutants for several AAP genes to test for their influence on nematode development. We found that mutants of AAP1, AAP2, and AAP8 significantly reduced the number of female nematodes developing on these plants. Our study showed that amino acid transport into syncytia is important for the development of the nematodes.

Functional characterization of cis-elements conferring vascular vein expression of At4g34880 amidase family protein gene in Arabidopsis

The expression of At4g34880 gene encoding amidase in Arabidopsis was characterized in this study. A promoter region of 1.5 kb on the upstream of the start codon of the gene (referred as AmidP) was fused with uidA (GUS) reporter gene, and transformed into Arabidopsis plant for determining its spatial expression. The results indicated that AmidP drived GUS expression in vascular system, predominately in leaves. Truncation analysis of AmidP demonstrated that VASCULAR VEIN ELEMENT (VVE) motif with a region of 176 bp sequence (−1500 to −1324) was necessary and sufficient to direct the vascular vein specific GUS expression in the transgenic plant. Tandem copy of VVE increased vascular system expression, and 5′- and 3′- deletions of VVE motif in combination with a truncated −65 CaMV 35S minimal promoter showed that 11bp cis-acting element, naming DOF2 domain, played an essential role for the vascular vein specific expression. Meanwhile, it was also observed that the other cis-acting elements among the VVE region are also associated with specificity or strength of GUS activities in vascular system.

Two Arabidopsis thaliana dihydrodipicolinate synthases, DHDPS1 and DHDPS2, are unequally redundant

In Arabidopsis thalinana (L.) Heynh., DHDPS1 and DHDPS2 encode orthologous dihydrodipicolinate synthases (DHDPS), the first enzyme of the lysine (Lys) biosynthesis pathway. A TDNA insertion mutant of dhdps2 was previously reported to be viable and to accumulate free threonine (Thr). Analysis of additional TDNA insertion lines showed that dhdps1 and dhdps2 mutants are both viable and that whereas dhdps2 mutants accumulate Thr, dhdps1 plants do not. Thr-accumulation was complemented by heterologous expression of Escherichia coli DapA, indicating that the phenotype is due to reduced DHDPS activity in dhdps2. DHDPS1 contributes ~30% towards the total DHDPS activity in leaves of young plants and DHDPS2 contributes 70%; therefore, the threshold of activity resulting in Thr accumulation lies within this narrow range. dhdps1-dhdps2 double mutants could not be isolated, even after exogenous feeding with Lys. Segregation analysis indicated that gametes lacking functional DHDPS genes are defective, as are embryos. Plants carrying only a single DHDPS2 gene do not accumulate Thr, but they show a gametophytic defect that is partially rescued by Lys application. Despite the accumulation of Thr, dhdps2 seedlings are no more sensitive than wild-type plants to growth inhibition by Lys or the Lys precursor diaminopimelate. They also are not rescued by methionine at growth-inhibitory Lys concentrations. Exogenous application of Lys and methionine to dhdps2 mutants did not reduce the accumulation of Thr.

Genome-wide survey and expression analysis of amino acid transporter gene family in rice (Oryza sativa L.)

Background

Amino acid transporters (AATs) that transport amino acids across cellular membranes are essential for plant growth and development. To date, a genome-wide overview of the AAT gene family in rice is not yet available.

Methodology/Principal Findings

In this study, a total of 85 AAT genes were identified in rice genome and were classified into eleven distinct subfamilies based upon their sequence composition and phylogenetic relationship. A large number of OsAAT genes were expanded via gene duplication, 23 and 24 OsAAT genes were tandemly and segmentally duplicated, respectively. Comprehensive analyses were performed to investigate the expression profiles of OsAAT genes in various stages of vegetative and reproductive development by using data from EST, Microarrays, MPSS and Real-time PCR. Many OsAAT genes exhibited abundant and tissue-specific expression patterns. Moreover, 21 OsAAT genes were found to be differentially expressed under the treatments of abiotic stresses. Comparative analysis indicates that 26 AAT genes with close evolutionary relationships between rice and Arabidopsis exhibited similar expression patterns.

Conclusions/Significance

This study will facilitate further studies on OsAAT family and provide useful clues for functional validation of OsAATs.

