Carotenoid metabolism is induced in rice bran during very high temperature stress.

Background: Postharvest yellowing (PhY) causes yellowing in rice endosperm during conditions of high temperature and moisture. Rice bran was investigated as a model tissue for studying PhY and for determining the possibility of carotenoids as the source of the yellow color.

Results: A survey of different colored rice and wild (non-sativa) rice lines resulted in the identification of several purple bran lines having very low or no bran carotenoids. Transcription levels of phytoene synthase, the first committed step in carotenoid biosynthesis, were higher in carotenoid-containing bran, indicating that carotogenesis is an ongoing process in mature bran. Bran and endosperm subjected to PhY conditions had a similar temperature optimum and color responses, so bran was utilized to investigate carotenoid levels and transcription levels of genes for carotogenesis during PhY. During PhY, total carotenoid levels in bran increased while levels of the predominant xanthophyll carotenoid, lutein, decreased. This difference could be explained by carotenoids being metabolized into apocarotenoids, which have high antioxidant activities and can be highly colored. This mechanism is further supported by the long-term transcription during PhY of CCD1, which encodes an enzyme in apocarotenoid production.

Conclusion: We propose that PhY is an active, metabolic response to a very high temperature stress and that the increase in total carotenoids in bran during PhY is a result of the production of colored apocarotenoids.