Sandalwood (Santalum album L.) heartwood-derived essential oil contains a high content of sesquiterpenoids that are economically highly valued and widely used in the fragrance industry. Sesquiterpenoids are biosynthesized via the mevalonate acid and methylerythritol phosphate (MEP) pathways, which are also the sources of precursors for photosynthetic pigments. 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR) is a secondary rate-limiting enzyme in the MEP pathway. In this paper, the 1416-bp open reading frame of SaDXR and its 897-bp promoter region, which contains putative conserved cis-elements involved in stress responsiveness (HSE and TC-rich repeats), hormone signaling (abscisic acid, gibberellin and salicylic acid) and light responsiveness, were cloned from 7-year-old S. album trees. A bioinformatics analysis suggested that SaDXR encodes a functional and conserved DXR protein. SaDXR was widely expressed in multiple tissues, including roots, twigs, stem sapwood, leaves, flowers, fruit and stem heartwood, displaying significantly higher levels in tissues with photosynthetic pigments, like twigs, leaves and flowers. SaDXR mRNA expression increased in etiolated seedlings exposed to light, and the content of chlorophylls and carotenoids was enhanced in all 35S::SaDXR transgenic Arabidopsis thaliana lines, consistent with the SaDXR expression level. SaDXR was also stimulated by MeJA and H2O2 in seedling roots. α-Santalol content decreased in response to fosmidomycin, a DXR inhibitor. These results suggest that SaDXR plays an important role in the biosynthesis of photosynthetic pigments, shifting the flux to sandalwood-specific sesquiterpenoids.
Keywords: 1-deoxy-D-xylulose 5-phosphate reductoisomerase; MEP pathway; Santalum album; chlorophylls and carotenoids; sandalwood sesquiterpenoids.