Distinct Mechanisms of the ORANGE Protein in Controlling Carotenoid Flux
Beta-carotene adds nutritious value and determines the color of many fruits including melon
Abstract
Beta-carotene adds nutritious value and determines the color of many fruits including melon. In melon mesocarp, 尾-carotene accumulation is governed by the Orange gene (CmOr) 鈥榞olden鈥� SNP through a yet to be discovered mechanism. In Arabidopsis, OR increases carotenoid levels by posttranscriptionally regulating phytoene synthase (PSY).
Here we identified a CmOr nonsense mutation (Cmor-low尾), which lowered fruit 尾-carotene levels with impaired chromoplast biogenesis. Cmor-low尾 exerted a minimal effect on PSY transcripts but dramatically decreased PSY protein levels and enzymatic activity, leading to reduced carotenoid metabolic flux and accumulation. However, the 鈥榞olden鈥� SNP was discovered to not affect PSY protein levels and carotenoid metabolic flux in melon fruit, as shown by carotenoid and immunoblotting analyses of selected melon genotypes and by using chemical pathway inhibitors. The high 尾-carotene accumulation in 鈥榞olden鈥� SNP melons was found to be due to a reduced further metabolism of 尾-carotene. This was revealed by genetic studies with double mutants including crtiso (鈥榶ofi鈥�), a carotenoid-isomerase nonsense mutant, which arrests the turnover of prolycopene. The 鈥榶ofi鈥� F2 segregants accumulated prolycopene independently of the 鈥榞olden鈥� SNP. Moreover, Cmor-low尾 was found to inhibit chromoplast formation and chloroplast disintegration in fruits from 30 days after anthesis until ripening, suggesting that CmOr regulates chloroplast-to-chromoplast transition. Taken together, our results demonstrate that CmOr is required to achieve PSY protein levels for maintaining carotenoid biosynthesis metabolic flux, but that the mechanism of the CmOr 鈥榞olden鈥� SNP involves an inhibited metabolism downstream to 尾-carotene to dramatically affect both carotenoid content and plastid fate.
This work is an output of the HarvestPlus Programme. The Department for International Development is one of the main donors for HarvestPlus.
Citation
Chayut, Noam, Hui Yuan, Shachar Ohali, Ayala Meir, Uzi Sa鈥檃r, Galil Tzuri, Yi Zheng, Michael Mazourek, Shimon Gepstein, Xiangjun Zhou, Vitaly Portnoy, Efraim Lewinsohn, Arthur A. Schaffer, Nurit Katzir, Zhangjun Fei, Ralf Welsch, Li Li, Joseph Burger, and Yaakov Tadmor. 2017. 鈥淒istinct Mechanisms of the ORANGE Protein in Controlling Carotenoid Flux.鈥� Plant Physiology 173 (1): 376-389. https://doi.org/10.1104/pp.16.01256.