The role of tetrapyrroles in chloroplast-to-nucleus retrograde signaling

Chloroplasts contain their own genomes and therefore chloroplast biogenesis requires the coordination of both chloroplast and nuclear gene expression. This is achieved by the exchange of signals between the two organelles. The existence of signals from the chloroplast (chloroplast-to-nucleus retrograde signaling) can be demonstrated by inhibition of chloroplast development through mutation or chemical treatment. This chloroplast damage results in the reduced expression of hundreds of nuclear-encoded genes, including many encoding chloroplast proteins. A classic mutant screen in which nuclear gene expression was retained after chloroplast damage resulted in the isolation of a series of genomes uncoupled or gun mutants. In five out of six mutants, the mutations resided in components of the tetrapyrrole biosynthesis pathway, resulting in a number of different models for the role of tetrapyrroles as retrograde signals. The current model is that a positive retrograde signal is generated by the activity of the ferrochelatase 1 enzyme suggesting that haem or a product of haem is a signal. The evidence for such a model and the interaction of tetrapyrrole signals with other possible retrograde signals is discussed. In addition, tetrapyrroles can generate singlet oxygen on exposure to light and oxygen and there is accumulating evidence that a tetrapyrrole-derived, singlet oxygen-dependent retrograde signal is important during chloroplast biogenesis and for stress signaling from mature chloroplasts.