INTRODUCTION
Trichoderma is a genus of ubiquitous fungi, comprising beneficious species used as biocontrol agents (BCAs) in crop plant protection due to their ability to antagonize and mycoparasitize a wide range of phytopathogens (Harman et al., 2004). Some strains promote plant growth (Fiorini et al., 2016), and are also able to protect plants against pathogens indirectly by inducing the plant defence responses (Shoresh, Harman, & Mastouri, 2010). The beneficial effects ofTrichoderma spp. are supported and often rely on the secondary metabolites (SMs) they produce (Lorito et al., 1996; Vinale et al., 2008; Viterbo et al., 2007), and the biological roles associated to these metabolites has been extensively reviewed (Contreras-Cornejo, Macías-Rodríguez, del Val, & Larsen, 2016; Contreras-Cornejo et al., 2018; Hermosa et al., 2014; Patil, Patil, & Paikrao, 2016; Rai, Solanki, Solanki, & Surapathrudu, 2019; Salwan, Rialch, & Sharma, 2019). These fungi produce a wide variety of SMs in a strain-dependent manner (Yu & Keller, 2005), with peptaibols, polyketides and terpenes as the most relevant (Reino et al., 2008).
Trichoderma spp. are reported to produce a broad diversity of terpenoids, including volatile compounds (Pachauri, Sherkane, & Mukherjee, 2019). Terpenoids play important roles in the physiology ofTrichoderma and in the interactions with other organisms, acting as toxins, chemical messengers, structural components of membranes, regulators of genes related to stress and inducers of plant defence responses (Pachauri et al., 2019; Zeilinger, Gruber, Bansal, & Mukherjee, 2016). Despite their huge variety, all fungal terpenes are synthesized from few precursors by terpene synthase enzymes (TSs). Isopentenyl-pyrophosphate and its isomer dimethyl-allyl pyrophosphate, both synthetized from acetyl-coA, are the five carbons (C) isoprene building blocks for the biosynthesis of linear polyprenyl pyrophosphates: 10C geranyl pyrophosphate (GPP), 15C farnesyl pyrophosphate (FPP) and 20C geranylgeranyl pyrophosphate (GGPP) (Quin, Flynn, & Schmidt-Dannert, 2014). These molecules are synthesized by the isoprenyl pyrophosphate synthases (IPSs), and constitute the precursors that undergo further modifications by terpene cyclases (TCs) and prenyl transferases (PTs), the core enzymes mediating the committed steps in terpenoid biosynthesis (Guzmán-Chávez et al., 2018). According to the origin of their scaffolds, terpenes can be distinguished in those exclusively formed by isoprenyl units (C10 monoterpenes, C15 sesquiterpenes, C20 diterpenes, C25 sesterterpenes and C30 triterpenes), and those of mixed origin (meroterpenoids, indole terpenoids and indole alkaloids).
Although many terpenes have been isolated from Trichodermaspecies, there is no extensive information about TS genes involved in their biosynthesis, and only few members of the TS family have been experimentally characterized (Bansal & Mukherjee, 2016). Functional characterization of TS genes in Trichoderma has been mainly focused on the trichodiene synthase (TRI5)-encoding gene, which catalyses the first committed step in the biosynthesis of trichothecenes harzianum A and trichodermin in T. arundinaceum and T. brevicompactum , respectively (Cardoza et al., 2011; Malmierca et al., 2013, 2014, 2015; Tijerino et al., 2011a,b). Other Trichoderma TS genes experimentally characterized are erg-20 of T. reesei , encoding a farnesyl pyrophosphate synthase (Pilsyk et al., 2013), and vir4 , required for the biosynthesis of mono- and sesquiterpenes in T. virens (Crutcher et al., 2013). Furthermore, genome mining studies have assessed the complete TS-gene family inTrichoderma , however in those cases, the diversity of the genus has been mainly limited to three species – T. virens , T. atroviride and T. reesei – (Bansal & Mukherjee 2016), or the study has been merely quantitative (Mukherjee et al., 2013; Kubicek et al., 2011; Kubicek et al., 2019).
Given the importance of terpenoids in the ecology of Trichoderma , and the scarce information available about the diversity within the TS-gene family, we focused on the genomic characterization of the complete set of TS genes of 21 strains belonging to 17Trichoderma spp., providing an overview of the terpenoid biosynthetic potential of the genus. In addition, aimed to decipher the environmental signals regulating the TS genes in Trichoderma , we assessed the expression patterns of some TSs in different conditions associated to the ecology of these fungi, using T. gamsii T6085 as a model. Strain T6085 presents a versatile lifestyle. During the past ten years, it has been evaluated as BCA against Fusarium graminearum , the most aggressive causal agent of Fusarium Head Blight (FHB) on wheat. T6085 is able to reduce the growth of the pathogen as well as the production of deoxynivalenol (DON) (Sarrocco, Mauro, & Battilani, 2019, Sarrocco et al., 2013a), growing in presence of high DON concentrations (50 ppm) and reducing FHB symptoms and the development of F. graminearum perithecia on wheat straw (Matarese et al., 2010, 2012; Sarrocco et al., 2013b, Sarrocco et al., 2020 - unpublished). In addition, the fungus establishes a beneficial interaction with wheat roots, behaving as an endophyte and inducing the plant defence responses (Sarrocco et al., 2020 - unpublished).