| In higher plants, amino acids are the currency of nitrogen exchange between source to sink tissues during different phases of development. AAP1, as a prototypical example of plant proton-amino acid symporter on plasma membrane, was regulated by nitrogen sources including nitrate, ammonium and amino acids. Microarray data also suggested high expression level of AAP1 compared to other amino acid transporters, and AAP1 was the only amino acid transporter induced by nitrate, glutamine and glutamate. These suggest that nitrogen status, via unidentified signaling molecules, regulates amino acid distribution through modulating gene expression of amino acid transporters, and AAP1 is an important candidate in this regulation.; Inhibitors of nitrogen metabolism enzymes showed that only GOGAT inhibitor/glutamine analogue inhibited nitrate induced AAP1 expression, suggesting glutamine was not major signal inducing AAP1 expression. Semi-quantitative RT-PCR was used to examine the response of nitrogen assimilation mutants, suggested nitrate and glutamate as metabolite signals that induce AAP1 expression after nitrogen starvation, although signaling function of other amino acids (such as aspartate and asparagine) were yet to be tested. Microarray was also applied for understanding the nitrogen regulated expression. Gene classification demonstrated that nitrate, glutamate and glutamine differentially influenced genes involved in many aspects of plant growth and development. This differentiation was also supported by a T-DNA insertion line of AAP1, in which only a short fragment of AAP1 promoter inducible by nitrate, but not by glutamate. The nitrate induction was also demonstrated in AAP1:LUC and AAP1:GUS plants. Mutagenesis and mutant screen of transgenic plants for nitrate responsive mutants has identified two constitutive mutants.; Arabidopsis amino acid transporter family members are highly homologous and detailed characterization is necessary to understand the function of each member in plant growth and development. AAP1:GUS plants demonstrated that AAP1 expression is developmentally regulated in both source and sink tissues. In situ hybridization identified the cell-specific AAP1 expression in the phloem cells of minor veins, consistent with a role in phloem loading of amino acids. Overexpression of AAP1 results in shorter roots and altered leaf shape in seedlings, while RNAi knock out plants flower later than wild type. These data suggested the importance of AAP1 in plant development and growth. The reasons led to these phenotypes were discussed.; Taken together, the evidence suggests that nitrogen mediated regulatory pathways are key steps in regulating expression of amino acid transporter AAP1, this may lead to modulated phloem loading of amino acids and finally influence plant nitrogen status. |
