Research Molecular Biologist/Adjunct Associate Professor Agricultural Research Service Ithaca, New York
Body of Abstract: Chromoplasts are the plant organelle with a unique ability to sequester and store massive carotenoids. It has long been hypothesized that chromoplasts enable high levels of carotenoid accumulation due to enhanced sequestration ability or sequestration substructure formation. However, the direct evidence is still lacking. In melon (Cucumis melo) fruit, β-carotene accumulation in chromoplasts is governed by CmOr, a key regulator for carotenoid accumulation in chromoplasts. By using comparative proteomic analysis of a high β-carotene melon variety and its isogenic line low-β mutant that is defective in CmOr with impaired chromoplast formation, we identified a carotenoid sequestration protein fibrillin (CmFBN1) as a differentially expressed protein (CmFBN1). CmFBN1 expresses highly in melon fruit. Overexpression of CmFBN1 in a genetic background mimicking CmOr significantly enhances carotenoid accumulation, demonstrating its involvement in the CmOr-induced carotenoid accumulation. Both in vitro and in vivo evidence showed that CmOR physically interacts with CmFBN1. Such an interaction occurs in plastoglobules and results in promoting CmFBN1 accumulation. The enhanced CmFBN1 accumulation stimulates plastoglobule proliferation for sequestration substructure formation, leading to increased carotenoid accumulation in chromoplasts. This study reveals a novel mechanism underlying carotenoid accumulation in chromoplasts, which is greatly important as carotenoid accumulation in chromoplasts enables to not only enhance carotenoid levels but also stably store the synthesized carotenoids, an important issue in carotenoid biofortification in crops.