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The initiation of chloroplast development in the light is dependent on nuclear encoded components. The nuclear genes encoding key components in the photosynthetic machinery are regulated by signals originating in the plastids. These plastid signals play an essential role in the regulation of photosynthesis associated nuclear genes ( PhANGs ) when proplastids develop into chloroplasts. One of the plastid signals is linked to the tetrapyrrole biosynthesis and accumulation of the intermediates the Mg-ProtoIX and its methyl ester Mg-ProtoIX-ME. Phytochrome-Associated Protein Phosphatase 5 (PAPP5) was isolated in a previous study as a putative Mg-ProtoIX interacting protein. In order to elucidate if there is a biological link between PAPP5 and the tetrapyrrole mediated signal we generated double mutants between the Arabidopsis  papp5  and the  crd  mutants. The  crd  mutant over-accumulates Mg-ProtoIX and Mg-ProtoIX-ME and the tetrapyrrole accumulation triggers retrograde signalling. The  crd  mutant exhibits repression of  PhANG  expression, altered chloroplast morphology and a pale phenotype. However, in the  papp5crd  double mutant, the  crd  phenotype is restored and  papp5crd  accumulated wild type levels of chlorophyll, developed proper chloroplasts and showed normal induction of  PhANG  expression in response to light. Tetrapyrrole feeding experiments showed that PAPP5 is required to respond correctly to accumulation of tetrapyrroles in the cell and that PAPP5 is most likely a component in the plastid signalling pathway down stream of the tetrapyrrole Mg-ProtoIX/Mg-ProtoIX-ME. Inhibition of phosphatase activity phenocopied the  papp5crd  phenotype in the  crd  single mutant demonstrating that PAPP5 phosphatase activity is essential to mediate the retrograde signal and to suppress  PhANG  expression in the  crd  mutant. Thus, our results suggest that PAPP5 receives an inbalance in the tetrapyrrole biosynthesis through the accumulation of Mg-ProtoIX and acts as a negative regulator of  PhANG  expression during chloroplast biogenesis and development.

PAPP5 Is Involved in the Tetrapyrrole Mediated Plastid Signalling during Chloroplast Development