Integrative Analyses of Nontargeted Volatile Profiling and Transcriptome Data Provide Molecular Insight into VOC Diversity in Cucumber Plants (Cucumis sativus)

Plant volatile organic compounds, which are generated in a tissue-specific manner, play important ecological roles in the interactions between plants and their environments, including the well-known functions of attracting pollinators and protecting plants from herbivores/fungi attacks. However, to date, there have not been reports of holistic volatile profiling of the various tissues of a single plant species, even for the model plant species. In this study, we qualitatively and quantitatively analyzed 85 volatile chemicals, including 36 volatile terpenes, in 23 different tissues of cucumber (Cucumis sativus) plants using solid-phase microextraction combined with gas chromatography-mass spectrometry. Most volatile chemicals were found to occur in a highly tissue-specific manner. The consensus transcriptomes for each of the 23 cucumber tissues were generated with RNA sequencing data and used in volatile organic compound-gene correlation analysis to screen for candidate genes likely to be involved in cucumber volatile biosynthetic pathways. In vitro biochemical characterization of the candidate enzymes demonstrated that TERPENE SYNTHASE11 (TPS11)/TPS14, TPS01, and TPS15 were responsible for volatile terpenoid production in the roots, flowers, and fruit tissues of cucumber plants, respectively. A functional heteromeric geranyl(geranyl) pyrophosphate synthase, composed of an inactive small subunit (type I) and an active large subunit, was demonstrated to play a key role in monoterpene production in cucumber. In addition to establishing a standard workflow for the elucidation of plant volatile biosynthetic pathways, the knowledge generated from this study lays a solid foundation for future investigations of both the physiological functions of cucumber volatiles and aspects of cucumber flavor improvement.