The complexity of volatile terpene biosynthesis in roses: Particular insights into β-citronellol production

The fascinating scent of rose (Rosa genus) flowers has captivated human senses for centuries, making them one of the most popular and widely used floral fragrances. Despite much progress over the last decade, many biochemical pathways responsible for rose scents remain unclear. We analyzed the floral scent compositions from various rose varieties and selected the modern cultivar Rosa hybrida "Double Delight" as a model system to unravel the formation of rose dominant volatile terpenes, which contribute substantially to the rose fragrance. Key genes involved in rose terpene biosynthesis were functionally characterized. Cytosolic geranyl diphosphate (GPP) generated by geranyl/farnesyl diphosphate synthase (G/FPPS1) catalysis played a pivotal role in rose scent production, and terpene synthases in roses play an important role in the formation of most volatile terpenes, but not for geraniol, citral, or β-citronellol. Subsequently, a series of enzymes, including geraniol dehydrogenase, geranial reductase, 12-oxophytodienoate reductase, and citronellal reductase, were characterized as involved in the transformation of geraniol to β-citronellol in roses through three successive steps. Interestingly, the β-citronellol biosynthesis pathway appears to be conserved in other horticultural plants like Lagerstroemia caudata and Paeonia lactiflora. Our findings provide valuable insights into the biosynthesis of rose volatile terpenoid compounds and offer essential gene resources for future breeding and molecular modification efforts.