DELLA proteins restrain the cell proliferation and enlargement that characterizes the growth of plant organs. Gibberellin stimulates growth via 26S proteasome–dependent destruction of DELLAs, thus relieving DELLA-mediated growth restraint. Here, we show that the Arabidopsis thaliana sleepy1 gar2-1 (sly1 gar2-1) mutant allele encodes a mutant subunit (sly1 gar2-1)ofan SCF SLY1 E3 ubiquitin ligase complex. SLY1 (the wild-type form) and sly1 gar2-1 both confer substrate specificity on this complex via specific binding to the DELLA proteins. However, sly1 gar2-1 interacts more strongly with the DELLA target than does SLY1. In addition, the strength of the SCF SLY1 –DELLA interaction is increased by target phosphorylation. Growthpromoting DELLA destruction is dependent on SLY1 availability, on the strength of the interaction between SLY1 and the DELLA target, and on promotion of the SCF SLY1 –DELLA interaction by DELLA phosphorylation
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