Deciphering Carotenoid Accumulation and Chromoplast Biogenesis via Expression of the or Transgene

Carotenoids are indispensable to plants and play a critical role in human nutrition and health. A comprehensive understanding of carotenoid metabolism is critical for development of carotenoid-enriched food crops. We have cloned a bona fide molecular switch, the Or gene, that triggers the formation of carotenoid storage sink of chromoplasts. Expression of the Or transgene in cauliflower confirms that Or mediates carotenoid accumulation and chromoplast formation. The Or transgene also confers carotenoid accumulation in transgenic potato tubers. Moreover, we found that the Or transgene promotes retention and continuous biosynthesis of b-carotene during long term of storage. Such increased accumulation was found to be associated with the formation of lipoprotein-carotenoid sequestering structures as well as with enhanced phytoene synthase protein stability. Examination of the Or transgenic crops suggests that sequestration of synthesized carotenoids within chromoplasts may offer plant a unique capacity to synthesize and stabilize carotenoids during crop growth and post-harvest storage. To establish a system to examine chromoplast biogenesis, the Or gene was transformed into Arabidopsis and the Arabidopsis calli exhibit orange color with chromoplast formation. This Arabidopsis Or transgenic callus system offers an effective and relatively high throughput way to functionally analyze a large number of candidate genes for their involvement in chromoplast development and carotenoid metabolism.