Cloning a plant amino acid transporter by functional complementation of a yeast amino acid transport mutant

Characterization and whole genome sequencing of Saccharomyces cerevisiae strains lacking several amino acid transporters: Tools for studying amino acid transport

Saccharomyces cerevisiae mutants have been used since the early 1980s as a tool for characterizing genes from other organisms by functional complementation. This approach has been extremely successful in cloning and studying transporters; for instance, plant amino acid, sugar, urea, ammonium, peptide, sodium, and potassium transporters were characterized using yeast mutants lacking these functions. Over the years, new strains lacking even more endogenous transporters have been developed, enabling the characterization of transport properties of heterologous proteins in a more precise way. Furthermore, these strains provide the added possibility of characterizing a transporter belonging to a family of proteins in isolation, and thus can be used to study the relative contribution of redundant transporters to the whole function. We focused on amino acid transport, starting with the yeast strain 22 ∆ 8AA, which was developed to clone plant amino acid transporters in the early 2000s. We recently deleted two additional amino acid permeases, Gnp1 and Agp1, creating 22 ∆ 10α. In the present work, five additional permeases (Bap3, Tat1, Tat2, Agp3, Bap2) were deleted from 22 ∆ 10α genome, in a combination of up to three at a time. Unexpectedly, the amino acid transport properties of the new strains were not very different from the parent, suggesting that these amino acid permeases play a minor role in amino acid uptake, at least in our conditions. Furthermore, the inability to utilize certain amino acids as sole nitrogen source did not correlate with reduced uptake activity, questioning the well-accepted relationship between lack of growth and loss of transport properties. Finally, in order to verify the mutations and the integrity of 22 ∆ 10α genome, we performed whole-genome sequencing of 22 ∆ 10α using long-read PacBio sequencing technology. We successfully assembled 22 ∆ 10α's genome de novo, identified all expected mutations and precisely characterized the nature of the deletions of the ten amino acid transporters. The sequencing data and genome will serve as a valuable resource to researchers interested in using these strains as a tool for amino acid transport study.

Effects of two amino acid transporter-like genes on potato growth

Amino acid transporters are membrane proteins that mediate amino acid transport across the plasma membrane. They play a significant role in plant growth and development. The amino acid permease (AAP) subfamily belongs to the activating transcription factor family, which is one of the main amino acid transporter families. Potato AAP genes were identified through simple bioinformatics, and the functions of StAAP1 and StAAP8 were verified by plant subcellular localization and potato transgenic technology. In this study, eight AAP-like genes in potato were separated into two subgroups based on the differences in the number of pore-lining residues. To identify the locations where the genes were expressed, we built green fluorescent protein expression vectors for two genes, StAAP1 and StAAP8, and found that these two genes were expressed on the plasma membrane. Meanwhile, we constructed overexpression vectors for these two genes to construct transgenic plants. By observing the phenotype of the transgenic plants, we concluded that StAAP1 and StAAP8 promoted leaf growth and increased leaf area and StAAP1 elongated the potato tubers. Overall, these two genes did not significantly affect tuber weight or number. However, the assessment of amino acid content in potato tubers showed that StAAP8 overexpression increased the content of amino acids, and some of these amino acids were related to protein synthesis. Therefore, StAAP8 overexpression may promote the accumulation of plant amino acids. Studies have shown that there are some differences in the functions of different transcription factor members. The studied AAP8 gene plays a role in amino acid transport and protein accumulation in potato tubers, which provides support for subsequent research on potato tuber nutrition.

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.

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

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

Genome-wide identification and comparative analysis of genes encoding AAPs in apple (Malus × domestica Borkh.)

Amino acid permeases (AAPs) play important roles in plant amino acid transport and nitrogen metabolism. In this study, we carried a comprehensive analysis for apple genes encoding AAPs using bioinformatics and molecular biology. Eleven MdAAPs were identified by a genome-wide search and comparative genomic analysis revealed relatively conserved gene composition, transmembrane characteristics, and protein structures. Phylogenetic tree construction and analysis of the conserved motifs of MdAAPs and AtAAPs showed that AAPs can be classified into three groups (I, II, and III). We compared the promoters of the identified genes and did gene functional annotation and qRT-PCR and found a relationship between apple AAPs and nitrogen deficiency. The expression profile data implied that MdAAPs exhibit diversified distributions and functions in different tissues.

To have or not to have: expression of amino acid transporters during pathogen infection

The interaction between plants and plant pathogens can have significant effects on ecosystem performance. For their growth and development, both bionts rely on amino acids. While amino acids are key transport forms of nitrogen and can be directly absorbed from the soil through specific root amino acid transporters, various pathogenic microbes can invade plant tissues to feed on different plant amino acid pools. In parallel, plants may initiate an immune response program to restrict this invasion, employing various amino acid transporters to modify the amino acid pool at the site of pathogen attack. The interaction between pathogens and plants is sophisticated and responses are dynamic. Both avail themselves of multiple tools to increase their chance of survival. In this review, we highlight the role of amino acid transporters during pathogen infection. Having control over the expression of those transporters can be decisive for the fate of both bionts but the underlying mechanism that regulates the expression of amino acid transporters is not understood to date. We provide an overview of the regulation of a variety of amino acid transporters, depending on interaction with biotrophic, hemibiotrophic or necrotrophic pathogens. In addition, we aim to highlight the interplay of different physiological processes on amino acid transporter regulation during pathogen attack and chose the LYSINE HISTIDINE TRANSPORTER1 (LHT1) as an example.

