Mode of action of Trichoderma asperellum SKT-1, a biocontrol agent against Gibberella fujikuroi

Trichoderma asperellum SKT- I and Gibberella fujikuroi, known as causal agents of '' Bakanae '' disease, were both transformed with genes encoding green fluorescent protein (GFP) and hygromycin B (hygB) by restriction enzyme- mediated integration (REMI). Rice seeds inoculated with GFP-tagged G.fijikuroi showed '' Bakanae '' symptoms. GFP-tagged SKT-1 maintained biocontrol activity against the pathogen by soaking seeds in SKT-1 spore suspension. Then, we monitored in situ interactions between SKT-1 and G. fijikuroi on rice seeds using GFP-tagged transformations under confocal scanning laser stereomicroscopy. G.fujikuroi disappeared from the embryo of rice seeds after treatment with SKT- 1, whereas SKT-1 was observed on the embryo 24 hr after initiation of germination. In addition, the hyphae of G. fujikuroi were penetrated by the hyphae of SKT- 1, and degradation of the cell walls of G. fujikuroi was observed under SEM in co-culture. The cell wall of G.fujikuroi on the embryo of rice seeds was lysed, suggesting that mycoparasitism is the mode of action of T asperellum SKT-1. (c) Pesticide Science Society of Japan.