Electron Microscope Study of Race-Specific and Age-Related Resistant and Susceptible Reactions of Soybeans toPhytophthora megaspermavar.sojae

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Abstract
Six-day old soybean (G. max ''Altona'') hypocotyls were inoculated with zoospores of race 6 or race 4 of P. megasperma var. sojae either at top (youngest tissue), which is a susceptible to race 6 but resistant to race 4, or at the bottom (older tissue), which is resistant to both races. In all cases a range of host cell responses was observed. These included necrosis of host cells without evidence of fungal invasion and the production of wall appositions with and without fungal invasion and necrosis. In a small number (< 5%) of cells in tissue inoculated with race 6, cell invasion occurred without wall apposition formation or necrosis. These were the only instances in which the interaction between individual host cells and hyphae was compatible. Compatibility is either a rare event or one of short duration. Differences between the susceptible reaction to race 6 at the top of the hypocotyl and the resistant reaction to this race at the bottom appeared to lie chiefly in the greater proportion of uninvaded necrotic cells at the bottom, especially in the epidermis and cell layers immediately beneath. The interactions with hyphae of race 4 were similar to those with race 6 except that no instances of invasion without necrosis were observed. At the bottom of the hypocotyl, hyphae of this race were restricted to the surface cell layers. The majority of hypocotyl cells react in an incompatible manner to hyphae of both races; only at the top of the hypocotyl following inoculation with race 6 is the frequency of incompatible reactions sufficiently reduced or delayed that visibly susceptible symptoms develop.

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