Gate crashing arbuscular mycorrhizas: in vivo imaging shows the extensive colonization of both symbionts by Trichoderma atroviride

Plant growth-promoting fungi include strains of Trichoderma species that are used in biocontrol, and arbuscular mycorrhizal (AM) fungi, that enhance plant nutrition and stress resistance. The concurrent interaction of plants with these two groups of fungi affects crop performance but has only been occasionally studied so far. Using in vivo imaging of green fluorescent protein-tagged lines, we investigated the cellular interactions occurring between Trichoderma atroviridePKI1, Medicago truncatula and two Gigaspora species under in vitro culture conditions. Trichoderma atroviride did not activate symbiotic-like responses in the plant cells, such as nuclear calcium spiking or cytoplasmic aggregations at hyphal contact sites. Furthermore, T.atroviride parasitized G.gigantea and G.margarita hyphae through localized wall breaking and degradation - although this was not associated with significant chitin lysis nor the upregulation of two major chitinase genes. Trichodermaatroviride colonized broad areas of the root epidermis, in association with localized cell death. The infection of both symbionts was also observed when T.atroviride was applied to a pre-established AM symbiosis. We conclude that - although this triple interaction is known to improve plant growth in agricultural environments - in vitro culture demonstrate a particularly aggressive mycoparasitic and plant-colonizing behaviour of a biocontrol strain of Trichoderma.