Volatile squalene from a nonseed plant Selaginella moellendorffii: Emission and biosynthesis

The triterpene squalene is a key metabolic intermediate for sterols, hopanoids and various other triterpenoids. The biosynthesis of squalene is catalyzed by squalene synthase (SQS), which converts two molecules of farnesyl diphosphate to squalene. In this study, a lycophyte Selaginella moellendorffii was found to emit squalene as a volatile compound under a number of conditions that mimic biotic stresses. Searching the genome sequence of S. moellendorffii led to the identification of a putative squalene synthase gene. It was designated as SmSQS. SmSQS is homologous to known squalene synthases from other plants and animals at both the amino acid level and structural level. Recombinant SmSQS expressed in Escherichia coli catalyzed the formation of squalene using farnesyl diphosphate as substrate. The expression of SmSQS was significantly induced by the same set of stress factors that induced the emission of volatile squalene from S. moellendorffii plants. Taken together, these results support that SmSQS is responsible for the biosynthesis of volatile squalene and volatile squalene may have a role in the defense of S. moellendorffii plants against biotic stresses.