Genetic approach to root colonization by Trichoderma virens
Trichoderma virens colonizes roots of a wide range of plants in a generally beneficial interaction, which can systemically prime plant defenses against infection by pathogens. The extent and outcome of the interaction varies on the Trichoderma-plant pair chosen for study. Nevertheless, comparison of different studies suggests that there is a core set of Trichoderma genes induced upon interaction with roots, encoding CAZymes and some small secreted proteins. To define the time course of colonization, we followed growth and ingress of T. virens in maize and tomato roots by staining with wheat germ agglutinin (WGA) Alexa-Fluor and live imaging by confocal microscopy. Image analysis of maize roots shows spore adhesion and germination within 24 hours post inoculation (hpi), followed by hyphal growth over the next 24 hpi. At 72 hpi there was massive hyphal development surrounding the root, with first indication of penetration. At 96 h and 120 hpi, T. virens colonized the maize epidermal and first cortical layers. Image analysis of tomato at 96 hpi shows the same colonization profile as in maize. In both experiments, Trichoderma hyphae appear to grow between the cells. A second approach is based on the hypothesis that ingress of Trichoderma requires increased extensibility of the plant cell wall. This hypothesis will be tested by constructing knockout mutants of genes encoding the expansin-like swollenin gene, and three polygalacturonidases, guided by studies showing colonization phenotypes by mutants in other Trichoderma species deleted for some of these genes.