The leaf reticulate mutant dov1 is impaired in the first step of purine metabolism

A series of reticulated Arabidopsis thaliana mutants were previously described. All mutants show a reticulate leaf pattern, namely green veins on a pale leaf lamina. They have an aberrant mesophyll structure but an intact layer of bundle sheath cells around the veins. Here, we unravel the function of the previously described reticulated EMS-mutant dov1 (differential development of vascular associated cells 1). By positional cloning, we identified the mutated gene, which encodes glutamine phosphoribosyl pyrophosphate aminotransferase 2 (ATase2), an enzyme catalyzing the first step of purine nucleotide biosynthesis. dov1 is allelic to the previously characterized cia1-2 mutant that was isolated in a screen for mutants with impaired chloroplast protein import. We show that purine-derived total cytokinins are lowered in dov1 and crosses with phytohormone reporter lines revealed differential reporter activity patterns in dov1. Metabolite profiling unraveled that amino acids that are involved in purine biosynthesis are increased in dov1. This study identified the molecular basis of an established mutant line, which has the potential for further investigation of the interaction between metabolism and leaf development.

Nucleoside transport across the plasma membrane mediated by equilibrative nucleoside transporter 3 influences metabolism of Arabidopsis seedlings

The metabolism of nitrogen-rich nucleosides in Arabidopsis seedlings was investigated at the level of import and subsequent salvage or degradation. Uptake and fate of nucleosides imported by equilibrative nucleoside transporter 3 (ENT3) was analysed and, furthermore, a comprehensive analysis of the effect of exogenously fed nucleosides at the level of metabolic as well as transcriptomic alterations was performed. Expression of nucleoside transporters ENT1 and ENT3, together with nucleoside import, was increased upon nitrogen limitation. Thereby a role for ENT3, which is expressed mainly in the vasculature of roots and leaves, as a major import route for nucleosides was supported. Exogenously fed nucleosides were able to attenuate nitrogen starvation effects such as chlorophyll breakdown, anthocyanin accumulation, RNA breakdown and reduced levels of amino acids. In response to nucleoside supply, up-regulation of genes involved in nitrogen distribution in plants was observed. In addition, genes involved in nucleoside metabolism were identified as regulated upon nitrogen limitation. In summary, an overall beneficial effect of nucleoside supply to Arabidopsis seedlings, especially under limiting nitrogen conditions, was observed.

Metabolic profiling of a mapping population exposes new insights in the regulation of seed metabolism and seed, fruit, and plant relations

To investigate the regulation of seed metabolism and to estimate the degree of metabolic natural variability, metabolite profiling and network analysis were applied to a collection of 76 different homozygous tomato introgression lines (ILs) grown in the field in two consecutive harvest seasons. Factorial ANOVA confirmed the presence of 30 metabolite quantitative trait loci (mQTL). Amino acid contents displayed a high degree of variability across the population, with similar patterns across the two seasons, while sugars exhibited significant seasonal fluctuations. Upon integration of data for tomato pericarp metabolite profiling, factorial ANOVA identified the main factor for metabolic polymorphism to be the genotypic background rather than the environment or the tissue. Analysis of the coefficient of variance indicated greater phenotypic plasticity in the ILs than in the M82 tomato cultivar. Broad-sense estimate of heritability suggested that the mode of inheritance of metabolite traits in the seed differed from that in the fruit. Correlation-based metabolic network analysis comparing metabolite data for the seed with that for the pericarp showed that the seed network displayed tighter interdependence of metabolic processes than the fruit. Amino acids in the seed metabolic network were shown to play a central hub-like role in the topology of the network, maintaining high interactions with other metabolite categories, i.e., sugars and organic acids. Network analysis identified six exceptionally highly co-regulated amino acids, Gly, Ser, Thr, Ile, Val, and Pro. The strong interdependence of this group was confirmed by the mQTL mapping. Taken together these results (i) reflect the extensive redundancy of the regulation underlying seed metabolism, (ii) demonstrate the tight co-ordination of seed metabolism with respect to fruit metabolism, and (iii) emphasize the centrality of the amino acid module in the seed metabolic network. Finally, the study highlights the added value of integrating metabolic network analysis with mQTL mapping.

Seeds represent 70% of the food source for man and livestock. However, as a result of millennia of domestication, crop plants have undergone major genetic deterioration, leading to a loss of important quality traits. Thus, the reintroduction of these quality traits is the key to the improvement of crops in modern agriculture. Seed quality traits include nutritional components, such as proteins and amino acids, and seed germination and storability, which are, in turn, inherently related to metabolism. To understand the genetic basis of seed metabolism—a strategic need in the improvement of seed crops—we studied a collection of offspring plants stemming from the cross between a domesticated tomato cultivar Solanum lycopersicum cv M82 and its distant wild relative S. pennellii. We monitored the changes in metabolism and studied the mode of regulation of the concentration of metabolites in the seeds as a result of genetic introgression, by taking advantage of state-of-the-art technologies and methods of data elaboration such as network-based analysis. We identified a number of candidate genes that may be useful in manipulations to enhance nutritional values in seeds. Finally, in an effort to study the relation among the seed, the fruit, and the mother plant, we determined potential yield-associated metabolic markers.