Force in numbers: high-throughput screening approaches to unlock microbial transport

Biological membranes are inherently complex, making transport processes in microbial cell factories a significant bottleneck. Lack of knowledge on transport proteins' characteristics and the need for advanced technical equipment often hamper transporter identification and optimization. For these reasons, moving away from individual characterization and towards high-throughput mining, engineering, and screening of transporters is an increasingly attractive approach. Superior transporters can be selected from large libraries by coupling their activity to growth, for substrates that function as feedstocks or toxic compounds. Other compounds can be screened thanks to recent advances in the design and deployment of synthetic genetic circuits (biosensors). Furthermore, novel strategies are rapidly increasing the repertoire of biomolecule transporters susceptible to high-throughput selection methods.

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.

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.

Molecular Cloning of an Amino Acid Permease Gene and Structural Characterization of the Protein in Common Bean (Phaseolus vulgaris L.)

Plants synthesize amino acids by collateral metabolic pathways using primary elements carbon and oxygen from air, hydrogen from water in soil and nitrogen from soil. Following synthesis, amino acids are immediately used for metabolism, transient storage or transported to the phloem. Different families of transporters have been identified for import of amino acids into plant cells. The first identified amino acid transporter, amino acid permease 1 (AAP1) in Arabidopsis belongs to a family of eight members and transports acidic, neutral, and basic amino acids. Legumes fix atmospheric nitrogen through a symbiotic relationship with root nodules bacteria. Following fixation, nitrogen is reduced to amino acids and is exported via different amino acid transporters. However, information is lacking about the structure of these important classes of amino acid transporter proteins in plant. We have amplified AAP from Phaseolus vulgaris, an economically important leguminous plant grown all over the world, and sequenced. The sequence has been characterized in silico and a three-dimensional structure of AAP has been predicted and validated. The information obtained not only enhances the knowledge about the structure of an amino acid permease gene in P. vulgaris, but will also help in designing protein-ligand studies using this protein as well.

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 influence of steric configuration of phenazine-1-carboxylic acid-amino acid conjugates on fungicidal activity and systemicity

BACKGROUND

Conjugating an amino acid onto existing fungicidal parent structures has been demonstrated to be an effective way to endow non-phloem mobile fungicides with phloem mobility. To alter the systemicity of the fungicide PCA (phenazine-1-carboxylic acid), 10 amino acids derivatives of this fungicide were designed and synthesized, and their synthesis, characterization, phloem and xylem mobility in Ricinus communis L, and their fungicidal activity in vitro are described.

RESULTS

The systemicity experiments in Ricinus communis system demonstrated that all conjugates exhibited obvious phloem mobility compared with non-phloem-mobile PCA, and the introduction of an L-amino acid to PCA more greatly enhanced the phloem mobility. The five D-amino acid conjugates exhibited higher xylem mobility than that of PCA and of each corresponding L-amino acid conjugate. Most conjugates were found to exhibit moderate in vitro fungicidal activities against six pathogenic fungi, which were lower than that of PCA. The results of the bioassay showed fungicidal activities of PCA-amino acid conjugates associated not only with different amino acids, but also with their conformation. Conjugation with D-amino acid contributed to the in vitro fungicidal activities of PCA-amino acid conjugates.

CONCLUSIONS

The current research offers a new strategy for enhancing the systemicity of non-phloem-mobile fungicides and presents some useful information on the effects of introducing amino acids of different steric configurations on the fungicidal activity, phloem and xylem mobility of the parent fungicide. © 2019 Society of Chemical Industry.

OsProT1 and OsProT3 Function to Mediate Proline- and γ-aminobutyric acid-specific Transport in Yeast and are Differentially Expressed in Rice (Oryza sativa L.)

BACKGROUND

Proline (Pro) and γ-aminobutyric acid (GABA) play important roles in plant development and stress tolerance. However, the molecular components responsible for the transport of these molecules in rice remain largely unknown.

RESULTS

Here we identified OsProT1 and OsProT3 as functional transporters for Pro and GABA. Transient expression of eGFP-OsProTs in plant protoplasts revealed that both OsProT1 and OsProT3 are localized to the plasma membrane. Ectopic expression in a yeast mutant demonstrated that both OsProT1 and OsProT3 specifically mediate transport of Pro and GABA with affinity for Pro in the low affinity range. qRT-PCR analyses suggested that OsProT1 was preferentially expressed in leaf sheathes during vegetative growth, while OsProT3 exhibited relatively high expression levels in several tissues, including nodes, panicles and roots. Interestingly, both OsProT1 and OsProT3 were induced by cadmium stress in rice shoots.

CONCLUSIONS

Our results suggested that plasma membrane-localized OsProT1 and OsProT3 efficiently transport Pro and GABA when ectopically expressed in yeast and appear to be involved in various physiological processes, including adaption to cadmium stress in rice plants.

