Protochlorophyllide reduction and greening in angiosperms: an evolutionary perspective

Synthesis of chlorophyll involves the conversion of a porphyrin-type molecule in the magnesium branch of the tetrapyrrole pathway to a dihydroporphyrin or chlorin. This is achieved in vivo mainly, if not exclusively, by the reduction of protochlorophyllide a to chlorophyllide a. At least two types of enzyme are involved, one which requires light (NADPH-protochlorophyllide oxidoreductase or FOR-type enzyme) and the other which does not (chlL, N, B-type enzyme). Both types have been identified in cyanobacteria, green algae and land plants up to and including gymnosperms. There is, however, no evidence of the former in anoxygenic purple photosynthetic bacteria or of the latter in angiosperms. It has been commonly accepted that angiosperms are incapable of synthesizing chlorophyll in darkness because they have lost the chloroplast-encoded chlL, N, B genes during the course of evolution. This review focuses on the biochemical and genetic attributes of the light-dependent and light-independent reductases and evidence for light-independent chlorophyll synthesis in angiosperms. In it we argue that because angiosperms which are synthesizing chlorophyll in light frequently continue to do so for hours or even days when light is withheld, angiosperms have the capacity for light-independent protochlorophyllide reduction (or some functional equivalent) and a mechanism for it needs to be found.