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Background  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.    Results  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”.    Conclusion  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.

bmc plant biology

The organ-specific differential roles of rice DXS and DXR, the first two enzymes of the MEP pathway, in carotenoid metabolism in Oryza sativa leaves and seeds