IDENTIFICATION OF NADPH-PROTOCHLOROPHYLLIDE OXIDOREDUCTASE-A AND OXIDOREDUCTASE-B - A BRANCHED PATHWAY FOR LIGHT-DEPENDENT CHLOROPHYLL BIOSYNTHESIS IN ARABIDOPSIS-THALIANA

Illumination releases the arrest in chlorophyll (Chi) biosynthesis in etiolated angiosperm seedlings through the enzymatic photoreduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide), the first light-dependent step in chloroplast biogenesis. NADPH: Pchlide oxidoreductase (FOR, EC 1.3.1.33), a nuclear-encoded plastid-localized enzyme, mediates this unique photoreduction. Paradoxically, light also triggers a drastic decrease in the amounts of FOR activity and protein before the Chl accumulation rate reaches its maximum during greening. While investigating this seeming contradiction, we identified two distinct Arabidopsis thaliana genes encoding FOR, in contrast to previous reports of only one gene in angiosperms. The genes, designated PorA and PorB, by analogy to the principal members of the phytochrome photoreceptor gene family, display dramatically different patterns of light and developmental regulation. PorA mRNA disappears within the first 4 h of greening, whereas PorB mRNA persists even after 16 h of illumination, mirroring the behavior of two distinct FOR protein species. Experiments designed to help define the functions of FOR A and FOR B demonstrate exclusive expression of PorA in young seedlings and of PorB both in seedlings and in adult plants. Accordingly, we propose the existence of a branched light-dependent Chl biosynthesis pathway in which FOR A performs a specialized function restricted to the initial stages of greening and FOR B maintains Chl levels throughout angiosperm development.