Carotenoid synthesis and function in plants: Insights from mutant studies in Arabidopsis*

From a molecular and genetic perspective, the decade of the 1990s was truly unparalleled in the study of carotenoids. A combination of new technologies and approaches allowed the isolation of bacterial carotenoid biosynthetic genes and the subsequent isolation of higher plant homologs based on colour complementation in Escherichia coli. These genes provided a basis for molecular and transgenic studies of the carotenoid pathway in various eukaryotic organisms which, although not detailed in this article, critically and synergistically impacted the work described. For in-depth information of plant carotenoid molecular biology, readers are directed to a recent comprehensive review [F.X. Cunningham, E. Gantt. Annu. Rev. Plant Physiol. Plant Mol. Biol. 49, 557±583 (1998)]. The primary focus on this article is to review the identi®cation and characterization of novel mutations in Arabidopsis, de®ning genes essential for xanthophyll synthesis in photosynthetic plant tissues. These mutations, both singly and in combination, allow dramatic alterations in the carotenoid composition of Arabidopsis photosystems in vivo, in the most extreme cases eliminating the synthesis of all wild-type xanthophylls (lutein, violaxanthin and neoxanthin). This work demonstrates a surprising plasticity in the carotenoid compositions that plant photosystems will accept in vivo and provides an insight into the functional role(s) of individual carotenoids in the photosystems.