Molecular evolution of terpene synthase underlying the diversification of isoprene emission in Fagaceae

Plants emit a wide range of volatile organic compounds, among which isoprene is the most abundant and atmospherically influential. Although oak species are major contributors to isoprene emission, there is considerable variation in isoprene emission capacity within the Fagaceae family. To unravel the evolutionary origins of isoprene emission, we investigated the molecular evolution of terpene synthase (TPS) genes across eight species from four genera within the Fagaceae. We identified a Fagaceae-specific TPS-b subclade in which potential isoprene synthase (IspS) activity evolved independently in two gene lineages within subgenus Quercus. Seasonal gene expression patterns showed that one lineage exhibited high expression particularly in summer, whereas the other displayed consistently low expression across all seasons. Ancestral sequence reconstruction revealed that the acquisition of a diagnostic amino acid residue for IspS function (a substitution from Thr338 to Phe338) arose convergently in these lineages and was subject to positive selection, suggesting adaptive evolution. Ancestral-enzyme assays targeting the gene lineage with high gene expression revealed that the early protein primarily produced monoterpenes—particularly trans-β-ocimene—from geranyl diphosphate (GPP), whereas their descendants shifted substrate preference to dimethylallyl diphosphate (DMAPP), evolving into dedicated isoprene synthases. Our results indicate that IspS activity was not ancestral in Fagaceae, but evolved approximately 56 million years ago within the subgenus Quercus, and has been retained ever since. These findings emphasize the roles of enzyme structural innovation and regulatory shifts in the diversification of volatile terpenoid biosynthesis.