Department of Genetic Engineering and Graduate School of Biotechnology, College of Life Sciences, Kyung Hee University, Yongin, 17104, Republic of Korea.
Division of Life Sciences and Bio-Resource and Environmental Center, Incheon National University, Incheon, 22012, Republic of Korea.
BMC Plant Biol. 2020 Apr 15;20(1):167. doi: 10.1186/s12870-020-02357-9.
Deoxyxylulose 5-phosphate synthase (DXS) and deoxyxylulose 5-phosphate reductoisomerase (DXR) are the enzymes that catalyze the first two enzyme steps of the methylerythritol 4-phosphate (MEP) pathway to supply the isoprene building-blocks of carotenoids. Plant DXR and DXS enzymes have been reported to function differently depending on the plant species. In this study, the differential roles of rice DXS and DXR genes in carotenoid metabolism were investigated.
The accumulation of carotenoids in rice seeds co-expressing OsDXS2 and stPAC was largely enhanced by 3.4-fold relative to the stPAC seeds and 315.3-fold relative to non-transgenic (NT) seeds, while the overexpression of each OsDXS2 or OsDXR caused no positive effect on the accumulation of either carotenoids or chlorophylls in leaves and seeds, suggesting that OsDXS2 functions as a rate-limiting enzyme supplying IPP/DMAPPs to seed carotenoid metabolism, but OsDXR doesn't in either leaves or seeds. The expressions of OsDXS1, OsPSY1, OsPSY2, and OsBCH2 genes were upregulated regardless of the reductions of chlorophylls and carotenoids in leaves; however, there was no significant change in the expression of most carotenogenic genes, even though there was a 315.3-fold increase in the amount of carotenoid in rice seeds. These non-proportional expression patterns in leaves and seeds suggest that those metabolic changes of carotenoids were associated with overexpression of the OsDXS2, OsDXR and stPAC transgenes, and the capacities of the intermediate biosynthetic enzymes might be much more important for those metabolic alterations than the transcript levels of intermediate biosynthetic genes are. Taken together, we propose a 'Three Faucets and Cisterns Model' about the relationship among the rate-limiting enzymes OsDXSs, OsPSYs, and OsBCHs as a "Faucet", the biosynthetic capacity of intermediate metabolites as a "Cistern", and the carotenoid accumulations as the content of "Cistern".
Our study suggests that OsDXS2 plays an important role as a rate-limiting enzyme supplying IPP/DMAPPs to the seed-carotenoid accumulation, and rice seed carotenoid metabolism could be largely enhanced without any significant transcriptional alteration of carotenogenic genes. Finally, the "Three Faucets and Cisterns model" presents the extenuating circumstance to elucidate rice seed carotenoid metabolism.
脱氧木酮糖 5-磷酸合酶（DXS）和脱氧木酮糖 5-磷酸还原异构酶（DXR）是催化甲基赤藓醇 4-磷酸（MEP）途径的前两个酶步骤的酶，为类胡萝卜素提供异戊烯基构建块。据报道，植物 DXR 和 DXS 酶根据植物物种的不同而具有不同的功能。在这项研究中，研究了水稻 DXS 和 DXR 基因在类胡萝卜素代谢中的差异作用。
共表达 OsDXS2 和 stPAC 的水稻种子中类胡萝卜素的积累比 stPAC 种子增加了 3.4 倍，比非转基因（NT）种子增加了 315.3 倍，而每个 OsDXS2 或 OsDXR 的过表达对叶片和种子中类胡萝卜素或叶绿素的积累没有产生积极影响，表明 OsDXS2 作为限速酶，为种子类胡萝卜素代谢提供 IPP/DMAPPs，但在叶片或种子中 OsDXR 则不然。无论叶片中叶绿素和类胡萝卜素的减少如何，OsDXS1、OsPSY1、OsPSY2 和 OsBCH2 基因的表达均上调；然而，大多数类胡萝卜素生物合成基因的表达没有明显变化，尽管水稻种子中的类胡萝卜素含量增加了 315.3 倍。这些叶片和种子中不成比例的表达模式表明，这些类胡萝卜素代谢的变化与 OsDXS2、OsDXR 和 stPAC 转基因的过表达有关，而中间生物合成酶的能力对于这些代谢改变可能比中间生物合成基因的转录水平更为重要。综上所述，我们提出了一个关于限速酶 OsDXSs、OsPSYs 和 OsBCHs 之间关系的“三个水龙头和蓄水池模型”，将限速酶比作“水龙头”，将中间代谢物的生物合成能力比作“蓄水池”，将类胡萝卜素积累比作“蓄水池”的含量。
我们的研究表明，OsDXS2 作为限速酶，为种子-类胡萝卜素积累提供 IPP/DMAPPs，起着重要作用，并且在不显著改变类胡萝卜素生物合成基因转录的情况下，可大大增强水稻种子类胡萝卜素代谢。最后，“三个水龙头和蓄水池模型”为阐明水稻种子类胡萝卜素代谢提供了一种缓解情况。
