A bZIP transcription factor, G-box binding factor 1, regulates blue light mediated photomorphogenic growth in Arabidopsis

Several transcriptional regulators have been identified and demonstrated to play either positive or negative regulatory roles in seedling development. However, the regulatory coordination between hypocotyl elongation and cotyledon expansion during early seedling development in plants remains unknown. We report the identification of a Z-box binding factor (ZBF2) and its functional characterization in cryptochrome mediated blue light signaling. ZBF2 encodes a G-box binding factor (GBF1), which is a basic leucine zipper transcription factor. Our DNA-protein interaction studies reveal that ZBF2/GBF1 also interacts with the Z-box light responsive element of light regulated promoters. Genetic analyses of gbf1 mutants and overexpression studies suggest that GBF1 acts as a repressor of blue light mediated inhibition in hypocotyl elongation, however it acts as a positive regulator of cotyledon expansion during photomorphogenic growth. Furthermore, whereas GBF1 acts as a positive regulator of lateral root formation, it differentially regulates the expression of light inducible genes. Taken together, these results demonstrate that GBF1 is a unique transcriptional regulator of photomorphogenesis in blue light