Noncanonical terpene cyclases for the biosynthesis of diterpenoids regulating chlamydospore formation in plant-associated Trichoderma

Terpene cyclases catalyze exquisite and complicated cyclization reactions to generate diverse terpenoid skeletons. Trichoderma fungi are important biocontrol agents, characteristic of producing complex bioactive tetracyclic diterpenoids named harzianes and trichodermanins, but their biosynthesis and biological functions have long been enigmatic. Here we identify TriDTCs, an unprecedented family of terpene cyclases in Trichoderma, responsible for cyclizing geranylgeranyl diphosphate (GGPP) into major diterpenes harzianol I and wickerol A, via heterologous expressions, gene deletion, and in vitro assays. TriDTCs represent a brand new class of terpene cyclases, lacking known motifs and diverging from all known enzymes. Mechanistically, TriDTCs likely employ a unique DxxDxxxD aspartate triad for cyclization initiation, a critical valine residue modulating product specificity, and "gatekeeper" residues for activity. Phylogenetic analysis shows TriDTCs have a narrow distribution in three fungal genera and are highly functionally specific within Trichoderma, suggesting a genus-specific acquisition and independent evolution. Functional studies implicated TriDTCs in fungal survival strategies by regulating formation of resistant propagules (chlamydospore in Trichoderma, sclerotia in Aspergillus oryzae). These findings expand the knowledge of terpene cyclase diversity and biological significance, herald a strategy to enhance Trichoderma's biocontrol efficacy, and open avenues for pharmacological investigation of these diterpenoids.