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MOLECULAR AND GENETIC ANALYSIS OF A RETROGRADE SIGNALING PATHWAY FROM THE PLASTID TO THE NUCLEUS

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 five Arabidopsis nuclear genes (GUN1‐GUN5) 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. GUN1 mutants have no visible phenotype in white light, but GUN1 mutations interfere with the switch from heterotrophic to photosynthetic growth in young seedlings. GUN2‐5 are pale. Double mutant studies suggest that gun1 affects a separate pathway from GUN2, 3, 4, and 5. GUN2 and GUN3 are allelic to the known photomorphogenetic mutants, hy1 and hy2, involved in phytochromobilin biosynthesis downstream from heme. GUN5 encodes the ChlH subunit of Mg‐chelatase, and GUN4 encodes a novel chloroplast protein that is essential for chlorophyll accumulation. GUN4 does not appear to be required for the synthesis of protochlorophyllide from 5‐aminolevulinic acid, which suggests that GUN4 might be required for the early steps and/or the late steps of chlorophyll synthesis or another process that is required for chlorophyll accumulation. Our data suggest that certain perturbations of the tetrapyrrole biosynthetic pathway generate a signal from chloroplasts that causes transcriptional repression of nuclear genes encoding plastid‐localized proteins. The precise nature of this signal and the mechanism by which this signal is transduced to the nucleus is under investigation.