Regulation and activation of phytoene synthase, a key enzyme in carotenoidbiosynthesis, during photomorphogenesis

During photomorphogenesis in higher plants, a coordinated increase occurs i n the chlorophyll and carotenoid contents. The carotenoid level is tinder p hytochrome control, as reflected by the light regulation of the mRNA level of phytoene synthase (PSY), the first enzyme in the carotenoid biosynthetic pathway. We investigated PSY protein levels, enzymatic activity and topolo gical localization during photomorphogenesis. The results revealed that PSY protein levels and enzymatic activity increase during de-etiolation and th at the enzyme is localized at thylakoid membranes in mature chloroplasts. H owever, under certain light conditions (e.g., far-rid light) the increases in PSY mRNA and protein levels are not accompanied by an increase in enzyma tic activity. Under those conditions, PSY is localized in the prolamellar b ody fraction in a mostly enzymatically inactive form. Subsequent illuminati on of dark-gl-own and/or in far-red light grown seedlings with white light causes the decay of these structures and a topological relocalization of PS Y to developing thylakoids which results in its enzymatic activation. This light-dependent mechanism of enzymatic activation of PSY in carotenoid bios ynthesis shares common features with the regulation of the NADPH:protochlor ophyllide oxidoreductase, the first light-regulated enzyme in chlorophyll b iosynthesis. The mechanism of regulation described here may contribute to e nsuring a spatially and temporally coordinated increase in both carotenoid and chlorophyll contents.