The Arabidopsis Mutant sleepy1^gar2-1 Protein Promotes Plant Growth by Increasing the Affinity of the SCF^SLY1 E3 Ubiquitin Ligase for DELLA Protein Substrates

Abstract

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) of an 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. Growth-promoting 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.