Ultrastructure of susceptible, host resistant, and nonhost resistant interactions of alfalfa with Phytophthora megasperma

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Abstract
Zoospores of Phytophthora megasperma Drechs. f. sp. medicaginis (Pmm), an alfalfa (Medicago sativa L.) pathogen, and P. megasperma f. sp. glycinea (Pmg), a pathogen of soybean (Glycine max (L.) Merr.), were used to inoculate alfalfa seedlings from the Pmm-susceptible cultivar 'Vernal' and the Pmm-resistant breeding line M193. The susceptible interaction ('Vernal'–Pmm) was characterized by rapid disruption of host mitochondria in cells adjacent to intercellular hyphae, dissolution of host cell walls, and death of host cells 12–24 h after inoculation. Colonization of roots was extensive, and hyphae usually appeared healthy. Both host resistant (M193–Pmm) and nonhost resistant (M193 or 'Vernal' – Pmg) interactions were characterized by hyphal growth inhibition, production of cell wall appositions in root cells in contact with hyphae, and rapid necrosis of root cells. Wall appositions were more numerous in root cells in contact with Pmg than with Pmm, and rapid cell necrosis was more extensive in the nonhost resistant interaction than in the host resistant interaction. Numerous abnormalities were observed in Pmg hyphae. Nonhost resistance was expressed more rapidly and appeared to be more effective than host resistance in limiting hyphal growth. An examination of a compatible interaction of Pmg with soybean indicated that its mode of pathogenesis may be fundamentally different from that of Pmm in alfalfa.

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