Beta-carotene adds nutritious value and determines the color of many fruits including melon. In melon mesocarp, β-carotene accumulation is governed by the Orange gene (CmOr) ‘golden’ 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-lowβ), which lowered fruit β-carotene levels with impaired chromoplast biogenesis. Cmor-lowβ 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 ‘golden’ 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 β-carotene accumulation in ‘golden’ SNP melons was found to be due to a reduced further metabolism of β-carotene. This was revealed by genetic studies with double mutants including crtiso (‘yofi’), a carotenoid-isomerase nonsense mutant, which arrests the turnover of prolycopene. The ‘yofi’ F2 segregants accumulated prolycopene independently of the ‘golden’ SNP. Moreover, Cmor-lowβ 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 ‘golden’ SNP involves an inhibited metabolism downstream to β-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.
Chayut, Noam, Hui Yuan, Shachar Ohali, Ayala Meir, Uzi Sa’ar, 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. “Distinct Mechanisms of the ORANGE Protein in Controlling Carotenoid Flux.” Plant Physiology 173 (1): 376-389. https://doi.org/10.1104/pp.16.01256.
Distinct Mechanisms of the ORANGE Protein in Controlling Carotenoid Flux