The Effect of Light on the Biosynthesis and Function of Nadph-Protochlorophyllide Oxidoreductases (PORs) A and B in Seedlings of Arabidopsis Thaliana and Hordeum Vulgare

Abstract

Most oxygenic photosynthetic organisms have two options to complete the synthesis of chlorophyll. In a light-independent reaction the immediate precursor of chlorophyllide, protochlorophyllide (Pchlide), can be reduced to chlorophyllide through a Pchlidereducing enzyme that consists of three subunits Ch L, Ch B and Ch N that in plants are encoded by chloroplast DNA (1). In addition to this light-independent enzyme they also contain a nuclear DNA-encoded light-dependent NADPH-Pchlide oxidoreductase (POR) (2, 3) (Fig. 1). In angiosperms, however, only one of these two routes is active. Through the loss of the light-independent enzyme angiosperms are no longer able to use this reaction during the night to avoid the accumulation of Pchlide, a photosensitizing pigment, that upon illumination may lead to severe photooxidative damage (Fig. 1). In order to compensate for the lack of the light-independent Pchlide-reducing enzyme angiosperms seem to have evolved at least two protection mechanisms that minimize the risk of photooxidative damage during the transition from the dark to the light. First, in contrast to other plants, some angiosperms — and also some of the gymnosperms - possess a second POR enzyme (2–6). It is tempting to speculate that angiosperms use this additional POR enzyme as part of a protection strategy against reactive oxygen species, that in particular during seedling development may lead to the rapid bleaching and death of etiolated seedlings after they have been exposed to light. Second, a feedback inhibitory loop has been described in higher plants through which further synthesis of S-aminolevulinic acid, a precursor common to all porphyrins, will be stopped once a critical level of Pchlide has been reached in the dark (7).