Comparative Transcriptome Analysis of Different Chemotypes of Withania somnifera to Elucidate Withanolide Biosynthesis

Withania somnifera is one of the most valuable medicinal plants synthesizing secondary metabolites known as withanolides. Despite pharmaceutical importance, limited information is available about the biosynthesis of withanolides. Chemo-profiling of leaf and root tissues suggests differences in the content and/or nature of withanolides in different chemotypes. To identify genes involved in chemotype and/or tissue-specific withanolide biosynthesis, we established transcriptome sequences of leaf and root tissues of distinct chemotypes. The transcriptome data obtained was in range of 172 to 260 MB with at least 598000 ESTs in each library. For digital differential expression analysis, reads from each library were tagged, pooled and assembled into 43287 contigs with average length of 617 bps. Through annotation based approach, genes encoding enzymes for each intermediate steps of terpenoid backbone biosynthesis were identified with their paralogs. Analysis identified a large number of differentially expressed genes among leaf and root tissues of the chemotypes. The study also identified differentially expressing transcripts encoding cytochrome P450s, glycosyltransferases, methyltransferases and transcription factors which might be involved in chemodiversity in Withania. Virus induced gene silencing of the gene encoding sterol D7-reductase (WsDWF5) suggest role of this enzyme in biosynthesis of central key precursor for biosynthesis of variety of withanolides in different chemotypes. Information generated in this study provides a rich resource for functional analysis of withanolide-specific genes to elucidated chemotype- as well as tissue-specific withanolide biosynthesis. This genomic resource will also help in development of new tools for functional genomics and metabolic engineering.