Amino Acid transporter inventory of the selaginella genome

Amino acids play fundamental roles in a multitude of functions including protein synthesis, hormone metabolism, nerve transmission, cell growth, production of metabolic energy, nucleobase synthesis, nitrogen metabolism, and urea biosynthesis. Selaginella as a member of the lycophytes is part of an ancient lineage of vascular plants that had arisen ∼400 million years ago. In angiosperms, which have attracted most of the attention for nutrient transport so far, we have been able to identify many of the key transporters for nitrogen. Their role is not always fully clear, thus an analysis of Selaginella as a representative of an ancient vascular plant may help shed light on the evolution and function of these diverse transporters. Here we annotated and analyzed the genes encoding putative transporters involved in cellular uptake of amino acids present in the Selaginella genome.

New perspectives on nodule nitrogen assimilation in actinorhizal symbioses

Nitrogen-fixing root nodules are plant organs specialised for symbiotic transfer of nitrogen and carbon between microsymbiont and host. The organisation of nitrogen assimilation, storage and transport processes is partitioned at the subcellular and tissue levels, in distinctive patterns depending on the symbiotic partners. In this review, recent advances in understanding of actinorhizal nodule nitrogen assimilation are presented. New findings indicate that Frankia within nodules of Datisca glomerata (Presl.) Baill. carries out both primary nitrogen assimilation and biosynthesis of arginine, rather than exporting ammonium. Arginine is a typical storage form of nitrogen in plant tissues, but is a novel nitrogen carrier molecule in root nodule symbioses. Thus Frankia within D. glomerata nodules exhibits considerable metabolic independence. Furthermore, nitrogen reassimilation is likely to take place in the host in the uninfected nodule cortical cells of this root nodule symbiosis, before amino acid export to host sink tissues via the xylem. The role of an augmented pericycle in carbon and nitrogen exchange in root nodules deserves further attention in actinorhizal symbiosis, and further highlights the importance of a comprehensive, structure-function approach to understanding function in root nodules. Moreover, the multiple patterns of compartmentalisation in relation to nitrogen flux within root nodules demonstrate the diversity of possible functional interactions between host and microsymbiont that have evolved in the nitrogen-fixing clade.

Phloem-specific expression of Yang cycle genes and identification of novel Yang cycle enzymes in Plantago and Arabidopsis

The 5-methylthioadenosine (MTA) or Yang cycle is a set of reactions that recycle MTA to Met. In plants, MTA is a byproduct of polyamine, ethylene, and nicotianamine biosynthesis. Vascular transcriptome analyses revealed phloem-specific expression of the Yang cycle gene 5-METHYLTHIORIBOSE KINASE1 (MTK1) in Plantago major and Arabidopsis thaliana. As Arabidopsis has only a single MTK gene, we hypothesized that the expression of other Yang cycle genes might also be vascular specific. Reporter gene studies and quantitative analyses of mRNA levels for all Yang cycle genes confirmed this hypothesis for Arabidopsis and Plantago. This includes the Yang cycle genes 5-METHYLTHIORIBOSE-1-PHOSPHATE ISOMERASE1 and DEHYDRATASE-ENOLASE-PHOSPHATASE-COMPLEX1. We show that these two enzymes are sufficient for the conversion of methylthioribose-1-phosphate to 1,2-dihydroxy-3-keto-5-methylthiopentene. In bacteria, fungi, and animals, the same conversion is catalyzed in three to four separate enzymatic steps. Furthermore, comparative analyses of vascular and nonvascular metabolites identified Met, S-adenosyl Met, and MTA preferentially or almost exclusively in the vascular tissue. Our data represent a comprehensive characterization of the Yang cycle in higher plants and demonstrate that the Yang cycle works primarily in the vasculature. Finally, expression analyses of polyamine biosynthetic genes suggest that the Yang cycle in leaves recycles MTA derived primarily from polyamine biosynthesis.

Amino acid export in plants: a missing link in nitrogen cycling

The export of nutrients from source organs to parts of the body where they are required (e.g. sink organs) is a fundamental biological process. Export of amino acids, one of the most abundant nitrogen species in plant long-distance transport tissues (i.e. xylem and phloem), is an essential process for the proper distribution of nitrogen in the plant. Physiological studies have detected the presence of multiple amino acid export systems in plant cell membranes. Yet, surprisingly little is known about the molecular identity of amino acid exporters, partially due to the technical difficulties hampering the identification of exporter proteins. In this short review, we will summarize our current knowledge about amino acid export systems in plants. Several studies have described plant amino acid transporters capable of bi-directional, facilitative transport, reminiscent of activities identified by earlier physiological studies. Moreover, recent expansion in the number of available amino acid transporter sequences have revealed evolutionary relationships between amino acid exporters from other organisms with a number of uncharacterized plant proteins, some of which might also function as amino acid exporters. In addition, genes that may regulate export of amino acids have been discovered. Studies of these putative transporter and regulator proteins may help in understanding the elusive molecular mechanisms of amino acid export in plants.

