Pathways of intracellular communication: Tetrapyrroles and plastid‐to‐nucleus signaling

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Abstract
Retrograde plastid‐to‐nucleus signaling plays a central role in coordinating nuclear and plastid gene expression. The gun (genomes uncoupled) mutants of Arabidopsis have been used to demonstrate that Mg‐protoporphyrin (Mg‐Proto) acts as a plastid signal to repress the transcription of nuclear photosynthesis genes.1 It is unclear how Mg‐Proto triggers repression, but several components of this pathway have been recently identified. These include the products of GUN4 and GUN5. GUN5 is the ChlH subunit of Mg‐chelatase, which produces Mg‐Proto, and GUN4 is a regulator of ChlH activity.2 GUN4 might also play a role in photoprotection and in the trafficking of Mg‐Proto. BioEssays 25:631–636, 2003.