Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 9/21/2005
Publication Date: 9/15/2006
Citation: Steber, C.M. 2006. Chapter 11: De-repression of seed germination by ga signaling. In: Seed Development, Dormancy and Germination. Eds. Kent Bradford and Hiroyuki Nonogaki. pgs 248-263. Blackwell Publishing, London, UK. Interpretive Summary: The plant hormone GA stimulates seed germination and mobilization of nutrients stored in the seed. GA does this by causing the destruction of a protein called RGL2 that normally blocks seed germination in Arabidopsis. Based on its amino acid sequence, RGL2 is considered a member of the DELLA protein family which is turn is a subfamily of the GRAS family of putative transcription factors. GA causes the destruction of RGL2 putting a small protein called ubiquitin on RGL2. Ubiquitin acts as a tag that sends RGL2 (and other tagged proteins) to a structure called the 26S proteasome which degrades proteins into amino acids.
Technical Abstract: This chapter explores evidence that the ubiquitin-proteasome pathway plays a role in gibberellin (GA) stimulation of germination via proteolysis of DELLA proteins. 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 domain amino acid sequences. It is clear that the ubiquitin-proteasome pathway relieves DELLA repression of stem elongation in response to GA signaling. However, interpretation of the evidence for DELLA regulation of seed germination has been somewhat contentious. The paradigm is that GA responses result from the disappearance of the DELLA proteins, negative regulators of GA response, and that this disappearance results from the action of a GA-stimulated SCF E3 ubiquitin ligase complex. Work in Arabidopsis thaliana suggests that the DELLA protein RGL2 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.