Functional characterization of RGA downstream targets in the GA signaling pathway

Abstract

Gibberellins (GA) are an important family of endogenous plant growth regulators, essential for many aspects of plant growth and development. In the GA signaling pathway, the action of GA is opposed by a group of DELLA repressors. RGA (REPRESSOR OF GA1-3), a member of the DELLA family proteins, is a major repressor of plant growth in Arabidopsis. Although the components of GA signaling and the mechanism of action of the DELLA repressor proteins have been studied in great detail, the effectors which bring about GA-responsive growth and development remain largely unknown. In this study, we characterized the novel gene, STUNTED (STU), a RLCK VI family protein expressed in multiple plant tissues, whose expression is repressed by RGA in a GA-dependent manner. STU loss-of-function mutant displayed multiple defects including retarded growth and smaller plant stature, which were attributed to a reduction in cell division. Furthermore, the effect of STU on cell division was found to occur through two cyclin-dependent kinase inhibitors, SIM and SMR1. Our results lead to the proposal of a novel mechanism of GA regulation of plant stature with STU as an important intermediate.