Chloroplast development requires coordinate nuclear and chloroplast gene expression. A putative signal from the chloroplast couples the transcription of certain nuclear genes encoding photosynthesis-related proteins with chloroplast function. We have identified at least three Arabidopsis nuclear genes (GUN1, GUN2, and GUN3) necessary for coupling the expression of some nuclear genes to the functional state of the chloroplast. Homozygous recessive gun mutations allow nuclear gene expression in the absence of chloroplast development and furthermore may interfere with the switch from dark-grown to light-grown development. Other reports suggest this intracellular cross-talk also involves mitochondrial interactions. The GUN genes thus define steps in one specific branch of a complex interorganellar regulatory network.