Histidine biosynthesis

Histidine (His) is one of the standard amino acids in proteins, and plays a critical role in plant growth and development. The chemical properties of the imidazole side group allow His to participate in acid-base catalysis, and in the co-ordination of metal ions. Despite the biological importance of this molecule, His biosynthesis has been somewhat neglected in plants, in stark contrast to micro-organisms where the study of this pathway was fundamental in the discovery of operon structure and regulation by attenuation. With the recent isolation of histidinol-phosphate phosphatase, all the enzymes of His biosynthesis have now been identified in Arabidopsis, and several lines of evidence have implicated ATP-phosphoribosyl transferase (which catalyses the first committed step of the pathway) as playing an important role in the regulation of this pathway. However, little is known about the transcriptional regulation of the His biosynthetic genes, nor how demand for this amino acid is balanced with other metabolic requirements in plants. Similarly, the pathway of His catabolism has yet to be determined.

Uptake and partitioning of amino acids and peptides

Plant growth, productivity, and seed yield depend on the efficient uptake, metabolism, and allocation of nutrients. Nitrogen is an essential macronutrient needed in high amounts. Plants have evolved efficient and selective transport systems for nitrogen uptake and transport within the plant to sustain development, growth, and finally reproduction. This review summarizes current knowledge on membrane proteins involved in transport of amino acids and peptides. A special emphasis was put on their function in planta. We focus on uptake of the organic nitrogen by the root, source-sink partitioning, and import into floral tissues and seeds.

The presence of amino acids affects inorganic N uptake in non-mycorrhizal seedlings of European beech (Fagus sylvatica)

To investigate the impact of organic N compounds for inorganic nitrogen uptake in the rhizosphere, we fed ammonium nitrate with or without amino acids (i.e., glutamine or arginine) to the roots of non-mycorrhizal beech (Fagus sylvatica L.) seedlings under controlled conditions at different levels of N availability. Uptake of individual N sources was determined from ¹⁵N (inorganic N) and ¹⁵N ¹³C (organic N) accumulation in the roots. In addition, gene fragments encoding proteins involved in N uptake and metabolism were cloned from beech for gene expression analyses by quantitative real-time PCR in the roots. Generally, ammonium was preferred over nitrate as N source. Organic N sources were taken up by beech roots as intact molecules. Uptake of organic N was significantly higher than inorganic N uptake, thus contributing significantly to N nutrition of beech. Depending on the level of N availability, inorganic N uptake was negatively affected by the presence of organic N sources. This result indicates an overestimation of the contribution of inorganic N uptake to N nutrition of beech in previous studies. Apparently, association with mycorrhizal fungi is not essential for organic N uptake by beech roots. Gene expression analyses showed that transcriptional regulation of the amino acid transporters FsCAT3, FsCAT5, FsAAT and FsAAP and the ammonium transporter FsAMT1.2 in the roots is involved in N nutrition of beech.

Glutamine, arginine and the amino acid transporter Pt-CAT11 play important roles during senescence in poplar

BACKGROUND AND AIMS

Nitrogen (N) availability in the forest soil is extremely low and N economy has a special importance in woody plants that are able to cope with seasonal periods of growth and development over many years. Here we report on the analysis of amino acid pools and expression of key genes in the perennial species Populus trichocarpa during autumn senescence.

METHODS

Amino acid pools were measured throughout senescence. Expression analysis of arginine synthesis genes and cationic amino acid transporter (CAT) genes during senescence was performed. Heterologous expression in yeast mutants was performed to study Pt-CAT11 function in detail.

KEY RESULTS

Analysis of amino acid pools showed an increase of glutamine in leaves and an accumulation of arginine in stems during senescence. Expression of arginine biosynthesis genes suggests that arginine was preferentially synthesized from glutamine in perennial tissues. Pt-CAT11 expression increased in senescing leaves and functional characterization demonstrated that Pt-CAT11 transports glutamine.

CONCLUSIONS

The present study established a relationship between glutamine synthesized in leaves and arginine synthesized in stems during senescence, arginine being accumulated as an N storage compound in perennial tissues such as stems. In this context, Pt-CAT11 may have a key role in N remobilization during senescence in poplar, by facilitating glutamine loading into phloem vessels.