Disruption of an amino acid transporter LHT1 leads to growth inhibition and low yields in rice

BACKGROUND

Research on plant amino acid transporters was mainly performed in Arabidopsis, while our understanding of them is generally scant in rice. OsLHT1 (Lysine/Histidine transporter) has been previously reported as a histidine transporter in yeast, but its substrate profile and function in planta are unclear. The aims of this study are to analyze the substrate selectivity of OsLHT1 and influence of its disruption on rice growth and fecundity.

RESULTS

Substrate selectivity of OsLHT1 was analyzed in Xenopus oocytes using the two-electrode voltage clamp technique. The results showed that OsLHT1 could transport a broad spectrum of amino acids, including basic, neutral and acidic amino acids, and exhibited a preference for neutral and acidic amino acids. Two oslht1 mutants were generated using CRISPR/Cas9 genome-editing technology, and the loss-of-function of OsLHT1 inhibited rice root and shoot growth, thereby markedly reducing grain yields. QRT-PCR analysis indicated that OsLHT1 was expressed in various rice organs, including root, stem, flag leaf, flag leaf sheath and young panicle. Transient expression in rice protoplast suggested OsLHT1 was localized to the plasma membrane, which is consistent with its function as an amino acid transporter.

CONCLUSIONS

Our results indicated that OsLHT1 is an amino acid transporter with wide substrate specificity and with preference for neutral and acidic amino acids, and disruption of OsLHT1 function markedly inhibited rice growth and fecundity.

Molecular Mechanisms Underlying Increase in Lysine Content of Waxy Maize through the Introgression of the opaque2 Allele

The opaque2 (o2) mutation in maize is associated with high lysine content in endosperm and good nutritional value. To improve the nutritional quality of waxy maize, the o2 allele was introgressed into the wxwx line using marker-assisted backcrossing selection technology. The lysine content of o2o2wxwx lines was higher than that of the wxwx line. To reveal the mechanism of increasing lysine content through introgression of the o2 in waxy maize, the transcriptome on kernels (18th day after pollination) of the o2o2wxwx and parent lines was analyzed using RNA-sequencing (RNA-Seq). The RNA-Seq analysis revealed 49 differentially expressed genes (DEGs). Functional analysis showed that these DEGs were mostly related to the catalytic activity and metabolic processes. The O2 gene regulated multiple metabolic pathways related to biological processes (BP) and molecular function (MP) during waxy maize endosperm development. In particular, in the o2o2wxwx lines, the two genes that encode the EF-1α and LHT1 were up-regulated, but the gene that encodes sulfur-rich proteins was down-regulated, raising the grain lysine content. These findings are of great importance for understanding the molecular mechanism underlying the lysine content increase due to o2 allele introgression into waxy maize.

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.

Genome-Wide analysis of the AAAP gene family in moso bamboo (Phyllostachys edulis)

BACKGROUND

Members of the amino acid/auxin permease (AAAP) gene family play indispensable roles in various plant metabolism and biosynthesis processes. Comprehensive analysis of AAAP genes has been conducted in Arabidopsis, rice, maize and poplar, but has not been reported from moso bamboo. Phylogenetics, evolutionary patterns and further expression profiles analysis of the AAAP gene family in moso bamboo (Phyllostachys edulis) will increase our understanding of this important gene family.

RESULTS

In this current study, we conducted phylogenetic, gene structure, promoter region, divergence time, expression patterns and qRT-PCR analysis of the 55 predicted AAAP genes in moso bamboo based on the availability of the moso bamboo genome sequence. We identified 55 putative AAAP (PeAAAP1-55) genes, which were divided into eight distinct subfamilies based on comparative phylogenetic analysis using 184 full-length protein sequences, including 55 sequences from moso bamboo, 58 sequences from rice and 71 sequences from maize. Analysis of evolutionary patterns and divergence showed that the PeAAAP genes have undergone a extensive duplication event approximately 12 million years ago (MYA) and that the split between AAAP family genes in moso bamboo and rice occurred approximately 27 MYA. The microarray analysis suggested that some genes play considerable roles in moso bamboo growth and development. We investigated the expression levels of the 16 AAP subfamily genes under abiotic stress (drought, salt and cold) by qRT-PCR to explore the potential contributions to stress response of individual PeAAAP genes in moso bamboo.

CONCLUSIONS

The results of this study suggest that PeAAAP genes play crucial roles in moso bamboo growth and development, especially in response to abiotic stress conditions. Our comprehensive, systematic study of the AAAPs gene family in moso bamboo will facilitate further analysis of the functions and evolution of AAAP genes in plants.

Recombinant Microorganisms as Tools for High Throughput Screening for Nonantibiotic Compounds

Microorganisms were among the first tools used for the discovery of biologically active compounds. Their utility reached a zenith during the era of antibiotic development in the 1950s and 1960s, then declined. Subsequently, a substantial role for microorganisms in the pharmaceutical industry developed with the realization that microbial fermentations were intriguing sources of nonantibiotic natural products. From recombinant DNA technology emerged another important role for microorganisms in pharmaceutical research: the expression of heterologous proteins for therapeutic products or for in vitro high throughput screens (HTSs). Recent developments in cloning, genetics, and expression systems have opened up new applications for recombinant microorganisms in screening for nonantibiotic compounds in HTSs. These screens employ microorganisms that depend upon the function of a heterologous protein for survival under defined nutritional conditions. Compounds that specifically target the heterologous protein can be identified by measuring viability of the microorganism under different nutrient selection. Advantages of this approach include a built-in selection for target selectivity, an easily measured end point that can be used for a multitude of different targets, and compatibility with automation required for HTSs. Mechanism-based HTSs using recombinant microorganisms can also address drug targets that are not readily approachable in other HTS formats, including certain enzymes; ion channels and transporters; and protein::protein, protein::DNA, and protein::RNA interactions.

