Minisymposium 3: Hormones I

Abs # 13003: Gibberellin signal transduction in Arabidopsis mediated by the F-box protein SLEEPY1 (SLY1)

Presenter:	Ritchie, Sian , sianrit@yahoo.com
Authors	Ritchie, Sian  (A)   Doherty, Lucia C (A)   Soule, Jonathan D (A)   McGinnis, Karen M (B)   Steber, Camille M (A)
Affiliations:	(A): Washington State University (B): University of Arizona

sleepy1 (sly1) is an Arabidopsis thaliana mutant with a gibberellin (GA)-insensitive phenotype. These dwarf plants exhibit poor germination and fertility, and neither of these characterstics can be rescued by adding GA. Both the sly1 alelles we have are recessive, and hence the SLY1 protein is a positive element in the GA response pathway of Arabidopsis. Cloning of SLEEPY1 revealed that it encodes an F-box protein that is presumably part of an SCF (SKP1,Cullin, F-box) E3 ubiquitin ligase complex (McGinnis et al., Plant Cell 15, in Press). A homologous F-box gene, GID2, was recently identified as a GA-response gene in rice (Sasaki et al., 2003 Science 229, p.1896). BLAST search of EST sequences identified homologues of SLY1/GID2 in most plant species. Ubiquitylation by the SCF complex targets proteins for degradation by the 26S proteasome. The F-box subunit determines target specificity. Arabidopsis has a large number of F-box genes, but only one other is similar to SLY1. Hence we have named it SLY2. We are investigating if this is also involved in the GA response pathway. The DELLA domain family of putative transcription factors including RGA, GAI, RGL1, RGL2, and RGL3 (Arabidopsis), SLR1 (rice), SLN1 (barley) are negative regulators of the GA response. Protein turnover is a key component in the regulation of many members of this gene family. Levels of RGA protein are elevated in sleepy1 mutants, and similarly SLR1 protein accumulates in gid2, the rice F-box mutant. Therefore our model is that the SCF ^SLY1 complex targets RGA directly, causing its degradation and eliminating its negative effect on GA-up-regulated genes. This hypothesis is supported by double mutant data. The rga-24 null mutant suppresses the dwarf phenotype of sly1-10. Interestingly, rga-24 does not rescue the reduced fertility of sly1 mutants. It is possible that the fertility phenotype of sly1 mutants results from overaccumulation of another DELLA protein. It will be important to determine if SCF ^SLY1 directly regulates other members of the DELLA family.