The role of lignification as a resistance mechanism in wheat to a toxin-producing isolate of Pyrenophora tritici-repentis

Brightfield, fluorescence, and scanning electron microscopy were used to observe the cellular effects of infection by a toxin-producing isolate (86-124) of Pyrenophora tritici-repentis, which causes tan spot of wheat (Triticum aestivum). Disruption of mesophyll cells surrounding or within close proximity to the intercellular hyphae was recognizable macroscopically as a distinct tan-brown lesion on the leaf surface. In a susceptible plant, spread of intercellular hyphae within the leaf was limited laterally only by the larger midveins. In a resistant plant, secondary hyphae were restricted to the infected epidermal cell and/or to a localized area of mesophyll cells at the infection site. Intercellular spaces of the mesophyll tissue surrounding the infection site were filled with material, confining the growth of the fungus. Histochemical staining of these regions indicated the presence of lignin or ligninlike material. Occlusion of the intercellular space and lignification appeared to be a response in the resistant host. Lesions in resistant wheat were observed as small brown necrotic spots. These observations are discussed in relation to the physical presence of the hyphae in the mesophyll tissue and the possible effect of the fungal toxin in the host-pathogen interactions of the susceptible and resistant plants.