DISTINCT ROLES FOR LIGHT-DEPENDENT NADPH-PROTOCHLOROPHYLLIDE OXIDOREDUCTASES (POR) A AND B DURING GREENING IN HIGHER-PLANTS

Light-dependent NADPH:protochlorophyllide oxidoreductase (POR), a nucl ear-encoded plastid-localized enzyme, catalyzes the photoreduction of protochlorophyllide (Pchlide) to chlorophyllide in higher plants, alga e and cyanobacteria. Angiosperms require light for chlorophyll (Chl) b iosynthesis and have recently been shown to contain two POR-encoding g enes, PorA and PorB,that are differentially regulated by light and dev elopmental state. PorA expression rapidly becomes undetectable after i llumination of etiolated seedlings, whereas PorB expression persists t hroughout greening and in adult plants. In order to study the in vivo functions of Arabidopsis POR A and POR B we have abolished the express ion of PorA through the use of the phytochrome A-mediated far-red high irradiance response. Wild-type seedlings grown in continuous far-red light (cFR) display the morphology of white light (WL)-grown seedlings , but contain only traces of Chl and do not green upon transfer to WL. This cFR-induced greening defect correlates with the absence of PorA mRNA, the putative POR A protein, phototransformable Pchlide-F-655, an d with strongly reduced POR enzymatic activity in plant extracts. In c ontrast, a cFR-grown phyA mutant expresses the PorA gene, accumulates Chl and visibly greens in WL. Furthermore, constitutive overexpression of POR A in cFR-grown transgenic Arabidopsis wild-type seedlings rest ores Chl accumulation and WL-induced greening. These data demonstrate that POR A is required for greening and that the availability of POR A limits Chl accumulation during growth in cFR. POR B apparently provid es a means to sustain light-dependent Chl biosynthesis in fully greene d, mature plants in the absence of phototransformable Pchlide-F-655.