Arabidopsis genomes uncoupled 5 (GUN5) mutant reveals the involvement of Mg-chelatase H subunit in plastid-to-nucleus signal transduction

A plastid-derived signal plays an important role in the coordinated express ion of both nuclear- and chloroplast-localized genes that encode photosynth esis-related proteins. Arabidopsis GUN (genomes uncoupled) loci have been i dentified as components of plastid-to-nucleus signal transduction, Unlike w ild-type plants. gun mutants have nuclear Lhcb1 expression in the absence o f chloroplast development. We observed a synergistic phenotype in some gun double-mutant combinations, suggesting there are at least two independent p athways in plastid-to-nucleus signal transduction. There is a reduction of chlorophyll accumulation in gun4 and gun5 muta nt plants, and a gun4gun5 do uble muta nt shows an albino phenotype, We cloned the GUNS gene, which enco des the ChlH subunit of Mg-chelatase. We also show that gun2 and gun3 are a lleles of the known photomorphogenic mutants, hy1 and hy2, which are requir ed for phytochromobilin synthesis from heme. These findings suggest that ce rtain perturbations of the tetrapyrrole biosynthetic pathway generate a sig nal from chloroplasts that causes transcriptional repression of nuclear gen es encoding plastid-localized proteins. The comparison of mutant phenotypes of gun5 and another Mg-chelatase subunit (ChlI) mutant suggests a specific : function for ChlH protein in the plastid-signaling pathway.