Insight into TPS-d Subfamily: The Evolutionary and Functional Diversity in Gymnosperms.

Gymnosperms represent an ancient lineage with diverse chemical profiles. Their terpenoids constitute a major class notable for substantial structural diversity. Terpene synthases (TPSs) are the driving force of terpene diversity. TPS-d is a gymnosperm-specific TPS subfamily, yet the evolution and functional diversity of TPS-d are still understudied. In this review, we analyze 134 functionally characterized gymnosperm TPSs to elucidate the evolutionary history and functional diversity of the predominant TPS-d subfamily. Phylogenetic analysis revealed substantial lineage-specific expansion and divergence within the TPS-d subfamily post-speciation. Crucially, we highlight the shifts in domain architecture during TPS-d evolution, providing structural evidence for the progressive attrition and eventual loss of the ancestral γ domain in TPS-d2 and TPS-d1 clades. Furthermore, we summarize the functional diversity of TPS-d enzymes in specialized metabolites, emphasizing their ecological roles in plant defense. Overall, this review provides a comprehensive overview of the evolutionary and functional landscape of the gymnosperm-specific TPS-d subfamily, serving as a valuable reference for future studies on TPSs in gymnosperms.