ARABIDOPSIS CAROTENOID MUTANTS DEMONSTRATE THAT LUTEIN IS NOT ESSENTIAL FOR PHOTOSYNTHESIS IN HIGHER-PLANTS

Lutein, a dihydroxy beta,epsilon-carotenoid, is the predominant carote noid in photosynthetic plant tissue and plays a critical role in light -harvesting complex assembly and function. To further understand lutei n synthesis and function, we isolated four lutein-deficient mutants of Arabidopsis that define two loci, lut1 and lut2 (for lutein deficient ). These loci are required for lutein biosynthesis but not for the bio synthesis of beta,beta-carotenoids. The lut1 mutations are recessive, accumulate high levels of zeinoxanthin, which is the immediate precurs or of lutein, and define lut1 as a disruption in epsilon ring hydroxyl ation. The lut2 mutations are semidominant, and their biochemical phen otype is consistent with a disruption of epsilon ring cyclization, The lut2 locus cosegregates with the recently isolated epsilon cyclase ge ne, thus providing additional evidence that the lut2 alleles are mutat ions in the epsilon cyclase gene. It appears likely that the epsilon c yclase is a key step in regulating lutein levels and the ratio of lute in to beta,beta-carotenoids. Surprisingly, despite the absence of lute in, neither the lut1 nor lut2 mutation causes a visible deleterious ph enotype or altered chlorophyll content, but both mutants have signific antly higher levels of beta,beta-carotenoids. In particular, there is a stable increase in the xanthophyll cycle pigments (violaxanthin, ant heraxan-thin, and zeaxanthin) in both lut1 and lut2 mutants as well as an increase in zeinoxanthin in lut1 and beta-carotene in lut2. The ac cumulation of specific carotenoids is discussed as it pertains to the regulation of carotenoid biosynthesis and incorporation into the photo synthetic apparatus. Presumably, particular beta,beta-carotenoids are able to compensate functionally and structurally for lutein in the pho tosystems of Arabidopsis.