Glycinergic-Fipronil Uptake Is Mediated by an Amino Acid Carrier System and Induces the Expression of Amino Acid Transporter Genes in Ricinus communis Seedlings

Phloem-mobile insecticides are efficient for piercing and sucking insect control. Introduction of sugar or amino acid groups to the parent compound can improve the phloem mobility of insecticides, so a glycinergic-fipronil conjugate (GlyF), 2-(3-(3-cyano-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-4-((trifluoromethyl)sulfinyl)-1H-pyrazole-5-yl)ureido) acetic acid, was designed and synthesized. Although the "Kleier model" predicted that this conjugate is not phloem mobile, GlyF can be continually detected during a 5 h collection of Ricinus communis phloem sap. Furthermore, an R. communis seedling cotyledon disk uptake experiment demonstrates that the uptake of GlyF is sensitive to pH, carbonyl cyanide m-chlorophenylhydrazone (CCCP), temperature, and p-chloromercuribenzenesulfonic acid (pCMBS) and is likely mediated by amino acid carrier system. To explore the roles of amino acid transporters (AATs) in GlyF uptake, a total of 62 AAT genes were identified from the R. communis genome in silico. Phylogenetic analysis revealed that AATs in R. communis were organized into the ATF (amino acid transporter) and APC (amino acid, polyaminem and choline transporter) superfamilies, with five subfamilies in ATF and two in APC. Furthermore, the expression profiles of 20 abundantly expressed AATs (cycle threshold (Ct) values <27) were analyzed at 1, 3, and 6 h after GlyF treatment by RT-qPCR. The results demonstrated that expression levels of four AAT genes, RcLHT6, RcANT15, RcProT2, and RcCAT2, were induced by the GlyF treatment in R. communis seedlings. On the basis of the observation that the expression profile of the four candidate genes is similar to the time course observation for GlyF foliar disk uptake, it is suggested that those four genes are possible candidates involved in the uptake of GlyF. These results contribute to a better understanding of the mechanism of GlyF uptake as well as phloem loading from a molecular biology perspective and facilitate functional characterization of candidate AAT genes in future studies.

50 years of Arabidopsis research: highlights and future directions

922 I. 922 II. 922 III. 925 IV. 925 V. 926 VI. 927 VII. 928 VIII. 929 IX. 930 X. 931 XI. 932 XII. 933 XIII. Natural variation and genome-wide association studies 934 XIV. 934 XV. 935 XVI. 936 XVII. 937 937 References 937 SUMMARY: The year 2014 marked the 25(th) International Conference on Arabidopsis Research. In the 50 yr since the first International Conference on Arabidopsis Research, held in 1965 in Göttingen, Germany, > 54 000 papers that mention Arabidopsis thaliana in the title, abstract or keywords have been published. We present herein a citational network analysis of these papers, and touch on some of the important discoveries in plant biology that have been made in this powerful model system, and highlight how these discoveries have then had an impact in crop species. We also look to the future, highlighting some outstanding questions that can be readily addressed in Arabidopsis. Topics that are discussed include Arabidopsis reverse genetic resources, stock centers, databases and online tools, cell biology, development, hormones, plant immunity, signaling in response to abiotic stress, transporters, biosynthesis of cells walls and macromolecules such as starch and lipids, epigenetics and epigenomics, genome-wide association studies and natural variation, gene regulatory networks, modeling and systems biology, and synthetic biology.

Transport of defense compounds from source to sink: lessons learned from glucosinolates

Plants synthesize a plethora of defense compounds crucial for their survival in a challenging and changing environment. Transport processes are important for shaping the distribution pattern of defense compounds, albeit focus hitherto has been mostly on their biosynthetic pathways. A recent identification of two glucosinolate transporters represents a breakthrough in our understanding of glucosinolate transport in Arabidopsis and has advanced knowledge in transport of defense compounds. In this review, we discuss the role of the glucosinolate transporters in establishing dynamic glucosinolate distribution patterns and source-sink relations. We focus on lessons learned from glucosinolate transport that may apply to transport of other defense compounds and discuss future avenues in the emerging field of defense compound transport.

