Transcriptome Sequencing Analysis Reveals a Difference in Monoterpene Biosynthesis between Scented Lilium ‘Siberia’ and Unscented Lilium ‘Novano’

Abstract

<i>Lilium</i> is a world famous fragrant bulb flower with high ornamental and economic values, and significant differences in fragrance are found among different <i>Lilium</i> genotypes. In order to explore the mechanism underlying the different fragrances, the floral scents of <i>Lilium</i> 'Sibeia', with a strong fragrance, and <i>Lilium</i> 'Novano', with a very faint fragrance, were collected <i>in vivo</i> using a dynamic headspace technique. These scents were identified using automated thermal desorption-gas chromatography/mass spectrometry (ATD-GC/MS) at different flowering stages. We used RNA-Seq technique to determine the petal transcriptome at the full-bloom stage and analyzed differentially expressed genes (DEGs) to investigate the molecular mechanism of floral scent biosynthesis. The results showed that a significantly higher amount of <i>Lilium</i> 'Siberia' floral scent was released compared with <i>Lilium</i> 'Novano'. Moreover, monoterpenes played a dominant role in the floral scent of <i>Lilium</i> 'Siberia'; therefore, it is believed that the different emissions of monoterpenes mainly contributed to the difference in the floral scent between the two <i>Lilium</i> genotypes. Transcriptome sequencing analysis indicated that ~29.24 Gb of raw data were generated and assembled into 124,233 unigenes, of which 35,749 unigenes were annotated. Through a comparison of gene expression between these two <i>Lilium</i> genotypes, 6,496 DEGs were identified. The genes in the terpenoid backbone biosynthesis pathway showed significantly different expression levels. The gene expressions of 1-deoxy-D-xylulose 5-phosphate synthase (DXS), 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR), 4-hydroxy-3-methylbut-2-enyl diphosphate synthase (HDS), 4-hydroxy-3-methylbut-2-enyl diphosphate reductase (HDR), isopentenyl diphosphate isomerase (IDI), and geranyl diphosphate synthase (GPS/GGPS), were upregulated in <i>Lilium</i> 'Siberia' compared to <i>Lilium</i> 'Novano', and two monoterpene synthase genes, ocimene synthase gene (<i>OCS</i>) and myrcene synthase gene (<i>MYS</i>), were also expressed at higher levels in the tepals of <i>Lilium</i> 'Siberia', which was consistent with the monoterpene release amounts. We demonstrated that the high activation levels of the pathways contributed to monoterpene biosynthesis in <i>Lilium</i> 'Siberia' resulting in high accumulations and emissions of monoterpenes, which led to the difference in fragrance between these two <i>Lilium</i> genotypes.