Ultrastructure of penetration by Helminthosporium maydis

Abstract

Penetration by Helminthosporium maydis race T into susceptible leaves from Texas male sterile and resistant ones from normal plants was studied with four different corn inbred lines. Six hours after inoculation, appressoria-like structures were present but were rare, and no evidence of penetration beneath such structures was found. All penetrations seen 6 h after inoculation took place between epidermal cells, and at least two-thirds of these occurred between cells adjacent to stomates. Breaks in the cuticle above subsidiary cells of the stomatal apparatus may account for the high frequency of penetration in this area. Eight to 10 h after inoculation, miniature infection cushions had formed at initial points of penetration, and from these, hyphae radiated out beneath the cuticle of epidermal cells. Haustoria-like branches produced by these subcuticular hyphae appeared to function in secondary infection. Hyphae of the fungus were seen occasionally within stomatal openings (apparent stomatal penetration), but these were consistently associated with penetrations which had occurred between nearby epidermal cells. No differences were observed at any stage of penetration or initial colonization between Texas male sterile and normal plants of the same inbred or among the four inbreds studied. These results suggest that resistance to this pathogen develops only after infection has occurred and that the selective pathotoxin produced by race T does not play an important rôle in the infection process.