Chloroplast development requires the coordinated expression of nuclear and chloroplastic genes. A hypothesized signal from the chloroplast couples the transcription of certain nuclear genes encoding photosynthetic proteins with chloroplast function. We have previously described an Arabidopsis thaliana mutant, gun1, which has a defect in the signal transduction pathway coupling such nuclear and plastidic gene expression. Here we show that gun1 seedlings are also defective in establishing photoautotrophic growth. gun1 seedlings develop normally in the dark, but, based on morphological criteria and the kinetics of chlorophyll accumulation, photosynthetic mRNA accumulation, and the differentiation of etioplasts to chloroplasts, are retarded in their ability to de-etiolate. Therefore, we propose that the GUN1 gene plays an important role in the transition from heterotrophic to photoautotrophic growth, suggesting an important physiological role for the plastid-nucleus signaling pathway during chloroplast biogenesis.