Cloning, molecular characterization and functional analysis of 1-hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate reductase (HDR) gene for diterpenoid tanshinone biosynthesis in Salvia miltiorrhiza Bge. f. alba

The enzyme 1-hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate reductase (HDR) is a terminal-acting enzyme in the plastid MEP pathway, which produce isoprenoid precursors. The full-length cDNA of HDR, designated SmHDR1 (Genbank Accession No. JX516088), was isolated for the first time from Salvia miltiorrhiza Bge. f. alba. SmHDR1 contains a 1389-bp open reading frame encoding 463 amino acids. The deduced SmHDR1 protein, which shows high identity to HDRs of other plant species, is predicted to possess a chloroplast transit peptide at the N-terminus and four conserved cysteine residues. Transcription pattern analysis revealed that SmHDR1 has high levels of transcription in leaves and low levels of transcription in roots and stems. The expression of SmHDR1 was induced by 0.1 mM methyl-jasmonate (MeJA) and salicylic acid (SA), but not by 0.1 mM abscisic acid (ABA), in the hairy roots of S. miltiorrhiza Bge. f. alba. Complementation of SmHDR1 in the Escherichia coli HDR mutant MG1655 ara < > ispH demonstrated the function of this enzyme. A functional color assay in E. coli showed that SmHDR1 accelerates the biosynthesis of β-carotene, indicating that SmHDR1 encodes a functional protein. Overexpression of SmHDR1 enhanced the production of tanshinones in cultured hairy roots of S. miltiorrhiza Bge. f. alba. These results indicate that SmHDR1 is a novel and important enzyme involved in the biosynthesis of diterpenoid tanshinones in S. miltiorrhiza Bge. f. alba.