PtAAP11, a high affinity amino acid transporter specifically expressed in differentiating xylem cells of poplar

Amino acids are the currency of nitrogen exchange between source and sink tissues in plants and constitute a major source of the components used for cellular growth and differentiation. The characterization of a new amino acid transporter belonging to the amino acid permease (AAP) family, AAP11, expressed in the perennial species Populus trichocarpa is reported here. PtAAP11 expression analysis was performed by semi-quantitative RT-PCR and GUS activity after poplar transformation. PtAAP11 function was studied in detail by heterologous expression in yeast. The poplar genome contains 14 putative AAPs which is quite similar to other species analysed except Arabidopsis. PtAAP11 was mostly expressed in differentiating xylem cells in different organs. Functional characterization demonstrated that PtAAP11 was a high affinity amino acid transporter, more particularly for proline. Compared with other plant amino acid transporters, PtAAP11 represents a novel high-affinity system for proline. Thus, the functional characterization and expression studies suggest that PtAAP11 may play a major role in xylogenesis by providing proline required for xylem cell wall proteins. The present study provides important information highlighting the role of a specific amino acid transporter in xylogenesis in poplar.

A mutation in amino acid permease AAP6 reduces the amino acid content of the Arabidopsis sieve elements but leaves aphid herbivores unaffected

The aim of this study was to investigate the role of the amino acid permease gene AAP6 in regulating phloem amino acid composition and then to determine the effects of this altered diet on aphid performance. A genotype of Arabidopsis thaliana (L.) was produced in which the function of the amino acid permease gene AAP6 (At5g49630) was abolished. Plants homozygous for the insertionally inactivated AAP6 gene had a significantly larger mean rosette width than the wild type and a greater number of cauline leaves. Seeds from the aap6 mutant were also significantly larger than those from the wild-type plants. Sieve element (SE) sap was collected by aphid stylectomy and the amino acids derivatized, separated, and quantified using Capillary Electrophoresis with Laser Induced Fluorescence (CE-LIF). In spite of the large variation across samples, the total amino acid concentration of SE sap of the aap6 mutant plants was significantly lower than that of the wild-type plants. The concentrations of lysine, phenylalanine, leucine, and aspartic acid were all significantly lower in concentration in the aap6 mutant plants compared with wild-type plants. This is the first direct demonstration of a physiological role for an amino acid transporter in regulating SE composition in vivo. The amino acid availability in sieve element sap is thought to be the major limiting factor for aphid growth and reproduction. Despite the changes in their diet, the aphid Myzus persicae (Sulzer) displayed only small changes in feeding behaviour on mutant plants when measured using the Electronic Penetration Graph (EPG) technique. Salivation by the aphid into the SE (E1 phase) was increased on mutant plants but there was no significant effect on other feeding EPG behaviours, or in the rate of honeydew production. Consistent with the small effect on aphid feeding behaviour, there was only a small effect of reduced sieve element amino acid concentration on aphid reproduction. The data are discussed in relation to the regulation of phloem composition and the role of phloem amino acids in regulating aphid performance.

Lysine racemase: a novel non-antibiotic selectable marker for plant transformation

A non-antibiotic based selection system using L-lysine as selection agent and the lysine racemase (lyr) as selectable marker gene for plant transformation was established in this study. L-lysine was toxic to plants, and converted by Lyr into D-lysine which would subsequently be used by the transgenic plants as nitrogen source. Transgenic tobacco and Arabidopsis plants were successfully recovered on L-lysine medium at efficiencies of 23 and 2.4%, respectively. Phenotypic characterization of transgenic plants clearly revealed the expression of normal growth and developmental characteristics as that of wild-type plants, suggesting no pleiotropic effects associated with the lyr gene. The specific activity of Lyr in transgenic tobacco plants selected on L: -lysine ranged from 0.77 to 1.06 mU/mg protein, whereas no activity was virtually detectable in the wild-type plants. In addition, the composition of the free amino acids, except aspartic acid, was not affected by the expression of the lyr gene in the transgenic tobacco plants suggesting very limited interference with endogenous amino acid metabolism. Interestingly, our findings also suggested that the plant aspartate kinases may possess an ability to distinguish the enantiomers of lysine for feedback regulation. To our knowledge, this is the first report to demonstrate that the lysine racemase selectable marker system is novel, less controversial and inexpensive than the traditional selection systems.

