Rice SVP-group MADS-box proteins, OsMADS22 and OsMADS55, are negative regulators of brassinosteroid responses

Most short vegetative phase (SVP)-group MADS-box genes control meristem identity and flowering time. Among the three SVP-group genes in rice, OsMADS47 has been reported as a negative regulator of brassinosteroid (BR) responses. Here, we investigated the functional roles of two close homologs, OsMADS22 and OsMADS55, by generating single, double and triple RNAi lines and overexpression lines. Analyses of the plants showed that their roles in regulating meristem identity are well conserved; however, the involvement of these genes in determining flowering time has diversified. Most importantly, OsMADS55 works as a major negative regulator of BR responses, and OsMADS22 functions to support OsMADS55. Whereas single OsMADS55 RNAi plants display weak BR responses in the lamina joint (LJ), OsMADS22-OsMADS55 double and OsMADS22-OsMADS47-OsMADS55 triple RNAi plants manifest dramatic BR responses with regard to LJ inclination, coleoptile elongation and senescence. Stem elongation is also notably reduced in the double and triple RNAi plants, probably because of BR oversensitivity. Expression analyses indicate the diversified roles in age-dependent BR responses. Altogether, our study demonstrates that all three rice SVP-group genes work as negative regulators of BR responses, but that their spatial and temporal roles are diversified.