Transport Function of Rice Amino Acid Permeases (AAPs)

The transport function of four rice (Oryza sativa) amino acid permeases (AAPs), OsAAP1 (Os07g04180), OsAAP3 (Os06g36180), OsAAP7 (Os05g34980) and OsAAP16 (Os12g08090), was analyzed by expression in Xenopus laevis oocytes and electrophysiology. OsAAP1, OsAAP7 and OsAAP16 functioned, similarly to Arabidopsis AAPs, as general amino acid permeases. OsAAP3 had a distinct substrate specificity compared with other rice or Arabidopsis AAPs. OsAAP3 transported the basic amino acids lysine and arginine well but selected against aromatic amino acids. The transport of basic amino acids was further analyzed for OsAAP1 and OsAAP3, and the results support the transport of both neutral and positively charged forms of basic amino acids by the rice AAPs. Cellular localization using the tandem enhanced green fluorescent protein (EGFP)-red fluorescent protein (RFP) reporter pHusion showed that OsAAP1 and OsAAP3 localized to the plasma membrane after transient expression in onion epidermal cells or stable expression in Arabidopsis.

Isolation and characterization of LHT-type plant amino acid transporter gene from Panax ginseng Meyer

A lysine histidine transporter (LHT) cDNA was isolated and characterized from the roots of Panax ginseng, designated PgLHT. The cDNA is 1,865 bp with an open reading frame that codes for a protein with 449 amino acids and a calculated molecular mass of 50.6 kDa with a predicted isoelectric point of 8.87. Hydropathy analysis shows that PgLHT is an integral membrane protein with 9 putative membrane-spanning domains. Multiple sequence alignments show that PgLHT shares a high homology with other plant LHTs. The expression profile of the gene was investigated by real-time quantitative polymerase chain reaction during various chemical treatments. PgLHT was up-regulated in the presence of abscisic acid, salicylic acid, methyl jasmonate, NaCl, and amino acids. To further explore the function of PgLHT gene, full-length cDNA of PgLHT was introduced into P. ginseng by Agrobacterium rhizogenes A4. The overexpression of PgLHT in the hairy roots led to an obviously increase of biomass compared to the controls, and after addition of the amino acids, the overexpressed-PgLHT hairy roots grew more rapidly than untreated controls during early stage of the culture cycle. The results suggested that the PgLHT isolated from ginseng might have role in the environmental stresses and growth response.

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.

Transporters for amino acids in plant cells: some functions and many unknowns

Membrane proteins are essential to move amino acids in or out of plant cells as well as between organelles. While many putative amino acid transporters have been identified, function in nitrogen movement in plants has only been shown for a few proteins. Those studies demonstrate that import systems are fundamental in partitioning of amino acids at cellular and whole plant level. Physiological data further suggest that amino acid transporters are key-regulators in plant metabolism and that their activities affect growth and development. By contrast, knowledge on the molecular mechanisms of cellular export processes as well as on intracellular transport of amino acids is scarce. Similarly, little is known about the regulation of amino acid transporter function and involvement of the transporters in amino acid signaling. Future studies need to identify the missing components to elucidate the importance of amino acid transport processes for whole plant physiology and productivity.

Functional analysis of OsPUT1, a rice polyamine uptake transporter

Polyamines are nitrogenous compounds found in all eukaryotic and prokaryotic cells and absolutely essential for cell viability. In plants, they regulate several growth and developmental processes and the levels of polyamines are also correlated with the plant responses to various biotic and abiotic stresses. In plant cells, polyamines are synthesized in plastids and cytosol. This biosynthetic compartmentation indicates that the specific transporters are essential to transport polyamines between the cellular compartments. In the present study, a phylogenetic analysis was used to identify candidate polyamine transporters in rice. A full-length cDNA rice clone AK068055 was heterologously expressed in the Saccharomyces cerevisiae spermidine uptake mutant, agp2∆. Radiological uptake and competitive inhibition studies with putrescine indicated that rice gene encodes a protein that functioned as a spermidine-preferential transporter. In competition experiments with several amino acids at 25-fold higher levels than spermidine, only methionine, asparagine, and glutamine were effective in reducing uptake of spermidine to 60% of control rates. Based on those observations, this rice gene was named polyamine uptake transporter 1 (OsPUT1). Tissue-specific expression of OsPUT1 by semiquantitative RT-PCR showed that the gene was expressed in all tissues except seeds and roots. Transient expression assays in onion epidermal cells and rice protoplasts failed to localize to a cellular compartment. The characterization of the first plant polyamine transporter sets the stage for a systems approach that can be used to build a model to fully define how the biosynthesis, degradation, and transport of polyamines in plants mediate developmental and biotic responses.

Transporters in Arabidopsis roots mediating uptake of amino acids at naturally occurring concentrations

Recent studies of Arabidopsis have identified several transporters as being important for amino acid uptake. We used Arabidopsis plants with altered expression of lysine histidine transporter 1 (LHT1), amino acid permease 1 (AAP1) and amino acid permease 5 (AAP5) with the aim of disentangling the roles of each transporter in the uptake of different amino acids at naturally occurring concentrations (2-50 μM). LHT1 mutants displayed reduced uptake rates of L-Gln, L-Ala, L-Glu and L-Asp but not of L-Arg or L-Lys, while AAP5 mutants were affected in the uptake of L-Arg and L-Lys only. Double mutants (lht1aap5) exhibited reduced uptake of all tested amino acids. In the concentration range tested, AAP1 mutants did not display altered uptake rates for any of the studied amino acids. Expression analysis of amino acid transporter genes with important root functions revealed no major differences in the individual mutants other than for genes targeted for mutation. We conclude that LHT1 and AAP5, but not AAP1, are crucial for amino acid uptake at concentrations typically found in soils. LHT1 and AAP5 displayed complementary affinity spectra, and no redundancy with respect to gene expression was found between the two transporters, suggesting these two transporters have separate roles in amino acid uptake.

