Transcriptomic and metabolic analyses reveal differences in monoterpene profiles and the underlying molecular mechanisms in six grape varieties with different flavors

Monoterpenes are important compounds determining grape floral and fruity aromas. In this study, free and bound monoterpene contents and the transcriptome profiles of six grape varieties with different flavors were determined using HS-SPME/GC-MS and RNA-sequencing technologies. The different aroma types of grape varieties had significantly different free and bound monoterpene profiles. Linalool, trans-β-ocimene, β-cis-ocimene, geraniol, and β-myrcene were the main free monoterpenes in Muscat-type grapes. In contrast, trans-β-ocimene, γ-terpinene, and β-citronellol were the main free monoterpenes in strawberry-type grapes, while limonene and β-myrcene were the main free monoterpenes in neutral grapes. The total odor activity values (OAVs) of the free monoterpenes with OAVs >1 were more than 10-times higher in Muscat-type grapes than in strawberry-type and neutral grapes, which explains the high intensity of floral, rose and sweet aromas of Muscat-type grapes. A total of 21 genes related to monoterpene biosynthesis were detected in the three types of grape varieties, among which the expression of DXS, CMK, HDS, HDR, GPPS, HMGS, HMGR, MPDC, TPS14, and GT14 was positively correlated with monoterpene accumulation, indicating these genes may contribute to the differential accumulation of monoterpene compounds in analyzed grape varieties. These results clarify the monoterpene accumulation in grapes and underlying molecular mechanism.