GENETIC DISSECTION OF CAROTENOID SYNTHESIS IN ARABIDOPSIS DEFINES PLASTOQUINONE AS AN ESSENTIAL COMPONENT OF PHYTOENE DESATURATION

Carotenoids are C-40 tetraterpenoids synthesized by nuclear-encoded mu ltienzyme complexes located in the plastids of higher plants. To under stand further the components and mechanisms involved in carotenoid syn thesis, we screened Arabidopsis for mutations that disrupt this pathwa y and cause accumulation of biosynthetic intermediates. Here, we repor t the identification and characterization of two nonallelic albino mut ations, pds1 and pds2 (for phytoene desaturation), that are disrupted in phytoene desaturation and as a result accumulate phytoene, the firs t C-40 compound of the pathway, Surprisingly, neither mutation maps to the locus encoding the phytoene desaturase enzyme, indicating that th e products of at least three loci are required for phytoene desaturati on in higher plants, Because phytoene desaturase catalyzes an oxidatio n reaction, it has been suggested that components of an electron trans port chain may be involved in this reaction, Analysis of pds1 and pds2 shows that both mutants are plastoquinone and tocopherol deficient, i n addition to their inability to desaturate phytoene, Separate steps o f the plastoquinone/tocopherol biosynthetic pathway are affected by th ese two mutations, The pds1 mutation affects the enzyme 4-hydroxypheny lpyruvate dioxygenase because it can be rescued by growth on the produ ct but not the substrate of this enzyme, homogentisic acid and 4-hydro xyphenylpyruvate, respectively The pds2 mutation most likely affects t he phenyl/phytyl transferase enzyme of this pathway, Because tocophero l-deficient mutants in the green alga Scenedesmus obliquus can synthes ize carotenoids, our findings demonstrate conclusively that plastoquin one is an essential component in carotenoid synthesis. We propose a mo del for carotenoid synthesis in photosynthetic tissue whereby plastoqu inone acts as an intermediate electron carrier between carotenoid desa turases and the photosynthetic electron transport chain.