Integrated metabolome and transcriptome analysis of Magnolia champaca identifies biosynthetic pathways for floral volatile organic compounds

Savitha Dhandapani, Jingjing Jin, Vishweshwaran Sridhar, Rajani Sarojam, Nam-Hai Chua, In-Cheol Jang

Magnolia champaca, commonly known as champak is a well-known tree due to its highly fragrant flowers. Champak floral scent is attributed to a complex mix of volatile organic compounds (VOCs). These aromatic flowers are widely used in flavors and fragrances industry. Despite its commercial importance, the VOC biosynthesis pathways in these flowers are largely unknown. Here, we combine metabolite and RNA sequencing (RNA-seq) analyses of fully opened champak flowers to discover the active VOC biosynthesis pathways as well as floral scent-related genes. Volatile collection by headspace method and analysis by gas chromatography-mass spectrometry (GC-MS) identified a total of 43 VOCs from fully opened champak flowers, of which 46.9% were terpenoids, 38.9% were volatile esters and 5.2% belonged to phenylpropanoids/benzenoids. Sequencing and de novo assembly of champak flower transcriptome yielded 47,688 non-redundant unigenes. Transcriptome assembly was validated using standard polymerase chain reaction (PCR) based approach for randomly selected unigenes. The detailed profiles of VOCs led to the discovery of pathways and genes involved in floral scent biosynthesis from RNA-seq data. Analysis of expression levels of many floral-scent biosynthesis-related unigenes in flowers and leaves showed that most of them were expressed higher in flowers than in leaf tissues. Moreover, our metabolite-guided transcriptomics, in vitro and in vivo enzyme assays and transgenic studies identified (R)-linalool synthase that is essential for the production of major VOCs of champak flowers, (R)-linalool and linalool oxides. As our study is the first report on transcriptome analysis of Magnolia champaca, this transcriptome dataset that serves as an important public information for functional genomics will not only facilitate better understanding of ecological functions of champak floral VOCs, but also provide biotechnological targets for sustainable production of champak floral scent.