Microtubule dynamics in compatible and incompatible interactions of soybean hypocotyl cells with Phytophthora sojae

The arrangement of microtubules in soybean (Glycine max) cells was examined during compatible and incompatible interactions of hypocotyls of soybean cv. Harosoy (susceptible) and cv. Haro 1272 (resistant) with race 1 of the soybean-specific pathogen Phytophthora sojae. Both reaction types were similar during the first 3 h after zoospore inoculation in terms of the number of cells penetrated, and depth penetrated into the cortex. By 3 h postinoculation, clear differences had developed between the two interaction types: incompatible interactions were characterized by a hypersensitive response that was confined to single penetrated cells; while compatibly responding cells appeared unchanged. Both types of response were characterized by autofluorescence of cell walls or cytoplasm and, at 6 h after inoculation, complete disorganization of cell cytoplasm. Reorientation and loss of microtubules was seen in the early stages of the incompatible interaction in association with cellular hypersensitivity, but not in compatible responses. In cells adjacent to those that reacted hypersensitively, there was little evidence of change in microtubule orientation. Treatment of hypocotyls with the microtubule depolymerizer oryzalin prior to inoculation did not alter the compatible response, but led to breakdown of the incompatible response. Changes in microtubule orientation and state are thus among the first structural changes that are visible within cells during incompatibility in this system.