Two AP2 family genes, supernumerary bract (SNB) and Osindeterminate spikelet 1 (OsIDS1), synergistically control inflorescence architecture and floral meristem establishment in rice.

Meristem identity is crucial in determining the inflorescence architecture of grass species. We previously reported that SUPERNUMERARY BRACT (SNB) regulates the transition of spikelet meristems into floral meristems in rice (Oryza sativa). Here we demonstrated that SNB and Oryza sativa INDETERMINATE SPIKELET 1 (OsIDS1) together play important roles in inflorescence architecture and the establishment of floral meristems. In snb osids1 double mutants, the numbers of branches and spikelets within a panicle are significantly decreased, and the transition to a floral meristem is further delayed compared with the snb single mutant. Expression analyses showed that SNB and OsIDS1 are required for spatio-temporal expression of B- and E-function floral organ identity genes in the lodicules. In addition, the AP2 family genes are important for determining the degree of ramification in branch meristems, regulating the spatio-temporal expression of spikelet meristem genes, such as FRIZZY PANICLE (FZP). Furthermore, overexpression of microRNA172 (miR172) causes reductions in SNB and OsIDS1 transcript levels, and phenotypes of the transgenic plants are more severe than for snb osids1. This indicates that additional gene(s) participate in the development of branch and floral meristems. Preferential expression of mature miR172s in the area around the spikelet meristems implies that depletion of the AP2 family genes in those meristems via miR172 is an important step in controlling inflorescence branching and the formation of floral organs.