OsMADS50 and OsMADS56 function antagonistically in regulating long day (LD)-dependent flowering in rice

In much of the tropics and subtropics, rice (Oryza sativa L.) is grown under long days (LDs). Therefore, LD must play a major role in inducing flowering signal in rice. However, little is known on LD-dependent flowering signal in the species. We previously reported that OsMADS50, which is highly homologous to Arabidopsis SOC1, functions as a positive regulator for flowering. However, its detailed photoperiodic mechanism was not yet elucidated. Here, we report the functional analysis of OsMADS50 and its closely related gene OsMADS56. Knock-out of OsMADS50 caused a late-flowering phenotype only under LD conditions. Overexpression of OsMADS56 (56OX) also resulted in delayed flowering under LD. In the osmads50 mutants and 56OX transgenic plants, transcripts of Ehd1, Hd3a and RFT1 were reduced, although that of OsLFL1 increased. On the other hand, mRNA levels of OsGI, Hd1, OsId1, OsDof12, Ghd7, Hd6 and SE5 were unchanged. These observations imply that OsMADS50 and OsMADS56 function antagonistically through OsLFL1-Ehd1 in regulating LD-dependent flowering. Yeast two-hybrid and co-immunoprecipitation analyses indicated an interaction between those two proteins as well as their formation of homodimers. These results suggest that OsMADS50 and OsMADS56 may form a complex that regulates downstream target genes.