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.

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.

TransportTP: a two-phase classification approach for membrane transporter prediction and characterization

BACKGROUND

Membrane transporters play crucial roles in living cells. Experimental characterization of transporters is costly and time-consuming. Current computational methods for transporter characterization still require extensive curation efforts, especially for eukaryotic organisms. We developed a novel genome-scale transporter prediction and characterization system called TransportTP that combined homology-based and machine learning methods in a two-phase classification approach. First, traditional homology methods were employed to predict novel transporters based on sequence similarity to known classified proteins in the Transporter Classification Database (TCDB). Second, machine learning methods were used to integrate a variety of features to refine the initial predictions. A set of rules based on transporter features was developed by machine learning using well-curated proteomes as guides.

RESULTS

In a cross-validation using the yeast proteome for training and the proteomes of ten other organisms for testing, TransportTP achieved an equivalent recall and precision of 81.8%, based on TransportDB, a manually annotated transporter database. In an independent test using the Arabidopsis proteome for training and four recently sequenced plant proteomes for testing, it achieved a recall of 74.6% and a precision of 73.4%, according to our manual curation.

CONCLUSIONS

TransportTP is the most effective tool for eukaryotic transporter characterization up to date.

AAP1 regulates import of amino acids into developing Arabidopsis embryos

The embryo of Arabidopsis seeds is symplasmically isolated from the surrounding seed coat and endosperm, and uptake of nutrients from the seed apoplast is required for embryo growth and storage reserve accumulation. With the aim of understanding the importance of nitrogen (N) uptake into developing embryos, we analysed two mutants of AAP1 (At1g58360), an amino acid transporter that was localized to Arabidopsis embryos. In mature and desiccated aap1 seeds the total N and carbon content was reduced while the total free amino acid levels were strongly increased. Separately analysed embryos and seed coats/endosperm of mature seeds showed that the elevated amounts in amino acids were caused by an accumulation in the seed coat/endosperm, demonstrating that a decrease in uptake of amino acids by the aap1 embryo affects the N pool in the seed coat/endosperm. Also, the number of protein bodies was increased in the aap1 endosperm, suggesting that the accumulation of free amino acids triggered protein synthesis. Analysis of seed storage compounds revealed that the total fatty acid content was unchanged in aap1 seeds, but storage protein levels were decreased. Expression analysis of genes of seed N transport, metabolism and storage was in agreement with the biochemical data. In addition, seed weight, as well as total silique and seed number, was reduced in the mutants. Together, these results demonstrate that seed protein synthesis and seed weight is dependent on N availability and that AAP1-mediated uptake of amino acids by the embryo is important for storage protein synthesis and seed yield.

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.

AAP1 transports uncharged amino acids into roots of Arabidopsis

Amino acids are available to plants in some soils in significant amounts, and plants frequently make use of these nitrogen sources. The goal of this study was to identify transporters involved in the uptake of amino acids into root cells. Based on the fact that high concentrations of amino acids inhibit plant growth, we hypothesized that mutants tolerating toxic levels of amino acids might be deficient in the uptake of amino acids from the environment. To test this hypothesis, we employed a forward genetic screen for Arabidopsis thaliana mutants tolerating toxic concentrations of amino acids in the media. We identified an Arabidopsis mutant that is deficient in the amino acid permease 1 (AAP1, At1g58360) and resistant to 10 mm phenylalanine and a range of other amino acids. The transporter was localized to the plasma membrane of root epidermal cells, root hairs, and throughout the root tip of Arabidopsis. Feeding experiments with [(14)C]-labeled neutral, acidic and basic amino acids showed significantly reduced uptake of amino acids in the mutant, underscoring that increased tolerance of aap1 to high levels of amino acids is coupled with reduced uptake by the root. The growth and uptake studies identified glutamate, histidine and neutral amino acids, including phenylalanine, as physiological substrates for AAP1, whereas aspartate, lysine and arginine are not. We also demonstrate that AAP1 imports amino acids into root cells when these are supplied at ecologically relevant concentrations. Together, our data indicate an important role of AAP1 for efficient use of nitrogen sources present in the rhizosphere.

Characterization of plant aquaporins

Plants have been reported to contain a large set of aquaporins (38 for Arabidopsis), which has been divided into four subfamilies on the basis of similarities in their amino acid sequences. They belong to the large superfamily of major intrinsic proteins (MIP), which was the basis for the nomenclature PIP, TIP, and NIP, also indicating the subcellular localization plasma membrane, tonoplast, and nodule of the respective founding member. The fourth subfamily of small and basic intrinsic proteins is not well characterized so far. The increasing number of reports dealing with various aspects of plant aquaporins is starting to advance our understanding of aquaporin biology in plants. Fundamental questions include: what is the basic function of the different plant aquaporins, what is their primary substrate, and what is the consequence of function/malfunction of a particular aquaporin for the overall function of the plant? Biochemical and biophysical techniques can be employed to get information on the basic functional characteristics of plant aquaporins. An impressive set of techniques has been used to study aquaporin function on molecular, subcellular, and cellular levels in plants, as well as in heterologous expression systems. The physiological role of aquaporins in plants is much less well understood, but reports unraveling the physiological role of aquaporins, mainly employing genetic techniques and functional measurement on the whole plant level, are emerging. The goal of this chapter is to give an overview on the applied methods, together with some exemplary findings.

