TAWAWA1, a regulator of rice inflorescence architecture, functions through the suppression of meristem phase transition

Inflorescencestructures result from the activities of meristems, which coordinate both the renewal of stem cells in the center and organ formation at the periphery. The fate of a meristemis specified at its initiation and changes as the plant develops. During rice inflorescencedevelopment, newly formed meristemsacquire a branch meristem(BM) identity, and can generate further meristemsor terminate as spikelets. Thus, the form of rice inflorescenceis determined by a reiterative pattern of decisions made at the meristems. In the dominant gain-of-function mutant tawawa1-D, the activity of the inflorescence meristem(IM) is extended and spikelet specification is delayed, resulting in prolonged branch formation and increased numbers of spikelets. In contrast, reductions in TAWAWA1 (TAW1) activity cause precocious IM abortion and spikelet formation, resulting in the generation of small inflorescences. TAW1 encodes a nuclear proteinof unknown function and shows high levels of expression in the shoot apical meristem, the IM, and the BMs. TAW1 expression disappears from incipient spikelet meristems(SMs). We also demonstrate that members of the SHORT VEGETATIVE PHASE subfamily of MADS-boxgenes function downstream of TAW1. We thus propose that TAW1 is a unique regulator of meristemactivity in rice and regulates inflorescencedevelopment through the promotion of IM activity and suppression of the phase change to SM identity.