OsPIL15-Induced Delay in Rice Heading Date via Direct Binding to the OsLF Promoter is Dependent on Functional Phytochrome B.

Heading date of rice (Oryza sativa) is a key factor determining rice production and regional adaptability. We analysed the molecular mechanism of OsPIL15, encoding phytochrome-interacting factor-like protein, in delaying rice heading date. Overexpression of OsPIL15 delayed rice heading date by upregulating Hd1 and inhibiting Hd3a and RFT1 expression. OsLF, encoding one rice heading repressor, was found to be the putative candidate regulated by OsPIL15 through a chromatin immunoprecipitation sequencing assay and a transcriptome sequencing assay. OsPIL15 could directly bind to the OsLF promoter and activated its expression. Knocking-out OsLF in OsPIL15-overexpressing lines resulted in flowering 2-3 days earlier, partially rescuing the delayed phenotype. This indicates that overexpression of OsPIL15 overexpression delays heading date partially through OsLF. Protein-protein interaction assay of OsPIL15 or OsPIL15-∆APB (OsPIL15 lacking the active phytochrome B [phyB]-binding [APB] motif) with PHYB showed that the APB motif was required for the interaction between OsPIL15 and PHYB. Furthermore, overexpression of either OsPIL15-∆APB in the wild type or OsPIL15 in the phyB mutant did not delay rice heading date under natural long-day conditions, suggesting that phyB influences OsPIL15-mediated delay in rice heading date.