Tiller Outgrowth in Rice ( Oryza sativa L.) is Controlled by OsGT1 , Which Acts Downstream of FC1 in a PhyB -Independent Manner

Tillering is one of the most important determinants of biomass and yield in rice (Oryza sativa L.). The capacity of plants to develop tillers from primordial meristems or buds is determined not only by the genotype but also by environmental cues. Here, we characterized the function of rice grassy tiller1 (OsGT1) and its interaction with other genetic and biological factors involved in tiller bud outgrowth in rice by generating OsGT1 RNA interference (RNAi) and overexpression (OX) lines. The tiller number was increased in OsGT1-RNAi mutants but strongly suppressed in OsGT1-OX lines. Expression analysis of OsGT1 in rice phyB mutants and in genotypes carrying various genetic combinations of GT1 RNAi and phyB demonstrated that OsGT1 is not involved in phyB-mediated suppression of tiller development in rice. Expression analysis of fine culm1 (fc1), a rice tb1 homolog, and molecular assays demonstrated that FC1 enhances the expression of OsGT1 by directly binding to its promoter. Comparison of the transcriptomic profiles of fc1 and OsGT1-RNAi mutants revealed differentially expressed genes (DEGs) common to both genotypes. Finally, analysis of tillering phenotypes of OX and RNAi seedlings treated with various phytohormones implied a possible role of OsGT1 in strigolactone-mediated tiller outgrowth. Overall, this study enhances our understanding of the diverse mechanisms of tiller development in grasses.