The East Indian sandalwood tree, Santalum album L. is known for its fragrant heartwood and essential oil. The major bioactive principles of sandalwood oil, i.e., sesquiterpenoids (C15 isoprenoids), are known as ‘santalols’ and are globally used in medicinal, cosmetic, dietary, and aromatherapeutic applications. However, there are no available reports on the biosynthesis and metabolism of isoprenoids in this forest tree. Hence, we provide detailed insights into sesquiterpenoid metabolism across several in vitro and in vivo developmental stages. Since no molecular information was available, several genes encoding enzymes participating in early and critical steps of isoprenoid biosynthetic pathways were isolated using degenerate primers, and their expression patterns across the developmental stages were studied by semi-quantitative reverse transcription PCR. Results indicate that the isoprenoid biosynthetic pathway is differentially regulated with development and in tissue-specific manner. Accumulation of plastidial isoprenoid pigments increased with development, while the amounts of farnesylated intermediates decreased with maturation, thereby possibly indicating conversion into sesquiterpenoids. A differential expression pattern was observed for hydroxy-3-methylglutaryl coenzyme A reductase and 1-deoxyxyulose-5-phosphate synthase at the levels of transcripts and proteins, indicating post-transcriptional regulation. Transcript levels of farnesyl pyrophsophate, sesquiterpene and monoterpene synthases were quantitatively higher in callus, and lower in matured tree leaves. Sesquiterpene synthase activity across different developmental stages indicated a tissue-specific conversion and accumulation. Henceforth, the results would facilitate characterization of routes of sandalwood oil biosynthesis and for future improvement of sesquiterpenoid content in this tree.
Communicated by M. Buckeridge