The OsHAPL1-DTH8-Hd1 complex functions as the transcription regulator to repress heading date in rice

Heading date is an important agronomic trait related to crop yield. Many genes related to heading date have already been identified in rice (Oryza sativa), and a complicated, preliminary regulatory genetic network has also already been established, but the protein regulatory network is poorly understood. We have identified a novel heading date regulator, Heme Activator Protein like 1 (OsHAPL1), which inhibits flowering under long-day conditions. OsHAPL1 is a nuclear-localized protein that is highly expressed in leaves in a rhythmic manner. OsHAPL1 can physically interact with Days To Heading on chromosome 8 (DTH8), which physically interacts with Heading date 1 (Hd1) both in vitro and in vivo. OsHAPL1 forms a complex with DTH8 and Hd1 in Escherichia coli. OsHAPL1, DTH8, and Hd1 physically interact with the HAP complex, and also with general transcription factors in yeast (Saccharomyces cerevisiae). Further studies showed that OsHAPL1 represses the expression of the florigen genes and FLOWERING LOCUS T 1 (RFT1) and Hd3a through Early heading date 1 (Ehd1). We propose that OsHAPL1 functions as a transcriptional regulator and, together with DTH8, Hd1, the HAP complex, and general transcription factors, regulates the expression of target genes and then affects heading date by influencing the expression of Hd3a and RFT1 through Ehd1.