CYTOLOGICAL AND BIOCHEMICAL FACTORS INVOLVED IN THE SUSCEPTIBLE, HOST RESISTANT AND NONHOST RESISTANT INTERACTIONS OF ALFALFA WITH PHYTOPHTHORA MEGASPERMA

Two plant/pathogen systems were developed in order to investigate the cytological and biochemical factors involved in the susceptible, host resistant, and nonhost resistant interactions of alfalfa (Medicago sativa L.) with a pathogen, Phytophthora megasperma Drechs. f. sp. medicaginis (Pmm), and a nonpathogen of alfalfa, P. megasperma f. sp. glycinea (Pmg). The cytological studies were carried out using alfalfa seedlings highly and uniformly resistant or susceptible to Pmm, while the biochemical studies involved Pmm-resistant and Pmm-susceptible callus tissue cultures. The results of the cytological studies demonstrated that host and nonhost resistance of alfalfa seedlings to Pmm and Pmg were not expressed prior to root penetration; zoospore attraction to roots, encystment, cyst germination and early penetration were the same in susceptible, host resistant and nonhost resistant interactions. Resistance was expressed during the colonization of seedling roots, and was characterized by a restriction in hyphal development in the host and nonhost resistant interactions. Colonization was more limited in the nonhost resistant than in the host resistant interactions.; Ultrastructural studies demonstrated that the susceptible and resistant interactions were fundamentally different, but that host and nonhost resistant interactions differed mainly in the timing and intensity of the resistant responses. Susceptible interactions were characterized by a rapid disruption of host cell mitochondria and death of host cells in contact with hyphae within 12 hr after inoculation. In the nonhost resistant interactions, plant cell mitochondria were not affected, but wall appositions were produced within 1-2 hr after inoculation in cells in contact with hyphae, and hypersensitive plant cell death was observed within 2 hr after inoculation. Many Pmg hyphae were abnormal within 24 hr after inoculation. Similar responses were observed in the host resistant interaction, but they generally occurred later and less often than in the nonhost resistant interactions.; Studies with alfalfa callus tissue demonstrated that the isoflavonoid phytolexins, medicarpin and sativan, accumulated in Pmm- and Pmg-challenged alfalfa callus tissue. Although neither was determined to play a primary role in host resistance, the possibility that medicarpin is an important factor in nonhost resistance could not be ruled out.