DE-repression of Seed Germination by GA Signaling

Abstract

This chapter explores evidence that the ubiquitin–proteasome pathway plays a role in gibberellin (GA) stimulation of germination via proteolysis of DELLA proteins (named after conserved amino acids). GA stimulates germination, stem elongation, transition to flowering, and fertility. DELLA proteins are repressors of GA responses defined by the presence of conserved DELLA and GRAS (named after GAI [GA-INSENSITIVE], RGA [REPRESSOR OF GAI], and SCARECROW proteins) domain amino acid sequences. It is clear that the ubiquitin–proteasome pathway relieves DELLA repression of stem elongation in response to GA signaling. The paradigm is that GA stimulates an SCF (named after the Skp1, Cullin, and F-box subunits) E3 ubiquitin ligase complex, which in turn stimulates degradation of the DELLA proteins, negative regulators of GA response, by the proteasome pathway. This induces the downstream events necessary for seed germination, which were under repression in the absence of GA. However, interpretation of the evidence for DELLA regulation of seed germination has been somewhat contentious. Work in Arabidopsis thaliana suggests that the DELLA protein RGL2 (RGALIKE2) is the main negative regulator of GA response in germination, and that the SCFSLY1 E3 ubiquitin ligase complex is required for GA-stimulated disappearance of RGL2.