The Arabidopsis COP9 SIGNALOSOME INTERACTING F-BOX KELCH 1 protein forms an SCF ubiquitin ligase and regulates hypocotyl elongation
Running title:CFK1 controls hypocotyl elongation
We report the isolation and characterization of a new plant-specific, kelch domain-containing F-box protein co-purifying with the COP9 signalosome (CSN) in cauliflower,which we denominated CFK (COP9 INTERACTING F-BOX KELCH). Arabidopsis thalianahas two potential CFK orthologs, CFK1 and -2.CFK1 is a component of a functional SCF ubiquitin ligase complex, and its stability is regulated by the CSN and by proteasome-dependent proteolysis. Light induces accumulation of the CFK1 transcript in the hypocotyl (the embryonic stem). Knocking down the levels of CFK1 and -2 mRNAs simultaneously by double-stranded RNA interference results in defects in light-dependent inhibition of hypocotyl elongation. On the contrary, overexpression of CFK1 causes a constitutive longer hypocotyl, regardless of light conditions, suggesting that CFK1 regulates hypocotyl elongation but not light sensitivity. In addition, CFK1-overexpressing seedlings display an increase in hypocotyl cell size but not in cell number. We further demonstrate that CFK1 acts downstream of gibberellins. Taken together, these results suggest that CFK1 might act as a positive regulator of cell expansion. Indeed, CFK1- overexpressing seedlings display enlarged cotyledons, further supporting this hypothesis.