LIGHT-DEPENDENT REGULATION OF CAROTENOID BIOSYNTHESIS OCCURS AT THE LEVEL OF PHYTOENE SYNTHASE EXPRESSION AND IS MEDIATED BY PHYTOCHROME INSINAPIS-ALBA AND ARABIDOPSIS-THALIANA SEEDLINGS

In chloroplasts, carotenoids are essential pigments involved in photos ynthesis. During photomorphogenesis, a coordinated increase in the amo unts of chlorophylls and carotenoids, in conjugation with other compon ents, leads to the formation of a functional photosynthetic apparatus. To investigate the regulation of carotenoid biosynthesis during this process at the molecular level, GGPS, PSY and PDS cDNAs have been clon ed from white mustard (Sinapis alba L.). GGPS encodes a key enzyme in plastid isoprenoid metabolism, while the products of PSY and PDS catal yse the subsequent steps in carotenoid biosynthesis. Due to the low mR NA levels of the genes involved, the use of a RT-PCR protocol was nece ssary to measure gene expression during photomorphogenesis. With light , there is an upregulation of PSY expression, the first gene within th e carotenoid biosynthetic pathway, while PDS and GGPS expression level s remain constant. Treatment with different light qualities reveals a phytochrome-mediated regulation of PSY expression in developing white mustard seedlings. To obtain more detailed information on the light-re gulation, Arabidopsis thaliana wild-type and phytochrome mutants were utilized. Continuous far-red and red light both increase the expressio n of PSY in wild-type seedlings, demonstrating that both light-labile and light-stable phytochromes are involved in PSY regulation. The resp onse to far-red light is completely abolished in the phyA mutant, show ing that PHYA mediates the increase in PSY transcript levels under the se light conditions. In the phyB mutant, the red light response is nor mal, indicating that PSY expression is not controlled by PHYB but by o ther light-stable phytochromes. Measurement of chlorophylls and carote noids under the same light regimes shows that the up-regulation of PSY expression does not necessarily result in an increase of the caroteno id content. Only those light conditions which allow chlorophyll biosyn thesis lead to a significant increase of the carotenoid content. There fore, it is proposed that up-regulation of PSY mRNA levels leads to an increased capacity for the formation of carotenoids. However, this on ly takes place under light conditions leading to protochlorophyllide p hotoconversion.