Abstract

Geranyl diphosphate synthase (GPPS) is a key enzyme in the plant isoprenoid metabolic pathway and regulates the biosynthesis of volatile monoterpenes. It plays an important role in the biosynthesis of floral volatile terpenoids (FVTs) and inter-cultivar variation in snapdragon. Despite its importance in floral scent formation, theGPPS/GGPPSgene family in snapdragon (Antirrhinum majusL.) has not been systematically characterized. In this study, nine GPPS/GGPPS family members were identified at genome-wide level. These include sixAmGPPSand threeAmGGPPSgenes. Phylogenetic analysis grouped them into distinct subfamilies. We further analyzed their chromosomal locations, gene structures, conserved protein motifs, and promoter cis-acting elements. These results revealed both conservation and functional divergence within the gene family. To explore their functional roles, we compared gene expression profiles at the full flowering stage. This comparison was performed between strongly scented cultivar (Am3) and the weakly scented cultivar (Am5). Among all candidates,AmGPPS6showed the most significant differential expression. Further, functional validation was conducted using transient overexpression and virus-induced gene silencing (VIGS). Overexpression ofAmGPPS6significantly increased terpenoid production. Total floral volatile terpenoids (FVTs) increased by 1.4 fold. Both monoterpene and sesquiterpene emissions were enhanced. In contrast, silencing ofAmGPPS6markedly reduced the emission of key monoterpenes such as ocimene and its isomers. Sequence analysis showed thatAmGPPS6shares 67.04% identity with canonical GPPS small subunit (GPPS.SSU). However, it lacks the conserved catalytic DDx2-D motif. This suggests thatAmGGPPS2is not catalytically active. Instead, it likely functions through heterodimer withAmGGPPS2. This interaction is supported by coordinated transcriptional expression patterns. Additionally, natural sequence polymorphisms were identified in GPPS.SSU. These