Comparison of Carotenoid Content, Gene Expression and Enzyme Levels in Tomato (Lycopersicon esculentum) Leaves

Physiological conditions which lead to changes in total carotenoid content in tomatoplantlets were identified. Carotenoid levels were found to increase after the onset of a darkperiod during a normal 24h cycle. This rapid initial increase is followed by a steady decreasein carotenoid content throughout the night. A decrease in the expression of several caroteno-genic genes, namelypds,zds(carotenoid desaturases) andptox(plastid terminal oxidase),was observed following the removal of the light (when carotenoid content is at its highest).An increase in gene expression was observed before the return to light forpdsandzds(whencarotenoid levels were at their lowest), or following the return to light forptox.The phytoenedesaturation inhibitor norflurazon leads to a decrease coloured carotenoid content and, inthe light, this correlated withpdsandzdsgene induction. In the dark, norflurazon treatmentled to only a weak decrease in carotenoid content and only a small increase inpdsandzdsgene expression. The striking absence of phytoene accumulation under norflurazon treatmentin the dark suggests a down-regulation of carotenoid formation in darkness. However, pro-longed dark conditions, or treatment with photosynthetic inhibitors, surprisingly led to highercarotenoid levels, which correlated with decreased expression of most examined genes. Inaddition to light, which acts in a complex way on carotenoid accumulation and gene expres-sion, our results are best explained by a regulatory effect of carotenoid levels on the expres-sion of several biosynthetic genes. In addition, monitoring of protein amounts for phytoenedesaturase and plastid terminal oxidase (which sometimes do not correlate with gene expres-sion) indicate an even more complex regulatory pattern

Dual role of the plastid terminal oxidase in tomato

International audienc

Dual role of the plastid terminal oxidase in tomato

International audienc

Dual Role of the Plastid Terminal Oxidase in Tomato

The plastid terminal oxidase (PTOX) is a plastoquinol oxidase whose absence in tomato (Solanum lycopersicum) results in the ghost (gh) phenotype characterized by variegated leaves (with green and bleached sectors) and by carotenoid-deficient ripe fruit. We show that PTOX deficiency leads to photobleaching in cotyledons exposed to high light primarily as a consequence of reduced ability to synthesize carotenoids in the gh mutant, which is consistent with the known role of PTOX as a phytoene desaturase cofactor. In contrast, when entirely green adult leaves from gh were produced and submitted to photobleaching high light conditions, no evidence for a deficiency in carotenoid biosynthesis was obtained. Rather, consistent evidence indicates that the absence of PTOX renders the tomato leaf photosynthetic apparatus more sensitive to light via a disturbance of the plastoquinone redox status. Although gh fruit are normally bleached (most likely as a consequence of a deficiency in carotenoid biosynthesis at an early developmental stage), green adult fruit could be obtained and submitted to photobleaching high light conditions. Again, our data suggest a role of PTOX in the regulation of photosynthetic electron transport in adult green fruit, rather than a role principally devoted to carotenoid biosynthesis. In contrast, ripening fruit are primarily dependent on PTOX and on plastid integrity for carotenoid desaturation. In summary, our data show a dual role for PTOX. Its activity is necessary for efficient carotenoid desaturation in some organs at some developmental stages, but not all, suggesting the existence of a PTOX-independent pathway for plastoquinol reoxidation in association with phytoene desaturase. As a second role, PTOX is implicated in a chlororespiratory mechanism in green tissues