Identification of genes from Trichoderma virens involved in the colonization of maize roots, and the relation of colonization to induced systemic resistance
As ubiquitously distributed plant beneficial symbionts, members of the genus Trichoderma provide experimental models to understand the molecular mechanisms underpinning the delivery of plant benefits by root-associated symbionts. A critical process that initiates this symbiotic relationship is the colonization of host roots. We propose a model of root colonization by T. virens that partitions the process into discrete units that are experimentally tractable: Recognition, contact/attachment, penetration, and ingress. Based on microarray and proteomic data, we have targeted genes encoding hydrophobins, expansins, glycoside hydrolases, and small secreted cysteine-rich proteins, putatively involved in individual components of the model, for characterization. As a proof of concept that individual genes involved in these components can be identified, we initiated studies with the gene (TvHyd1) encoding a hydrophobin to test the hypothesis that hydrophobins are necessary for attachment of hyphae to the root cell wall. ΔTvHyd1 mutants showed a significant decrease in colonization ability compared to the wild type strain. We also hypothesized that reduction of colonization at the early phases (recognition and contact/attachment) would influence the induction of beneficial effects in the plant. Using induced systemic resistance (ISR) as a model of host benefits, we demonstrated that ΔTvHyd1 mutants were also impaired in their ability to induce ISR in maize plants, exhibiting more protection than the untreated plants, but less protection than the wild type treated plants. These proof of concept experiments provide support for our model and help us to better understand how components of colonization allow the fungus to convey beneficial attributes to the plant host.