Carotenoid biosynthesis during tomato fruit development: regulatory role of 1-deoxy-D-xylulose 5-phosphate synthase

Plant isoprenoids represent a heterogeneous group of compounds which play e ssential roles not only in growth and development, but also in the interact ion of plants with their environment. Higher plants contain two pathways fo r the biosynthesis of isoprenoids: the mevalonate pathway, located in the c ytosol/endoplasmic reticulum, and the recently discovered mevalonate-indepe ndent pathway (Rohmer pathway), located in the plastids. In order to evalua te the function of the Rohmer pathway in the regulation of the synthesis of plastidial isoprenoids, we have isolated a tomato cDNA encoding 1-deoxy-D- xylulose 5-phosphate synthase (DXS), the first enzyme of the pathway. We de monstrate in vivo activity and plastid targeting of plant DXS. Expression a nalysis of the tomato DXS gene indicates developmental and organ-specific r egulation of mRNA accumulation and a strong correlation with carotenoid syn thesis during fruit development. 1-Deoxy-D-xylulose feeding experiments, to gether with expression analysis of DXS and PSY1 (encoding the fruit-specifi c isoform of phytoene synthase) in wild-type and yellow flesh mutant fruits , indicate that DXS catalyses the first potentially regulatory step in caro tenoid biosynthesis during early fruit ripening. Our results change the cur rent view that PSY1 is the only regulatory enzyme in tomato fruit carotenog enesis, and point towards a coordinated role of both DXS and PSY1 in the co ntrol of fruit carotenoid synthesis.