Reciprocal degradation of PIF3 and HY5 in phytochrome-mediated light signaling

Abstract

Phytochromes regulate various plant developmental processes through many signaling components, including phytochrome-interacting proteins. The functional relationships among these signaling components, however, have not been clearly established. Here, I sought to identify a functional relationship between PIF3, a phytochrome-interacting bHLH protein, and HY5, a COP1-interacting bZIP protein, which have previously been shown to antagonistically regulate hypocoty1 elongation. Analysis of a pif3/hy5 double mutant indicated that these two components regulate hypocoty1 elongation independently. Furthermore, I showed that phytochromes act on these two factors in opposing manners: light-stimulated phytochromes increase degradation of PIF3 by the 26S proteasome, and decrease degradation of HY5. Taken together, my data suggest that the reciprocal light-dependent, PHY-mediated degradation of PIF3 and HY5 and their binding to the same G-box elements form the basis for their antagonistic regulation of hypocoty1 elongation.