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.