Proline Accumulation Is Regulated by Transcription Factors Associated with Phosphate Starvation.

Pro accumulation in plants is a well-documented physiological response to osmotic stress caused by drought or salinity. In Arabidopsis (), the stress and ABA-induced Δ1-PYRROLINE-5-CARBOXYLATE SYNTHETASE1 () gene was previously shown to control Pro biosynthesis in such adverse conditions. To identify regulatory factors that control the transcription of , Y1H screens were performed with a genomic fragment of , containing 1.2-kB promoter and 0.8-kb transcribed regions. The myeloblastosis (MYB)-type transcription factors PHOSPHATE STARVATION RESPONSE1 (PHR1) and PHR1-LIKE1 (PHL1) were identified to bind to regulatory sequences in the first intron, which carries a conserved PHR1-binding site (P1BS) motif. Binding of PHR1 and PHL1 factors to P1BS was confirmed by Y1H, electrophoretic mobility assay and chromatin immunoprecipitation. Phosphate starvation led to gradual increase in Pro content in wild-type Arabidopsis plants as well as transcriptional activation of and PRO DEHYDROGENASE2 genes. Induction of transcription and Pro accumulation during phosphate deficiency was considerably reduced by and mutations and was impaired in the ABA-deficient and ABA-insensitive mutants. Growth and viability of double mutant was significantly reduced in phosphate-depleted medium, while growth was only marginally affected in the mutants, suggesting that ABA is implicated in growth retardation in such nutritional stress. Our results reveal a previously unknown link between Pro metabolism and phosphate nutrition and show that Pro biosynthesis is target of cross talk between ABA signaling and regulation of phosphate homeostasis through PHR1- and PHL1-mediated transcriptional activation of the gene.