Submitted to: Plant Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/30/2001
Publication Date: N/A
Citation: Interpretive Summary: Carotenoids are important for human health and well-being, because they serve as precursors of vitamin A synthesis, and they may have chemoprotective activity. Thus it would be desirable to modify crop carotenoid content to make our foods more nutritious. The ability to do so in an efficient fashion will require a firm understanding of the mechanisms sthat regulate carotenoid synthesis in crops. Unfortunately, to date we simply do not understand how crops regulate this process. We are studying a novel single gene mutant of cauliflower that may shed light on this question, and in turn lead to the identification of strategies for carotenoid modification. This mutant possesses a gene that causes many tissues of the plant such as the curd to turn orange. Thus, it is an excellent potential model for identifying simple strategies for enhancing carotenoid content in crops. In this study, we found that the orange color ris due exclusively to the massive accumulation of the carotenoid beta-carotene. The beta-carotene accumulates in large sheet-like structures. Many tissues exhibit some level of beta-carotene accumulation, including the curd, developing seeds, roots, and the stems. No clear association between the mutant gene and any of the genes that participate in carotenoid synthesis was detected. We speculate that the gene mutant causes beta-carotene accumulation by inducing the formation of a site in the cell in which the beta-carotene can be stored. Importantly, we observed that the beta-carotene can accumulate in cell organelles similar to those found in the seeds of rice and wheat. Thus, if we can isolate this gene, it may have broad applications for increasing carotenoid in these important crops.
Technical Abstract: The Or gene of cauliflower (Brassica oleracea L. var. botrytis) causes many tissues of the plant to accumulate carotenoids and turn orange, which is suggestive of a perturbation of the normal spatial regulation of carotenogenesis. A series of experiments aimed at gaining exploring the cellular basis of the carotenoid accumulation induced by the Or gene was completed. The Or gene causes obvious carotenoid accumulation in weakly o unpigmented tissues such as the curd, pith, leaf bases, and shoot meristems, and cryptically in some cells of other organs, including the roots and developing fruits. The dominant carotenoid is beta-carotene, which accumulates in plastids mainly within large, highly ordered sheets, and can reach levels that are several-hundred fold higher than those in comparable wildtype tissues. In contrast, the carotenoid composition of leaf tissues of Or and wildtype plants was the same, as was the color and chromoplast appearance in flower petals, suggesting that the Or gene only exerts its effect in the absence of signals dictating normal chloroplast or chromoplast development. Interestingly, mRNA for carotenogenic and other isoprenoid biosynthesis genes upstream of the carotenoid pathway was detected not only in orange tissues of the mutant, but also in comparable unpigmented wildtype tissues. Thus, the unpigmented wildtype tissues are likely to be competent to synthesize carotenoids, but this process is suppressed by an as-yet unidentified mechanism. Our results suggest that the Or gene may circumvent this mechanism by inducing the synthesis of a carotenoid deposition sink in the form of the large carotenoid-containing sheets, which provides a driving force for carotenoid synthesis and accumulation.