Editorial: Molecular intricacies of Trichoderma-plant-pathogen interactions

Trichoderma spp. are widely used as plant disease biocontrol agents around the world (Guzmán-Guzmán, 2019). Initial research on Trichoderma focused on their biocontrol ability mediated by mycoparasitism and antibiosis. Trichoderma spp. are known for direct antagonistic action on plant pathogenic fungi, and more recently for indirect suppression via induced defense. Thus, with the discovery that Trichoderma spp. can internally colonize roots and invoke defense responses in plants, attention of the research community largely shifted toward understanding how the plants and Trichoderma communicate with each other leading to a symbiosis-like relationship (Harman et al., 2004; Hohmann, 2012). At the initial attachment stage, Trichoderma spp. are known to secrete hydrophobins that could aid in adhesion to the root surface (Viterbo and Chet, 2006). Following attachment, secretion of plant cell wall degrading enzymes like pectate lyase (Morán Diez et al., 2009) and swollenin (Brotman et al., 2008; Andberg et al., 2015; Cosgrove, 2017) could facilitate penetration. It is important to note that penetration into the root is limited, and indeed genes encoding plant cell wall degrading enyzymes are downregulated early in root colonization (Estrada-Rivera et al., 2020). This occurs along with high representation of glycosyl hydrolases in the secretome (Nogueira-Lopez et al., 2018). Soluble enzymes may still be present in the secretome after the corresponding transcripts decrease, highlighting the need for obtaining well-resolved time course experiments for both gene expression and protein abundance during early colonization events. Trichoderma fungi are known to secrete a large number of small secreted cysteine-rich proteins (SSCPs) that might be involved in modulation of plant defense, fine tuning of which may be responsible for the outcome of this association. Though not clearly established, it seems possible that Trichoderma SSCPs initially suppress plant defense, and once the colonization is complete, induces plant defense to enter into a symbiosis-like relationship. Unlike mycorrhiza, there seems to exist no specificity in Trichoderma-plant association, which appears to be quite universal. For example, AM mycorrhizal fungi cannot colonize cruciferous roots exhibiting a level of specificity, but Trichoderma can effectively colonize such roots, indicating a generalist type of lifestyle