Coordination of plastid and nuclear gene expression.

The coordinated expression of genes distributed between the nuclear and plastid genomes is essential for the assembly of functional chloroplasts. Although the nucleus has a pre-eminent role in controlling chloroplast biogenesis, there is considerable evidence that the expression of nuclear genes encoding photosynthesis-related proteins is regulated by signals from plastids. Perturbation of several plastid-located processes, by inhibitors or in mutants, leads to decreased transcription of a set of nuclear photosynthesis-related genes. Characterization of arabidopsis gun (genomes uncoupled) mutants, which express nuclear genes in the presence of norflurazon or lincomycin, has provided evidence for two separate signalling pathways, one involving tetrapyrrole biosynthesis intermediates and the other requiring plastid protein synthesis. In addition, perturbation of photosynthetic electron transfer produces at least two different redox signals, as part of the acclimation to altered light conditions. The recognition of multiple plastid signals requires a reconsideration of the mechanisms of regulation of transcription of nuclear genes encoding photosynthesis-related proteins.