Ds9 was isolated encoding as OsHAP3H and its C-terminus was required for interaction with HAP2 and HAP5

Heading date is one of the most important agronomic traits in rice. Here, we analyzed an early flowering mutant ds9 under long day (LD) conditions and found that the early flowering phenotype was controlled by a single dominant gene. Mapping the Ds9 revealed that it was located near SSR marker RM5432 on chromosome 8, close to the known heading date-related DTH8/Ghd8 locus. Sequence analysis indicated that Ds9 was allelic to Ghd8 , encoding the OsHAP3H subunit of CCAAT-box binding proteins (HAPs), and transformation of ds9 mutant with the OsHAP3H gene from wild type XS11 plants significantly increased the heading date under LD conditions. Yeast two hybrid and three hybrid assays revealed that the C-terminus of OsHAP3H is required for its interactions with OsHAP2 and OsHAP5. The data demonstrated that OsHAP3H plays a critical role as a repressor of photoperiodic flowering in rice.