Role of PIF3 in arabidopsis phytochrome signaling

Abstract

Phytochromes are red/far-red light receptors that regulate various light responses by initiating transcriptional cascades responsible for changing the expression patterns of 10 - 30% of the entire plant transcriptome. Several transcription factors that are thought to participate in this process have been identified, but the underlying molecular functions have not yet been fully elucidated. Here, I investigate the role of such transcription factor, PIF3, on the anthocyanin and chlorophyll biosynthesis. PIF3 is a nuclear localized bHLH protein whose activity is regulated by active phytochromes. PIF3 is known to regulate hypocotyl elongation, anthocyanin biosynthesis, shoot gravitropism, and photosynthetic activity. I found that PIF3 positively regulate anthocyanin biosynthesis by activating transcription of anthocyanin biosynthetic genes, but the positive effects of PIF3 required functional HY5. Chromatin immunoprecipitation analyses indicated that both PIF3 and HY5 regulate anthocyanin biosynthetic gene expression by directly binding to different regions of the gene promoters in vivo. Additional experiments revealed that PIF3 bound to the promoters regardless of light and HY5. I additionally investigate the role of PIF3 in chlorophyll biosynthesis. I found that PIF3 negatively regulates the expression of photosynthetic genes and chlorophyll biosynthetic genes including CAB and GUN5. Consistent with the higher expression of chlorophyll biosynthetic genes in the pif3 mutant, etiolated pif3 seedlings showed severe photooxidative damage upon the transfer to light. The photooxidative damage is partly due to the overexpression of chlorophyll biosynthetic genes including GUN5 as gun5pif3 double mutant does not show any photooxidative damage. Interestingly, PIF3 is necessary for plastid-to-nucleus signaling, when the tetrapyrrole intermediates are overproduced. Moreover, tetrapyrrole intermediates stabilize the PIF3 protein to form a negative feedback loop in the ch...