The small interfering RNA production pathway is required for shoot meristem initiation in rice

The shoot apical meristem (SAM) is a group of stem cells that are responsible for plant development. Mutations in riceSHOOTLESS2 (SHL2 ),SHL4/SHOOT ORGANIZATION2 (SHO2 ), andSHO1 cause complete deletion or abnormal formation of the SAM. In this study we showed that defects in SAM formation inshl mutants are associated with the loss of expression of the homeodomain–leucine zipper (HD-ZIPIII ) family genes. RiceSHL2 ,SHL4/SHO2 , andSHO1 encoded orthologues ofArabidopsis RNA-dependent RNA polymerase 6, ARGONAUTE (AGO) 7, and DICER-like 4, respectively, whose mutations affect leaf development through the trans-acting siRNA (ta-siRNA) pathway. This suggested that the ta-siRNA pathway regulates the critical step of SAM formation during rice embryogenesis. The gain-of-function experiment by the ectopic expression ofSHL4 resulted in reduced accumulation of an microRNA,miR166 , and partial adaxialization of leaves, supporting a role for the ta-siRNA pathway in the maintenance of leaf polarity as previously reported in maize. Analysis of the spatiotemporal expression patterns ofHD-ZIPIII andmiR166 in wild-type andshl mutant embryos suggested that the loss ofHD-ZIPIII expression in the SAM region of the developing embryo is the result of ectopic expression ofmiR166 . Our analysis ofshl mutants demonstrated thatHD-ZIPIII expression regulated bymiR166 is sensitive to the ta-siRNA pathway during SAM formation in rice embryogenesis.