Differential Regulation of an OsIspH1, the Functional 4-Hydroxy-3-Methylbut-2-Enyl Diphosphate Reductase, for Photosynthetic Pigment Biosynthesis in Rice Leaves and Seeds

The methylerythritol 4-phosphate (MEP) pathway is responsible for providing common precursors for the biosynthesis of diverse plastidial terpenoids, including chlorophylls, carotenoids, and phytohormones, in plants. In rice ( ), the last-step genes encoding 4-hydroxy-3-methylbut-2-enyl diphosphate reductase [HDR/isoprenoid synthesis H (IspH)] have been annotated in two genes ( and ) in the rice genome. The spatial transcript levels indicated that is highly expressed in all tissues at different developmental stages, whereas is barely expressed due to an early stop in exon 1 caused by splicing error. OsIspH1 localized into plastids and , a T-DNA inserted knockout mutant, showed an albino phenotype, indicating that is the only functional gene. To elucidate the role of in the MEP pathway, we created two single (H145P and K407R) and double (H145P/K407R) mutations and performed complementation tests in two mutants, including DLYT1 strains and rice plants. The results showed that every single mutation retained HDR function, but a double mutation lost it, proposing that the complementary relations of two residues might be important for enzyme activity but not each residue. When overexpressed in rice plants, the double-mutated gene, , reduced chlorophyll and carotenoid biosynthesis in the leaves and seeds. It confirmed the crucial role of in plastidic terpenoid biosynthesis, revealing organ-specific differential regulation of in rice plants.