The cross-talk between Trichoderma gamsii and Fusarium graminearum at distance: a genome-wide transcriptome analysis

The present thesis reported on the cross-talk at distance between the beneficial fungus Trichoderma gamsii T6085 and the mycotoxigenic fungal pathogen Fusarium graminearum ITEM 124 (one of the main causal agents of Fusarium Head Blight disease, FHB). The cross-talk was investigated both at the physiological and the molecular levels. Previous studies have already reported on the mycoparasitic behaviour showed by T. gamsii T6085 against F. graminearum ITEM 124 and on its ability to reduce both the incidence and severity of FHB in field conditions. Furthermore, T. gamsii T6085 is able to compete for substrate possession with the pathogen and to reduce deoxynivalenol (DON) production. We carried out a genome-wide transcriptomic analysis to elucidate the molecular cross-talk at the sensing phase, i.e. before contact, between the two fungi. To this end, their genomes were sequenced and used as platforms for transcriptomics. The gene expression patterns of the two fungi were regulated in opposite ways. While an overall up-regulation of gene expression was recorded in F. graminearum ITEM 124 (‘buzzing mode’), gene expression in T. gamsii T6085 was predominantly down-regulated (‘stealth mode’). Moreover, F. graminearum ITEM 124 accelerated in presence of T. gamsii T6085 before contact, supporting the observed increased metabolic activity. The annotation and functional characterization of genes differentially expressed during the sensing phase revealed that competition for iron occurred. In fact, while T. gamsii T6085 up-regulated a ferric reductase involved in iron uptake, F. graminearum ITEM 124 down-regulated several genes involved in stress responses having iron as cofactor. Furthermore, while F. graminearum ITEM 124 up-regulated its entire repertoire of killer toxin-like chitinases, T. gamsii T6085 decreased the degree of interaction at distance by down-regulating six extracellularly secreted chitinases, supporting the ‘stealth mode’. This study showed how the communication between Trichoderma and Fusarium is definitely regulated before contact. The study of gene expression profiles unravelled a strongly different behaviour of the two fungi when they face each other. The ‘buzzing’ and ‘stealth’ modes showed by F. graminearum ITEM 124 and T. gamsii T6085, respectively, at distance opened new fields of investigation towards the successful development of T. gamsii T6085 as biocontrol agent.