Proline accumulation is regulated by transcription factors associated with phosphate starvation

Proline accumulation in plants is a well-documented physiological response to osmotic stress caused by drought or salinity. In Arabidopsis thaliana the stress and ABA-induced Delta1-PYRROLINE-5-CARBOXYLATE SYNTHETASE 1 (P5CS1) gene was previously shown to control proline biosynthesis in such adverse conditions. To identify regulatory factors which control the transcription of P5CS1, yeast one hybrid (Y1H) screens were performed with a genomic fragment of P5CS1, containing 1.2 kB promoter and 0.8 kB transcribed regions. The transcription factors PHOSPHATE STARVATION RESPONSE 1 (PHR1) and PHR1-LIKE 1 (PHL1) were identified to bind to P5CS1 sequences in the first intron, which carry a conserved PHR1-binding site (P1BS) motif. PHR1 and PHL1 binding to P1BS was confirmed by Y1H, electrophoretic mobility assay (EMSA) and chromatin immune precipitation (ChIP). Phosphate starvation led to gradual increase in proline content in wild type Arabidopsis plants as well as transcriptional activation of P5CS1 and PROLINE DEHYDROGENASE 2 (PDH2) genes. Induction of P5CS1 transcription and proline accumulation during phosphate deficiency was considerably reduced by phr1 and phl1 mutations and was impaired in the ABA deficient aba2-3 and ABA insensitive abi4-1 mutants. Growth and viability of phr1phl1 double mutant was reduced in phosphate-depleted medium, it was slightly diminished in the aba2-3 mutant, suggesting that ABA mediates growth retardation in such stress. Our results reveal a previously unknown link between proline metabolism and phosphate nutrition, and show that proline biosynthesis is target of crosstalk between ABA signaling and regulation of phosphate homeostasis through PHR1 and PHL1-mediated transcriptional activation of the P5CS1 gene

Light Control of Salt-Induced Proline Accumulation is Mediated by Elongated Hypocotyl 5 in Arabidopsis

Plants have to adapt their metabolism to constantly changing environmental conditions, among which the availability of light and water is crucial in determining growth and development. Proline accumulation is one of the sensitive metabolic responses to extreme conditions; it is triggered by salinity or drought and is regulated by light. Here we show that red and blue but not far-red light is essential for salt-induced proline accumulation, upregulation of Delta 1-PYRROLINE-5-CARBOXYLATE SYNTHASE 1 (P5CS1) and downregulation of PROLINE DEHYDROGENASE 1 (PDH1) genes, which control proline biosynthetic and catabolic pathways, respectively. Chromatin immunoprecipitation and electrophoretic mobility shift assays demonstrated that the transcription factor ELONGATED HYPOCOTYL 5 (HY5) binds to G-box and C-box elements of P5CS1 and a C-box motif of PDH1. Salt-induced proline accumulation and P5CS1 expression were reduced in the hy5hyh double mutant, suggesting that HY5 promotes proline biosynthesis through connecting light and stress signals. Our results improve our understanding on interactions between stress and light signals, confirming HY5 as a key regulator in proline metabolism