SALT STRESS-INDUCED PROLINE TRANSPORTERS AND SALT STRESS-REPRESSED BROAD-SPECIFICITY AMINO-ACID PERMEASES IDENTIFIED BY SUPPRESSION OF A YEAST AMINO-ACID PERMEASE-TARGETING MUTANT

A yeast mutant lacking SHR3, a protein specifically required for corre ct targeting of plasma membrane amino acid permeases, was used to stud y the targeting of plant transporters and as a tool to isolate new SHR 3-independent amino acid transporters, For this purpose, an shr3 mutan t was transformed with an Arabidopsis cDNA library, Thirty-four clones were capable of growth under selective conditions, but none showed ho mology with SHR3. However, genes encoding eight different amino acid t ransporters belonging to three different transporter families were iso lated, Five of these are members of the general amino acid permease (A AP) gene family, one is a member of the NTR family, encoding an oligop eptide transporter, and two belong to a new class of transporter genes , A functional analysis of the latter two genes revealed that they enc ode specific proline transporters (ProT) that are distantly related to the AAP gene family, ProT1 was found to be expressed in all organs, b ut highest levels were found in roots, stems, and flowers, Expression in flowers was highest in the floral stalk phloem that enters the carp els and was downregulated after fertilization, indicating a specific r ole in supplying the ovules with proline, ProT2 transcripts were found ubiquitously throughout the plant, but expression was strongly induce d under water or salt stress, implying that ProT2 plays an important r ole in nitrogen distribution during water stress, unlike members of th e AAP gene family whose expression was repressed under the same condit ions, These results corroborate the general finding that under water s tress, amino acid export is impaired whereas proline export is increas ed.