Comparative Transcriptome Analysis and Identification of Candidate Genes Involved in Cucurbitacin IIa Biosynthesis in Hemsleya Macrosperma.

Hemsleya macrosperma (H. macrosperma) is widely used in southwestern China as folk medicine with various bioactivities. Cucurbitacin IIa is the main active component in H. macrosperma and draws increased attention for its potential pharmacological activities. In order to reveal the mechanism of cucurbitacin IIa biosynthesis and regulation in H. macrosperma, transcriptome analysis was performed to compare differentially expressed genes in three tissues (root tuber, stem and leaf). A total of 47 946 unigenes were generated from these tissues and 55 unigenes were identified as candidate genes involved in triterpenoid backbone biosynthesis. Three homologous genes encoding squalene epoxidase (HmSE) were discovered and successfully expressed in a prokaryotic system. HmSE1 was found to be responsible for oxidization of squalene. In addition, several cytochrome P450s and transcription factors were predicted as candidates associated to cucurbitacin IIa biosynthesis. Notably, the expression profiles of those putative genes showed a positive correlation with elevated curcurbitacin IIa production in methyl jasmonate-elicited suspension cells of H. macrosperma., suggesting probable functions of the candidates on curcurbitacin IIa biosynthesis. These findings provide insights on cucurbitacin IIa biosynthesis and regulation in H. macrosperma.