NITROGEN IS ONE OF THE MOST IMPORTANT FACTORS AFFECTING CROP YIELD. THEREFORE, IT IS CRITICAL TO UNDERSTAND HOW PLANTS SENSE, TAKE UP, USE, AND STORE NITROGEN. UPON SENSING THE SUPPLY OF NITROGEN, GENE EXPRESSION IN A PLANT CHANGES AT GENOME-WIDE SCALE, ALLOWING THE PLANT TO EFFECTIVELY UPTAKE AND ASSIMILATE NITROGEN. WE RECENTLY FOUND THAT A GENE ENCODING AN AMINO ACID TRANSPORTER UNDERGOES "ISOFORM SWITCHING" IN RESPONSE TO CHANGES AT NITROGEN LEVELS: WHEN NITROGEN IS ABUNDANT, THE GENE IS EXPRESSED AT FULL LENGTH, BUT WHEN NITROGEN IS DEPLETED, A SHORTER VERSION IS EXPRESSED. WE HYPOTHESIZE THAT THE FULL-LENGTH TRANSPORTER FUNCTIONS TO TRANSPORT AMINO ACIDS INTO THE VACUOLE TO STORE EXCESS NITROGEN WHEN NITROGEN IS ABUNDANT. WHEN NITROGEN IS LIMITING, THE SHORTER VERSION IS DEPLOYED TO SHUT OFF AMINO ACID TRANSPORT. THE GOAL OF THIS PROJECT IS TO TEST THIS HYPOTHESIS BY I) UNDERSTANDING THE REGULATORY MECHANISM OF ISOFORM SWITCHING; II) DETERMINING THE BIOCHEMICAL FUNCTION AND INTERACTIONS OF THE FULL LENGTH AND SHORTER VERSION ISOFORMS; AND III) UNDERSTANDING THE ROLE THIS GENE PLAYS IN MAINTAINING NITROGEN STATUS IN PLANTA.