FUNCTIONAL-INTEGRATION OF NONNATIVE CAROTENOIDS INTO CHLOROPLASTS BY VIRAL-DERIVED EXPRESSION OF CAPSANTHIN-CAPSORUBIN SYNTHASE IN NICOTIANA-BENTHAMIANA

The biosynthesis of leaf carotenoids in Nicotiana benthamiana was alte red by forced re-routing of the pathway to the synthesis of capsanthin , a non-native chromoplast-specific xanthophyll, using an RNA viral ve ctor containing capsanthin-capsorubin synthase (Ccs) cDNA. The cDNA en coding Ccs was placed under the transcriptional control of a tobamovir us subgenomic promoter. Leaves from transfected plants expressing Ccs developed an orange phenotype and accumulated high levels of capsanthi n (up to 36% of total carotenoids). This phenomenon was associated wit h thylakoid membrane distortion and reduction of grana stacking. In co ntrast to the situation prevailing in chromoplasts, capsanthin was not esterified and its increased level was balanced by a concomitant decr ease of the major leaf xanthophylls, suggesting an autoregulatory cont rol of chloroplast carotenoid composition. Capsanthin was exclusively recruited into the trimeric and monomeric light-harvesting complexes o f photosystem II (PSII) and shown to significantly contribute to the l ight-harvesting capacity. On a chlorophyll basis, the concentrations o f PSI and PSII reaction centres were not modified. This demonstration that higher plant antenna complexes can accommodate non native caroten oids provides compelling evidence for functional remodelling of photos ynthetic membranes toward a better photoreactivity by rational design of the incorporated carotenoid structures.