Chloroplast biogenesis 80. Proposal of a unified multibranched chlorophyll a/b biosynthetic pathway

Abstract

A unified multibranched chlorophyll (Chl) biosynthetic pathway is proposed. The proposed pathway takes into account the following considerations: (a) that the earliest putative precursor of monovinyl Chl b that has been detected in higher plants is monovinyl protochlorophyllide b, (b) that in most cases, Chl b biosynthesis has its roots in the Chl a biosynthetic pathway, (c) that the Chl a biosynthetic pathway exhibits extensive biosynthetic heterogeneity, (d) that Chl biosynthesis may proceed differently at different stages of greening and in different greening groups of plants. Integration of the Chl a and b biosynthetic pathways into a unified multibranched pathway offers the functional flexibility to account for the structural and biosynthetic complexity of photosynthetic membranes. In this context, it is proposed that the unified, multibranched Chl a/b biosynthetic pathway represents the template of a Chl-protein biosynthesis center where photosystem (PS) 1, PS2, and light-harvesting Chl-protein complexes are assembled into functional photosynthetic units. The individual biosynthetic routes or groups of two to three adjacent biosynthetic routes may constitute Chl-protein biosynthesis subcenters, where specific Chl-protein complexes are assembled.