Marchantia polymorpha is a common liverwort considered in recent years as a model plant for evolutionary studies on plant–microorganism interactions. Despite the lack of research, remarkable results have been reported regarding the understanding of metabolic and evolutionary processes of beneficial and/or harmful interactions, owing to a better understanding of the origin and evolution of different plant defense pathways. In this study, we have carried out work on the direct and indirect interactions (exudates and volatiles) of M. polymorpha with different species of the fungal genus Trichoderma. These interactions showed different outcomes, including resistance or even growth promotion and disease. We have analyzed the level of tissue colonization and defense-related gene expression. Furthermore, we have used the pteridophyte Dryopteris affinis and the angiosperm Arabidopsis thaliana, as subsequent steps in plant evolution, together with the plant pathogen Rhizoctonia solani as a control of plant pathogenicity. Trichoderma virens, T. brevicompactum and T. hamatum are pathogens of M. polymorpha, while exudates of T. asperellum are harmful to the plant. The analysis of the expression of several defense genes in M. polymorpha and A. thaliana showed that there is a correlation of the transcriptional activation of SA-related genes with resistance or susceptibility of M. polymorpha to Trichoderma. Moreover, exogenous SA provides resistance to the virulent Trichoderma species. This beneficial fungus may have had an evolutionary period of interaction with plants in which it behaved as a plant pathogen until plants developed a defense system to limit its colonization through a defense response mediated by SA.
