## 4 Discussion
Similarly, TS genes were regulated in opposite ways in response to oxidative stress. Association of oxidative stress with SMs biosynthesis in fungi has been extensively demonstrated, and it has been suggested that it is induced to prevent fungi from ROS damage [115]. In particular, up-regulation of _ts9_ suggests that biosynthesis of indole diterpenes might occur in _T. gamsii_ T6085 under oxidative stress conditions, as it has been observed in _Aspergillus_ spp. [116]. Interestingly, the addition of 0.9% sucrose or 0.5 mM H\({}_{2}\)O\({}_{2}\) to the medium did not induce _tri5_ expression in _T. gamsii_ T6085, unlike what has been observed in _T. brevicompactum_ when grown in the presence of 1% or 2% sucrose or in the presence of 0.5 mM H\({}_{2}\)O\({}_{2}\)[24]. This suggests different types of regulation of _tri5_ in _T. gamsii_ T6085 and _T. brevicompactum_. Nitrogen availability and saline stress have a considerable impact on fungal secondary metabolism [117; 118; 119]. N starvation and saline stresses negatively regulated TS genes in _T. gamsii_ T6085, suggesting terpenoid biosynthesis does not confer particular advantages to the fungus to overcome these stresses. In _T. arundinaceum_, _tri_ gene expression is affected when grown in dual cultures with _B. cinerea_, while polyketides and harzianum A (HA) produced by the first induce changes in some _B. cinerea_ genes linked to its virulence [23]. Since _T. gamsii_ T6085 is able to suppress _F. graminearum_ on wheat spikes and to reduce DON production by the pathogen [31; 32; 34; 35], we investigated whether a differential expression of TS genes of _T. gamsii_ T6085 occurs when the fungus interacts with _F. graminearum_ on wheat spikes. However, our results show that the presence of the pathogen did not induce prominent changes in TS expression in _T. gamsii_ T6085 when both fungi were on wheat spikes, although _ts11_ was found slightly up-regulated. Gene expression patterns are highly dynamic, and more extensive time-course experiments are needed to provide more information about the role of TSs in this scenario. Root colonization by _Trichoderma_ is an intimate relationship involving a tightly regulated exchange of molecular signals including SMs [120]. When _Trichoderma_ colonizes the roots, it releases a variety of SMs that promote substantial changes in plant biochemistry, which in turn, cause changes in the fungal physiology [8]. Our results indicate that a significative reprogramming of terpene biosynthesis occurs in _T. gamsii_ T6085 when it colonizes wheat roots. Down-regulation of _ts9_ suggests that root colonization induces a repression in indole diterpene biosynthesis in the fungus. In addition, the contrasting effects observed in the expression of sesquiTS and the up-regulation of the SQS suggest that root colonization induces a modulation on sesquiterpene and triterpene biosynthesis in _T. gamsii_ T6085 through FPP as the central node. The absence of _tri5_ expression in response to wheat spikes and the strong up-regulation observed during root colonization was remarkable, as it suggests that signals from the roots induce the expression of this gene in _T. gamsii_ T6085. Although activation of _tri5_ usually leads to the production of phytotoxic compounds, such as trichodermin in _T. brevicompactum_[24], HA produced by _T. arundinaceum_ lacks phytotoxic activity and has a crucial role in plant protection against _B. cinerea_[21]. This example illustrates how different ecological demands led to an adjustment in metabolic pathways governed by a single gene signature in species within the same fungal genus [28]. In this context, we can imagine the involvement of _tri5_ in the biosynthesis of a sesquiterpenoid related to the promotion of beneficial relationships between _T. gamsii_ T6085 and wheat plants, a question that must be further addressed by using _tri5_-deletion mutants of the fungus. Results strongly indicate the involvement of TS genes in the interaction of _T.