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.

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.

The SLC36 family: proton-coupled transporters for the absorption of selected amino acids from extracellular and intracellular proteolysis

Whilst Na(+) has replaced H(+) as a major transport driving force at the plasma membrane of animal cells, the evolutionarily older H(+)-driven systems persist on endomembranes and at the plasma membrane of specialized cells. The first member of the SLC36 family, present in both intracellular and plasma membranes, was identified independently as a lysosomal amino acid transporter (LYAAT1) responsible for the export of lysosomal proteolysis products into the cytosol and as a proton/amino acid transporter (PAT1) responsible for the absorption of amino acids in the gut. In addition to LYAAT1/PAT1, the family comprises another characterized member, PAT2, and two orphan transporters. Both PAT1 and PAT2 mediate 1:1 symport of protons and small neutral amino acids such as glycine, alanine, and proline. Their mRNAs are broadly and differentially expressed in mammalian tissues. The PAT1 protein localizes to lysosomes in brain neurons, but is also found in the apical membrane of intestinal epithelial cells with a role in the absorption of amino acids from luminal protein digestion. In both cases, protons supplied by the lysosomal H(+)-ATPase or by the acidic microclimate of the brush border membrane drive transport of the amino acids into the cytosol. The subcellular localization and physiological role of PAT2 have still to be determined. SLC36 transporters are related distantly to other proton-coupled amino acid transporters, such as the vesicular neurotransmitter transporter VIAAT/VGAT (SLC32) and system N transporters (SLC38 family).

Transport mechanisms for organic forms of carbon and nitrogen between source and sink

Sugars and amino acids are generated in plants by assimilation from inorganic forms. Assimilated forms cross multiple membranes on their way from production sites to storage or use locations. Specific transport systems are responsible for vacuolar uptake and release, for efflux from the cells, and for uptake into the vasculature. Detailed phylogenetic analyses suggest that only proton-coupled cotransporters involved in phloem loading have been identified to date, whereas systems for vacuolar transport and efflux still await identification. Novel imaging approaches may provide the means to characterize the cellular events and elucidate whole plant control of assimilate partitioning and allocation.

Reduced amino acid content in transgenic potato tubers due to antisense inhibition of the leaf H+/amino acid symporter StAAP1

Transport processes across the plasma membrane of leaf vascular tissue are essential for transport and distribution of assimilates. In potato, leaves are the predominant sites for nitrate reduction and amino acid biosynthesis. From there, assimilated amino acids are exported through the phloem to supply tubers with organic nitrogen. To study the role of amino acid transporters in long-distance transport and allocation of organic nitrogen in potato plants, a gene encoding a functional, leaf-expressed amino acid permease StAAP1 was isolated. Similar to the sucrose transporter SUT1, StAAP1 expression was induced during the sink-to-source transition, indicating a role in phloem loading. To test the role of StAAP1, expression was inhibited by an antisense approach. Transgenic plants with reduced StAAP1 expression were phenotypically indistinguishable from wild type, as were photosynthetic capacity and tuber yield. However, tubers from antisense StAAP1 plants showed up to 50% reduction in free amino acid contents. In comparison, starch content was not affected or tended to increase relative to wild type. The reduction in all amino acids except aspartate in the antisense plants is consistent with the properties of amino acid permeases (AAPs) found in heterologous systems. The results demonstrate an important role for StAAP1 in long-distance transport of amino acids and highlight the importance of plasma membrane transport for nutrient distribution in plants.

High affinity amino acid transporters specifically expressed in xylem parenchyma and developing seeds of Arabidopsis

Arabidopsis amino acid transporters (AAPs) show individual temporal and spatial expression patterns. A new amino acid transporter, AAP8 was isolated by reverse transcription-PCR. Growth and transport assays in comparison to AAP1-5 characterize AAP8 and AAP6 as high affinity amino acid transport systems from Arabidopsis. Histochemical promoter-beta-glucuronidase (GUS) studies identified AAP6 expression in xylem parenchyma, cells requiring high affinity transport due to the low amino acid concentration in xylem sap. AAP6 may thus function in uptake of amino acids from xylem. Histochemical analysis of AAP8 revealed stage-dependent expression in siliques and developing seeds. Thus AAP8 is probably responsible for import of organic nitrogen into developing seeds. The only missing transporter of the family AAP7 was nonfunctional in yeast with respect to amino acid transport, and expression was not detectable. Therefore, AAP6 and -8 are the only members of the family able to transport aspartate with physiologically relevant affinity. AAP1, -6 and -8 are the closest AAP paralogs. Although AAP1 and AAP8 originate from a duplicated region on chromosome I, biochemical properties and expression pattern diverged. Overlapping substrate specificities paired with individual properties and expression patterns point to specific functions of each of the AAP genes in nitrogen distribution rather than to mere redundancy.

