Figure 1.

Gibberellic acid (GA) signaling in spikelet development. Under GA-deficient conditions, the casein kinase I (EL1) phosphorylates SLENDER RICE1 (SLR1) to increase its activity and stability; then, SLR1 represses the expression of GAMYB. Under GA-sufficient conditions, GA is perceived by GID1 and the resulting conformational change of GID1 allows it to interact with and degrade SLR1 through a SCF^GID2-mediated proteasome pathway. In the absence of SLR1 activity, GAMYB transcription becomes de-repressed; then, GAMYB protein positively regulates the transcription of several downstream genes for formation of the Ubisch body and exine, as well as for cell death in the tapetum during spikelet development.