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).