Functional Analysis of the Rice Type-B Response Regulator RR22

The phytohormone cytokinin plays a critical role in regulating growth and development throughout the life cycle of the plant. The primary transcriptional response to cytokinin is mediated by the action of the type-B response regulators (RRs), with much of our understanding for their functional roles being derived from studies in the dicot Arabidopsis. To examine the roles played by type-B RRs in a monocot, we employed gain-of-function and loss-of-function mutations to characterize <em>RR22</em> function in rice. Ectopic overexpression of <em>RR22</em> in rice results in an enhanced cytokinin response based on molecular and physiological assays. Phenotypes associated with enhanced activity of <em>RR22</em> include effects on leaf and root growth, inflorescence architecture, and trichome formation. Analysis of four <em>Tos17</em> insertion alleles of <em>RR22</em> revealed effects on inflorescence architecture, trichomes, and development of the stigma brush involved in pollen capture. Both loss- and gain-of-function <em>RR22</em> alleles affected the number of leaf silica-cell files, which provide mechanical stability and improve resistance to pathogens. Taken together, these results indicate that a delicate balance of cytokinin transcriptional activity is necessary for optimal growth and development in rice.