SLENDER RICE1 and <i>Oryza sativa</i> INDETERMINATE DOMAIN2 Regulating OsmiR396 Are Involved in Stem Elongation

Abstract

GAs play key roles in controlling cell proliferation through the GIBBERELLIN INSENSITIVE DWARF1/DELLA-mediated pathway. However, how DELLA proteins affect downstream pathways is not well understood. Therefore, discovering the signaling events downstream of DELLAs is key to better understanding the roles of GAs in plant development. Here, we discovered that miR396 is regulated by <i>SLENDER RICE1</i> (<i>SLR1</i>) in controlling cell proliferation. The positive response of rice (<i>Oryza sativa</i>) <i>GROWTH-REGULATING FACTORs</i> (<i>OsGRFs</i>) to GAs was found to be caused by a negative response of miR396 to GAs. miR396 acts downstream of <i>SLR1</i> and upstream of GA-induced cell-cycle genes. Rice INDETERMINATE DOMAIN2 (OsIDD2) directly binds the promoter of OsmiR396a and can interact with SLR1 in vivo and in vitro. Rice lines overexpressing miR396a (miR396OE) or <i>OsIDD2</i> (<i>OsIDD2OE</i>) displayed dwarfism resulting from higher abundance of miR396 RNA. However, the stem elongation of <i>OsIDD2OE</i> plants could be significantly stimulated by applying exogenous GA₃, while that of miR396OE plants could not. Rice with <i>OsIDD2</i> knocked down by RNA interference showed a <i>slr1</i>-like phenotype, in which the expression of miR396 was inhibited while its targets were enhanced. The protein levels of OsIDD2 were unaffected by GA in wild-type and <i>OsIDD2OE</i> plants, implying that OsIDD2 promotes the expression of miR396 and likely requires the coactivator of SLR1. Taken together, these results provided a close link between <i>SLR1/OsIDD2</i> and <i>GRFs</i> via a negative regulator, miR396, and thus highlighted a molecular mechanism of GA-mediated cell proliferation in rice.