BAT1, a bidirectional amino acid transporter in Arabidopsis

The Arabidopsis thaliana At2g01170 gene is annotated as a putative gamma amino butyric acid (GABA) permease based on its sequence similarity to a yeast GABA transporting gene (UGA4). A cDNA of At2g01170 was expressed in yeast and analyzed for amino acid transport activity. Both direct measurement of amino acid transport and yeast growth experiments demonstrated that the At2g01170 encoded-protein exhibits transport activity for alanine, arginine, glutamate and lysine, but not for GABA or proline. Significantly, unlike other amino acid transporters described in plants to date, At2g01170 displayed both export and import activity. Based on that observation, it was named bidirectional amino acid transporter 1 (BAT1). Sequence comparisons show BAT1 is not a member of any previously defined amino acid transporter family. It does share, however, several conserved protein domains found in a variety of prokaryotic and eukaryotic amino acid transporters, suggesting membership in an ancient family of transporters. BAT1 is a single copy gene in the Arabidopsis genome, and its mRNA is ubiquitously expressed in all organs. A transposon--GUS gene-trap insert in the BAT1 gene displays GUS localization in the vascular tissues (Dundar in Ann Appl Biol, 2009) suggesting BAT1 may function in amino acid export from the phloem into sink tissues.

The amino acid permease AAP8 is important for early seed development in Arabidopsis thaliana

The development of seeds depends on the import of carbohydrates and amino acids supplied by the maternal tissue via the phloem. Several amino acid transporters have been reported to be expressed during seed and silique development in Arabidopsis thaliana (L.) Heynh. Here we show that mutants lacking the high affinity amino acid permease 8 (At1g10010) display a severe seed phenotype. The overall number of seeds and the number of normally developed seed is reduced by approximately 50% in siliques of the Ataap8 T-DNA insertion mutant. This result could be reproduced in plants where expression of AtAAP8 is targeted with an RNAi approach. The seed phenotype is correlated with a specifically altered amino acid composition of young siliques. Aspartic acid and glutamic acid are significantly reduced in young siliques of the mutants. In correlation with the fact that AAP8 is a high affinity transporter for acidic amino acids, translocation of (14)C-labelled aspartate fed via the root system to seeds of the mutants is reduced. AAP8 plays a crucial role for the uptake of amino acids into the endosperm and supplying the developing embryo with amino acids during early embryogenesis.

Genetic dissection of histidine biosynthesis in Arabidopsis

The biosynthesis of histidine (His) in microorganisms, long studied through the isolation and characterization of auxotrophic mutants, has emerged as a paradigm for the regulation of metabolism and gene expression. Much less is known about His biosynthesis in flowering plants. One limiting factor has been the absence of large collections of informative auxotrophs. We describe here the results of a systematic screen for His auxotrophs of Arabidopsis (Arabidopsis thaliana). Ten insertion mutants disrupted in four different biosynthetic genes (HISN2, HISN3, HISN4, HISN6A) were identified through a combination of forward and reverse genetics and were shown to exhibit an embryo-defective phenotype that could be rescued by watering heterozygous plants with His. Male transmission of the mutant allele was in several cases reduced. Knockouts of two redundant genes (HISN1B and HISN5A) had no visible phenotype. Another mutant blocked in the final step of His biosynthesis (hisn8) and a double mutant altered in the redundant first step of the pathway (hisn1a hisn1b) exhibited a combination of gametophytic and embryonic lethality in heterozygotes. Homozygous mutant seedlings and callus tissue produced from rescued seeds appeared normal when grown in the presence of His but typically senesced after continued growth in the absence of His. These knockout mutants document the importance of His biosynthesis for plant growth and development, provide valuable insights into amino acid transport and source-sink relationships during seed development, and represent a significant addition to the limited collection of well-characterized auxotrophs in flowering plants.

Transporters for uptake and allocation of organic nitrogen compounds in plants

Nitrogen is an essential macronutrient for plant growth. Following uptake from the soil or assimilation within the plant, organic nitrogen compounds are transported between organelles, from cell to cell and over long distances in support of plant metabolism and development. These translocation processes require the function of integral membrane transporters. The review summarizes our current understanding of the molecular mechanisms of organic nitrogen transport processes, with a focus on amino acid, ureide and peptide transporters.

Comprehensive screening of Arabidopsis mutants suggests the lysine histidine transporter 1 to be involved in plant uptake of amino acids

Plant nitrogen (N) uptake is a key process in the global N cycle and is usually considered a "bottleneck" for biomass production in land ecosystems. Earlier, mineral N was considered the only form available to plants. Recent studies have questioned this dogma and shown that plants may access organic N sources such as amino acids. The actual mechanism enabling plants to access amino acid N is still unknown. However, a recent study suggested the Lysine Histidine Transporter 1 (LHT1) to be involved in root amino acid uptake. In this study, we isolated mutants defective in root amino acid uptake by screening Arabidopsis (Arabidopsis thaliana) seeds from ethyl methanesulfonate-treated plants and seeds from amino acid transporter T-DNA knockout mutants for resistance against the toxic D-enantiomer of alanine (Ala). Both ethyl methanesulfonate and T-DNA knockout plants identified as D-Ala resistant were found to be mutated in the LHT1 gene. LHT1 mutants displayed impaired capacity for uptake of a range of amino acids from solutions, displayed impaired growth when N was supplied in organic forms, and acquired substantially lower amounts of amino acids than wild-type plants from solid growth media. LHT1 mutants grown on mineral N did not display a phenotype until at the stage of flowering, when premature senescence of old leaf pairs occurred, suggesting that LHT1 may fulfill an important function at this developmental stage. Based on the broad and unbiased screening of mutants resistant to D-Ala, we suggest that LHT1 is an important mediator of root uptake of amino acids. This provides a molecular background for plant acquisition of organic N from the soil.

