HY5 Regulates GLK and GNC Transcription Factors to Orchestrate Photomorphogenesis in Arabidopsis thaliana

Light induced de-etiolation is an important aspect of seedling photomorphogenesis. GOLDEN2 LIKE (GLK) and GATA NITRATE-INDUCIBLE CARBON-METABOLISM-INVOLVED (GNC) are master transcriptional regulators involved in chloroplast development, but whether they participate in photomorphogenesis is largely unknown. Here we show that ELONGATED HYPOCOTYL5 (HY5) binds to GLK and GNC promoters and activates their expression. Chlorophyll content in the de-etiolating seedlings of the hy5 glk2 and hy5 gnc double mutants was lower than that in the single mutants. It was further found that GLK inhibited hypocotyl elongation, whereas GNC promoted hypocotyl elongation, and that both could superimpose on the hy5 phenotype. Correspondingly, GLK and GNC differentially regulate the expression of cell elongation genes. DE-ETIOLATED 1 (DET1) destabilises GLK proteins. The light-grown traits exhibited by etioplast of det1 mutant were attenuated in det1 glk2 double mutant, while similar to det1 mutant, photosystem genes were up-regulated to different extent in etiolated seedlings overexpressing GLK2 or GNC . Our study reveals that GLK and GNC act downstream of HY5 and likely cooperate with HY5 and DET1 to orchestrate multiple developmental traits during the light-induced skotomorphogenesis to photomorphogenesis transition. One-sentence summary GLK and GNC act downstream of HY5, and cooperate with HY5 and DET1, to regulate both chloroplast development and hypocotyl elongation during the transition from skotomorphogenesis to photomorphogenesis.