Functional characterization of betaine/proline transporters in betaine-accumulating mangrove

Betaine is an important osmoprotectant in many plants, but its transport activity has only been demonstrated using a proline transporter from tomato, a betaine-nonaccumulating plant. In this study, two full-length and one partial transporter genes were isolated from betaine-accumulating mangrove Avicennia marina. Their homologies to betaine transporters from bacteria and betaine/4-aminobutyrate transporters from mammalian cells were low but were high to proline transporters from Arabidopsis and tomato. Two full-length transporters could complement the Na(+)-sensitive phenotype of the Escherichia coli mutant deficient in betT, putPA, proP, and proU. Both transporters could efficiently take up betaine and proline with similar affinities (K(m), 0.32-0.43 mm) and maximum velocities (1.9-3.6 nmol/min/mg of protein). The uptakes of betaine and proline were significantly inhibited by mono- and dimethylglycine but only partially inhibited by betaine aldehyde, choline, and 4-aminobutyrate. Sodium and potassium chloride markedly enhanced betaine uptake rates with optimum concentrations at 0.5 m, whereas sucrose showed only modest activation. The change of amino acids Thr(290)-Thr-Ser(292) in a putative periplasmic loop to Arg(290)-Gly-Arg(292) yielded the active transporter independent of salts, suggesting the positive charge induced a conformational change to the active form. These data clearly indicate that the betaine-accumulating mangrove contains betaine/proline transporters whose properties are distinct from betaine transporters of bacteria and mammalian cells.

Amino acid permeases in developing seeds of Vicia faba L.: expression precedes storage protein synthesis and is regulated by amino acid supply

Full length cDNAs encoding three amino acid permeases were isolated from seed-specific libraries of Vicia faba. The predicted proteins VfAAP1, VfAAP3 and VfAAP4 share up to 66% identity among themselves. Functional characterization of VfAAP1 and VfAAP3 in a yeast mutant showed that these permeases transport a broad range of amino acids. However, VfAAP1 had a preference for cysteine and VfAAP3 for lysine and arginine. VfAAP1 was highly expressed in cotyledons at early developmental stages and moderately in other sink tissues. Its peak of expression in cotyledons corresponded to the appearance of storage protein transcripts, suggesting that this transporter fulfills an important role in providing amino acids for storage protein biosynthesis. VfAAP3 was expressed most abundantly in maternal tissues, that is in roots, stems, gynoecia, pods and seed coats at different developmental stages. VfAAP4 transcripts could not be detected by northern hybridization. In situ hybridization showed that VfAAP1 mRNA is distributed throughout cotyledon storage parenchyma cells, but could not be detected in the abaxial epidermal cell layer. It also accumulate in the chlorenchyma and thin-walled parenchyma cells of seed coats. VfAAP1 mRNA levels were lower in cotyledons cultured in the presence of glutamine, whereas expression of a vicilin storage protein gene was up-regulated under similar conditions. Cysteine repressed the expression of the GUS reporter gene under control of the VfAAP1 promoter, suggesting that this transporter is modulated at the transcriptional level. Regulation of amino acid transport in relation to storage protein accumulation is discussed.

TRANSPORTERS RESPONSIBLE FOR THE UPTAKE AND PARTITIONING OF NITROGENOUS SOLUTES

The acquisition and allocation of nitrogenous compounds are essential processes in plant growth and development. The huge economic and environmental costs resulting from the application of nitrogen fertilizers make this topic very important. A diverse array of transporters varying in their expression pattern and also in their affinity, specificity, and capacity for nitrogenous compounds has been identified. Now the future challenge is to define their individual contribution to nitrogen nutrition and signalling processes. Here we have reviewed recent advances in the identification and molecular characterization of these transporters, concentrating on mechanisms existing at the plasma membrane. The review focuses on nitrate, ammonium, and amino acid transporter familes, but we also briefly describe what is known at the molecular level about peptide transporters and a recently identified family implicated in the transport of purines and their derivatives.

Functional characterization of a maize purine transporter by expression in Aspergillus nidulans

We have characterized the function of Leaf Permease1 (LPE1), a protein that is necessary for proper chloroplast development in maize, by functional expression in the filamentous fungus Aspergillus nidulans. The choice of this ascomycete was dictated by the similarity of its endogenous purine transporters to LPE1 and by particular genetic and physiological features of purine transport and metabolism in A. nidulans. When Lpe1 was expressed in a purine transport-deficient A. nidulans strain, the capacity for uric acid and xanthine transport was acquired. This capacity was directly dependent on Lpe1 copy number and expression level. Interestingly, overexpression of LPE1 from >10 gene copies resulted in transformants with pleiotropically reduced growth rates on various nitrogen sources and the absolute inability to transport purines. Kinetic analysis established that LPE1 is a high-affinity (K(m) = 30 +/- 2.5 microM), high-capacity transporter specific for the oxidized purines xanthine and uric acid. Competition studies showed that high concentrations of ascorbic acid (>30 mM) competitively inhibit LPE1-mediated purine transport. This work defines the biochemical function of LPE1, a plant representative of a large and ubiquitous transporter family. In addition, A. nidulans is introduced as a novel model system for the cloning and/or functional characterization of transporter genes.