PROTOCHLOROPHYLLIDE REDUCTION AND GREENING IN ANGIOSPERMS - AN EVOLUTIONARY PERSPECTIVE

Synthesis of chlorophyll involves the conversion of a porphyrin-type m olecule in the magnesium branch of the tetrapyrrole pathway to a dihyd roporphyrin or chlorin. This is achieved in vivo mainly, if not exclus ively, by the reduction of protochlorophyllide a to chlorophyllide a. At least two types of enzyme are involved, one which requires light (N ADPH-protochlorophyllide oxidoreductase or POR-type enzyme) and the ot her which does not (chlL, N, B-type enzyme). Both types have been iden tified in cyanobacteria, green algae and land plants up to and includi ng gymnosperms. There is, however, no evidence of the former in anoxyg enic purple photosynthetic bacteria or of the latter in angiosperms. I t has been commonly accepted that angiosperms are incapable of synthes izing chlorophyll in darkness because they have lost the chloroplast-e ncoded chlL, N, B genes during the course of evolution. This review fo cuses on the biochemical and genetic attributes of the light-dependent and light-independent reductases and evidence for light-independent c hlorophyll synthesis in angiosperms. In it we argue that because angio sperms which are synthesizing chlorophyll in light frequently continue to do so for hours or even days when light is withheld, angiosperms h ave the capacity for light-independent protochlorophyllide reduction ( or some functional equivalent) and a mechanism for it needs to be foun d. (C) 1997 Elsevier Science S.A.