Penetration of Zea mays by Helminthosporium carbonum

Abstract

Light microscopy showed that on corn leaves, 81–93% of appressoria of Helminthosporium carbonum races I and II are formed over junctions of epidermal cells. During the early stages of penetration of corn plants resistant and susceptible to race I, 63–83% of appressoria have an epidermal cell nucleus within 10 μm. Transmission electron microscopy of race II on inbred W187R showed that appressoria are attached to the cuticle by fibrillar material. Vesicles are present in the appressorium at the site of cuticle penetration and initial cuticle penetration appears to be enzymic; subsequent rupture may be mechanical as the penetration peg widens. A septum forms between the appressorium and the subcuticular hyphae. Epidermal cell cytoplasm is thicker beneath penetration sites than elsewhere under the epidermal wall. Changes in epidermal cytoplasm were observed 8 h after inoculation; by 18 h epidermal cells beneath subcuticular hyphae have electron-opaque contents. Hyphae are mainly subcuticular up to 48 h after inoculation, and underlying epidermal and mesophyll cells are frequently collapsed. Results indicate that H. carbonum races I and II have similar initial reactions on susceptible and resistant genotypes and that penetration occurs by degradation of the cuticle and host cell walls.