Submitted to: The Plant Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2004
Publication Date: 5/21/2004
Citation: Dill, A., Thomas, S.G., Hu, J., Steber, C.M., Sun, T.P. 2004. The arabidopsis f-box protein sleepy1 targets gibberellin signaling repressors for gibberellin-induced degradation. The Plant Cell. 16:1392-1405. Interpretive Summary: The plant hormone gibberellin (GA) controls many agronimically important traits including seed germination, plant stature, induction of flowering and fertility. The importance of GAs in modulating plant stature is highlighted by the recent finding that the semidwarf wheat cultivars from the "Green Revolution" in the 1960s contain mutations that affect GA signaling. Mutations in the DELLA genes are the dwarfing genes that caused the 20% increase in yield during the green revolution. This paper reports evidence that the DELLA genes are controlled by proteolysis. The SLEEPY1 (SLY1) gene of Arabidopsis encodes the protein in a ubiquitin ligase that specifically recognizes and targets DELLA proteins for destruction. SLY1 specifically binds to the DELLA proteins RGA (Repressor of GA) and GAI (GA-Insensitive). This affinity for GAI and RGA is enhanced by a gain-of-function mutation, sly1-d, that suppresses that dwarf phenotypes of rga-delta17 and gai-1.
Technical Abstract: Bioactive gibberellins (GAs) are a class of plant hormones that control growth and development throughout the life cycle of a plant. The importance of GAs in modulating plant stature is highlighted by the recent finding that the semidwarf wheat cultivars from the "Green Revolution" in the 1960s contain mutations that affect GA signaling. The nuclear localized DELLA proteins are highly conserved repressors of GA signaling. In Arabidopsis, GA de-represses its signaling pathway by inducing proteolysis of the DELLA protein RGA. SLEEPY1 (SLY1) encodes an F-box containing protein, and the loss-of-function sly1 mutant has a GA-insensitive dwarf phenotype and accumulates a high level of RGA. These findings suggested that SLY1 recruits RGA to the SCFSLY1 E3 ligase complex for ubiquitination and subsequent degradation by the 26S proteasome. In this report, we provide new insight into the molecular mechanism of how SLY1 interacts with the DELLA proteins for controlling GA response. We also show that not only RGA, but also GAI (a closely related DELLA protein) which had been reported otherwise, is degraded by SLY1-dependent mechanism. We demonstrate that SLY1 interacts directly with both RGA and GAI via their C-terminal GRAS domain in yeast two-hybrid and in vitro pull-down assays. Moreover, the rga and gai null mutations additively suppressed the recessive sly1 mutant phenotype, further supporting the model that SCFSLY1 targets both RGA and GAI for degradation. The N-terminal DELLA domain of RGA was previously shown to be essential for GA-induced degradation. However, we show that this DELLA domain is not required for protein-protein interaction with SLY1 in yeast, suggesting its role is in a GA-triggered conformational change of the DELLA proteins. We identified a novel gain-of-function sly1-d mutation that increased GA signaling by reducing the levels of the DELLA protein in plants. This effect of sly1-d appears to be caused by an enhanced interaction between sly1-d and the DELLA proteins.