Arabidopsis FIBRILLIN6 regulates carotenoid biosynthesis by directly promoting phytoene synthase activity

Carotenoids are health-promoting plastidial isoprenoids with essential functions in plants as photoprotectants and photosynthetic pigments in chloroplasts. They also accumulate in specialized plastids named chromoplasts, providing color to non-photosynthetic tissues such as flower petals and ripe fruit. Carotenoid accumulation in chromoplast requires specialized structures and proteins such as fibrillins. Although fibrillins were first reported as structural components of carotenoid sequestering structures in chromoplasts, later work revealed roles in chloroplasts and other plastid types. However, the association of fibrillins with carotenoids in plastids other than chromoplasts has remained unexplored. Here we show that a member of the fibrillin family, FBN6, interacts with phytoene synthase (PSY, the first committed and rate-determining step of the carotenoid pathway) to promote its enzymatic activity. Transient overexpression of FBN6 in Nicotiana benthamiana leaves results in a higher production of phytoene, the product of PSY activity, whereas loss of FBN6 activity in Arabidopsis thaliana mutants dramatically reduces the production of carotenoids during seedling deetiolation and after exposure to high light. Our work hence demonstrates that fibrillins not only promote the accumulation of carotenoids but also their biosynthesis.