SALT TOLERANCE HOMOLOG2, a B-Box Protein in Arabidopsis That Activates Transcription and Positively Regulates Light-Mediated Development

CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) and ELONGATED HYPOCOTYL5 (HY5) are two major regulators of light signaling in plants. Here, we identify SALT TOLERANCE HOMOLOG2 ( STH2 ) as a gene that interacts genetically with both of these key regulators. STH2 encodes a B-box–containing protein that interacts physically with HY5 in yeast and in plant cells. Whereas STH2 is uniformly nuclear by itself, it shows a COP1-dependent localization to speckles when coexpressed with COP1. We identified two independent T-DNA insertion lines in STH2 . Both alleles are hyposensitive to blue, red, and far-red light. The sth2 mutant, like hy5 , shows an enhanced number of lateral roots and accumulates less anthocyanin. Analysis of double mutants between sth2 and hy5 indicates that STH2 has both HY5 -dependent and -independent functions. Furthermore, besides partially suppressing the hypocotyl phenotype of dark-grown cop1 alleles, sth2 also suppresses the reduced number of lateral roots and high anthocyanin levels in light-grown cop1 alleles. Interestingly, we found that STH2 can activate transcription. Transient transfection assays in protoplasts using a LUC reporter driven by the chalcone isomerase promoter show that the B-boxes in STH2 and a functional G-box element in the promoter are required for this activity. In conclusion, we have identified STH2, a B-box protein in Arabidopsis thaliana , as a positive regulator of photomorphogenesis and report that the B-box domain plays a direct role in activating transcription in plants.