A novel gene mutation that confers abnormal patterns of beta-carotene accumulation in cauliflower (Brassica oleracea var. botrytis)

The Or gene of cauliflower (Brassica oleracea var. botrytis) causes many ti ssues of the plant to accumulate carotenoids and turn orange, which is sugg estive of a perturbation of the normal regulation of carotenogenesis. A ser ies of experiments to explore the cellular basis of the carotenoid accumula tion induced by the Or gene was completed. The Or gene causes obvious carot enoid accumulation in weakly or unpigmented tissues such as the curd, pith, leaf bases and shoot meristems, and cryptically in some cells of other org ans, including the roots and developing fruits. The dominant carotenoid acc umulated is beta -carotene, which can reach levels that are several hundred -fold higher than those in comparable wild-type tissues. The beta -carotene accumulates in plastids mainly as a component of massive, highly ordered s heets. The Or gene does not affect carotenoid composition of leaves, nor do es it alter color and chromoplast appearance in flower petals. Interestingl y, mRNA from carotenogenic and other isoprenoid biosynthetic genes upstream of the carotenoid pathway was detected both in orange tissues of the mutan t, and in comparable unpigmented wild-type tissues. Thus the unpigmented wi ld-type tissues are likely to be competent to synthesize carotenoids, but t his process is suppressed by an unidentified mechanism. Our results suggest that the Or gene may induce carotenoid accumulation by initiating the synt hesis of a carotenoid deposition sink in the form of the large carotenoid-s equestering sheets.