KIDARI, encoding a non-DNA Binding bHLH protein, represses light signal transduction in Arabidopsis thaliana.

Through activation tagging mutagenesis, we isolated a kidari-D (kdr-D) mutant, which exhibited a defect in blue and far-red light mediated photomorphogenesis. Under continuous blue light, the kdr-D mutant showed long hypocotyl phenotype, whereas it showed normal cotyledon opening and expansion. In addition, the kdr-D showed slightly longer hypocotyl under continuous far-red light, suggesting that KDR functions in a branch of cry signaling and mediates a cross-talk between cry and phyA. In the kdr-D mutant, a gene encoding a putative basic/Helix-Loop-Helix (bHLH) protein was overexpressed by the insertion of 35S enhancer into 10 kb upstream of the gene. Consistently, overexpression of this gene recapitulated the phenotype of kdr-D. KDR is composed of 94 amino acids with non-DNA binding HLH domain, a structure found in human Inhibitor of DNA binding 1 (Id-1) which functions as a negative regulator of bHLH proteins through heterodimerization with them. The KDR specifically interacted with HFR1, a bHLH protein regulating photomorphogenesis, in yeast two hybrid assay and the kdr-D was epistatic to 35S::HFR1 in the hypocotyl phenotype. Thus, it shows that KDR functions as a negative regulator of HFR1, similar to Id-1 in human. The KDR exhibited circadian expression pattern with an increase during the day. Taken together, our results suggest that KDR attenuates light mediated responses in day light condition through inhibition of the activity of bHLH proteins involved in light signaling.