AtCAT6, a sink-tissue-localized transporter for essential amino acids in Arabidopsis

Amino acids represent the major form of reduced nitrogen that is transported in plants. Amino acid transporters in plants often show tissue-specific expression patterns and are used by plants to transport these metabolites from source to sink during development and under changing environmental conditions. We identified one amino acid transporter, AtCAT6, which is expressed in sink tissues such as lateral root primordia, flowers and seeds. Additionally AtCAT6 was induced during infestation of roots by the plant-parasitic root-knot nematode, Meloidogyne incognita. Quantitative reverse-transcriptase PCR revealed nematode inducibility throughout the duration of nematode infestation and in nematode-induced feeding sites. Promoter analyses confirmed expression in endogenous sink tissues and nematode-induced feeding sites. In Xenopus oocytes, AtCAT6 mediated electrogenic transport of proteinogenic as well as non-proteinogenic amino acids with moderate affinity. AtCAT6 transported large, neutral and cationic amino acids in preference to other amino acids. Knockout mutants of this transporter failed to grow on medium containing l-glutamine as the sole nitrogen source. Our data suggest that AtCAT6 plays a role in supplying amino acids to sink tissues of plants and nematode-induced feeding structures.

Arabidopsis LHT1 is a high-affinity transporter for cellular amino acid uptake in both root epidermis and leaf mesophyll

Amino acid transport in plants is mediated by at least two large families of plasma membrane transporters. Arabidopsis thaliana, a nonmycorrhizal species, is able to grow on media containing amino acids as the sole nitrogen source. Arabidopsis amino acid permease (AAP) subfamily genes are preferentially expressed in the vascular tissue, suggesting roles in long-distance transport between organs. We show that the broad-specificity, high-affinity amino acid transporter LYSINE HISTIDINE TRANSPORTER1 (LHT1), an AAP homolog, is expressed in both the rhizodermis and mesophyll of Arabidopsis. Seedlings deficient in LHT1 cannot use Glu or Asp as sole nitrogen sources because of the severe inhibition of amino acid uptake from the medium, and uptake of amino acids into mesophyll protoplasts is inhibited. Interestingly, lht1 mutants, which show growth defects on fertilized soil, can be rescued when LHT1 is reexpressed in green tissue. These findings are consistent with two major LHT1 functions: uptake in roots and supply of leaf mesophyll with xylem-derived amino acids. The capacity for amino acid uptake, and thus nitrogen use efficiency under limited inorganic N supply, is increased severalfold by LHT1 overexpression. These results suggest that LHT1 overexpression may improve the N efficiency of plant growth under limiting nitrogen, and the mutant analyses may enhance our understanding of N cycling in plants.

Expression and transcriptional regulation of amino acid transporters in plants

Recent studies have shown that there are more than 50 amino acid transporter genes in the Arabidopsis genome. This abundance of amino acid transporters implies that they play a multitude of fundamental roles in plant growth and development. Current research on the expression and regulation (i.e., tissue-specific expression and regulation of expression in response to nutrient and environmental changes) of these genes has provided useful information about the functional significance of plant amino acid transport systems.

Are high-lysine cereal crops still a challenge?

The essential amino acids lysine and threonine are synthesized in higher plants via a pathway starting with aspartate that also leads to the formation of methionine and isoleucine. Lysine is one of most limiting amino acids in plants consumed by humans and livestock. Recent genetic, molecular, and biochemical evidence suggests that lysine synthesis and catabolism are regulated by complex mechanisms. Early kinetic studies utilizing mutants and transgenic plants that over-accumulate lysine have indicated that the major step for the regulation of lysine biosynthesis is at the enzyme dihydrodipicolinate synthase. Despite this tight regulation, recent strong evidence indicates that lysine catabolism is also subject to control, particularly in cereal seeds. The challenge of producing crops with a high-lysine concentration in the seeds appeared to be in sight a few years ago. However, apart from the quality protein maize lines currently commercially available, the release of high-lysine crops has not yet occurred. We are left with the question, is the production of high-lysine crops still a challenge?

