Plant Cell Advance Online Publication
Published on September 30, 2008; 10.1105/tpc.108.058487
Received February 4, 2008
Returned for revision July 24, 2008
Accepted September 11, 2008
Proteolysis-Independent Downregulation of DELLA Repression in Arabidopsis by the Gibberellin Receptor GIBBERELLIN INSENSITIVE DWARF1
Tohru Ariizumi 1, Kohji Murase 2, Tai-ping Sun 2, and Camille M. Steber 3*
1 Department of Crop and Soil Science, Washington State University, Pullman, Washington 99164-6420
2 Department of Biology, Duke University, Durham, North Carolina 27708
3 Department of Crop and Soil Science, Washington State University, Pullman, Washington 99164-6420; U.S. Department of Agriculture–Agricultural Research Service, Washington State University, Pullman, Washington 99164-6420
* To whom correspondence should be addressed. E-mail: csteber{at}wsu.edu.
This article presents evidence that DELLA repression of gibberellin (GA) signaling is relieved both by proteolysis-dependent and -independent pathways in Arabidopsis thaliana. DELLA proteins are negative regulators of GA responses, including seed germination, stem elongation, and fertility. GA stimulates GA responses by causing DELLA repressor degradation via the ubiquitin-proteasome pathway. DELLA degradation requires GA biosynthesis, three functionally redundant GA receptors GIBBERELLIN INSENSITIVE DWARF1 (GID1a, b, and c), and the SLEEPY1 (SLY1) F-box subunit of an SCF E3 ubiquitin ligase. The sly1 mutants accumulate more DELLA proteins but display less severe dwarf and germination phenotypes than the GA biosynthesis mutant ga1-3 or the gid1abc triple mutant. Interestingly, GID1 overexpression rescued the sly1 dwarf and infertility phenotypes without decreasing the accumulation of the DELLA protein REPRESSOR OF GA1-3. GID1 rescue of sly1 mutants was dependent on the level of GID1 protein, GA, and the presence of a functional DELLA motif. Since DELLA shows increasing interaction with GID1 with increasing GA levels, it appears that GA-bound GID1 can block DELLA repressor activity by direct protein–protein interaction with the DELLA domain. Thus, a SLY1-independent mechanism for GA signaling may function without DELLA degradation.
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