Carotenoid synthesis in mustard seedlings as controlled by phytochrome and inhibitors

Accumulation of carotenoids in the mustard seedling (Sinapis alba L.) is controlled by phytochrome (Pfr). Separation of the carotenoids shows that the control is quantitative rather than qualitative. Kinetic studies indicate that Pfr exerts a rapid and nearly reversible control over the rate of carotenoid accumulation. Whereas carotenoid accumulation between 36 and 60h after sowing is relatively insensitive towards Actinomycin D, the sensitivity towards cycloheximide and Puromycin is high. It is concluded that at least some of the enzymes required for carotenoid biosynthesis are made in the extraplastidal cytoplasm and it is suggested that Pfr acts at the level of carotenoid accumulation by providing a structural prerequisite for carotenoid accumulation in the plastid compartment. This latter suggestion is mainly based on the fact that carotenoid accumulation in light and dark is very sensitive towards chloramphenicol if the compound is applied at the time of sowing. If, however, the compound is applied 36 h after sowing, the effect of chloramphenicol is different in light and dark. In the dark there is no influence up to 200 μg·ml(-1), whereas in the light there is a significant inhibition of carotenoid accumulation.