Ectopic expression of an activated RAC in Arabidopsis disrupts membrane cycling

Rho GTPases regulate the actin cytoskeleton, exocytosis, endocytosis, and other signaling cascades. Rhos are subdivided into four subfamilies designated Rho, Racs, Cdc42, and a plant-specific group designated RACs/Rops. This research demonstrates that ectopic expression of a constitutive active Arabidopsis RAC, AtRAC10, disrupts actin cytoskeleton organization and membrane cycling. We created transgenic plants expressing either wild-type or constitutive active AtRAC10 fused to the green fluorescent protein. The activated AtRAC10 induced deformation of root hairs and leaf epidermal cells and was primarily localized in Triton X-100-insoluble fractions of the plasma membrane. Actin cytoskeleton reorganization was revealed by creating double transgenic plants expressing activated AtRAC10 and the actin marker YFP-Talin. Plants were further analyzed by membrane staining with N-[3-triethylammoniumpropyl]-4-[p-diethylaminophenylhexatrienyl] pyridinium dibromide (FM4-64) under different treatments, including the protein trafficking inhibitor brefeldin A or the actin-depolymeryzing agents latrunculin-B (Lat-B) and cytochalasin-D (CD). After drug treatments, activated AtRAC10 did not accumulate in brefeldin A compartments, but rather reduced their number and colocalized with FM4-64-labeled membranes in large intracellular vesicles. Furthermore, endocytosis was compromised in root hairs of activated AtRAC10 transgenic plants. FM4-64 was endocytosed in nontransgenic root hairs treated with the actin-stabilizing drug jasplakinolide. These findings suggest complex regulation of membrane cycling by plant RACs.

Selective expression of a novel high-affinity transport system for acidic and neutral amino acids in the tapetum cells of Arabidopsis flowers

Within the flower, microsporogenesis represents a major sink for nitrogen, but knowledge on how the imported nitrogen is transferred from the anther cell layers to developing pollen is lacking. Here, we provide information on characterization of a transporter (AtLHT2) that might play an important role in partitioning of amino acids for microspore development. Biochemical analysis in yeast showed that AtLHT2 transports proline and aspartate with high affinity. However, other neutral and acidic amino acids act as strong competitors for proline and aspartate uptake indicating that AtLHT2 generally transports uncharged and negatively charged amino acids. Comparison of the apparent K(m) values of AtLHT2 with previously characterized amino acid transporters clearly demonstrated that AtLHT2 represents a novel high-affinity system for neutral and acidic amino acids. Northern blot analysis showed strong expression of the amino acid transporter in flower buds. Cellular expression could be resolved by using RNA in situ hybridization and in situ RT-PCR methods, which localized AtLHT2 specifically to the tapetum tissue of the anthers. Developing pollen grains are symplasmically isolated from the sporophytic tissue and rely on the nutrients and other compounds secreted from the tapetum cells. Thus, the functional characterization of AtLHT2, together with our expression and localization studies, strongly suggest that in Arabidopsis flowers, AtLHT2 has a critical function in import of neutral and acidic amino acids into the tapetum cells for synthesis of compounds important for microspore structure and in transfer of organic nitrogen to the locule for pollen development.

Root phloem-specific expression of the plasma membrane amino acid proton co-transporter AAP3

Amino acids are regarded as the nitrogen 'currency' of plants. Amino acids can be taken up from the soil directly or synthesized from inorganic nitrogen, and then circulated in the plant via phloem and xylem. AtAAP3, a member of the Amino Acid Permease (AAP) family, is mainly expressed in root tissue, suggesting a potential role in the uptake and distribution of amino acids. To determine the spatial expression pattern of AAP3, promoter-reporter gene fusions were introduced into Arabidopsis. Histochemical analysis of AAP3 promoter-GUS expressing plants revealed that AAP3 is preferentially expressed in root phloem. Expression was also detected in stamens, in cotyledons, and in major veins of some mature leaves. GFP-AAP3 fusions and epitope-tagged AAP3 were used to confirm the tissue specificity and to determine the subcellular localization of AtAAP3. When overexpressed in yeast or plant protoplasts, the functional GFP-AAP3 fusion was localized in subcellular organelle-like structures, nuclear membrane, and plasma membrane. Epitope-tagged AAP3 confirmed its localization to the plasma membrane and nuclear membrane of the phloem, consistent with the promoter-GUS study. In addition, epitope-tagged AAP3 protein was localized in endodermal cells in root tips. The intracellular localization suggests trafficking or cycling of the transporter, similar to many metabolite transporters in yeast or mammals, for example, yeast amino acid permease GAP1. Despite the specific expression pattern, knock-out mutants did not show altered phenotypes under various conditions including N-starvation. Microarray analyses revealed that the expression profile of genes involved in amino acid metabolism did not change drastically, indicating potential compensation by other amino acid transporters.