Ultrastructural comparison of a compatible and incompatible interaction triggered by the presence of an avirulence gene during early infection of the smut fungus, Ustilago hordei, in barley

Abstract

Cell morphologies and reactions during infection of barley by the smut fungus, Ustilago hordei, were investigated by TEM. We compared compatible and incompatible interactions caused respectively by the absence or presence of an avirulence gene (V1) on cultivar ‘Hannchen’, harboring the cognate resistance gene ( Ruh1). In both interactions, U. hordei penetrated coleoptile epidermal cells directly within two days post inoculation (dpi). Upon penetration, an electron-opaque interfacial matrix formed around both inter- and intracellular hyphae in compatible interactions. Hyphae grew and extended into the host bundle sheath and invaded parenchyma cells. At 12 dpi, cell wall appositions formed surrounding the hyphae. Tubule-like structures of variable thickness and orientation were visible in the interfacial matrix mainly in the outer region bordering these appositions. In contrast, in incompatible interactions, cell wall appositions occurred as soon as hyphae penetrated host epidermal cells. Material, thicker and more granular in appearance compared to that in compatible interactions, was deposited around the invading hyphae and extended onto the inner surface of invaded epidermal cells. Upon penetration, host cell reactions included disorganization of cytoplasm and organelles leading to necrosis and cell death. This gene-for-gene combination triggers a very early hypersensitive response-like resistance reaction, extremely localized at sites of primary infection which involves only a few plant cells and may or may not be responsible for fungal arrest.