Biosynthesis of acyclic and cyclic carotenoids in higher plants and the control by phytochrome

Radish plants ( Raphanus sativus L. cv. Saxa treib) were grown in the presence of three different herbicides interfering with the biosynthesis of cyclic carotenoids. The herbicides caused an accumulation of acyclic biosynthetic intermediates. Plants were then irradiated using four different light programs in order to gain more insight into the first steps of carotenoid biosynthesis and their control by light and phytochrome. Plants grown in the dark in the presence of SAN 6706 or aminotriazole accumulated the acyclic intermediate phytoene, and those treated with J 852, the intermediates phytoene, phytofluene and zeta-carotene. In herbicide-treated plants short time irradiation with red light enhanced the formation of phytoene, phytofluene, zeta-carotene or lycopene, consistent with an effect of phytochrome on the early steps of carotenoid biosynthesis. Biosynthesis of cyclic carotenoids was also enhanced by red light in the untreated controls. In amitrole-treated plants formation of β-carotene, but not that of xanthophylls was stimulated by red light. In many cases neither the red light-induced biosynthesis of cyclic carotenoids nor the formation of acyclic intermediates could be prevented by a subsequent irradiation with far-red light. Similar enhancement as with red light was also obtained after treatment with far-red light only. Presented data may be taken as evidence that the biosynthesis and dehydrogenation of phytoene and the cyclization of lycopene are activated by a low threshold of active phytochrome. This may be further supported by the observation that far-red light itself stimulated carotenoid biosynthesis.