Early cytological differences between compatible and incompatible interactions of soybeans with Phytophthora megasperma f.sp. glycinea

The interaction of soybean cv. Harosoy 63 with Phytophthora megasperma f.sp. glycinea race 1 (incompatible) and race 4 (compatible) and of the near-isogenic cv. Harosoy with these races (compatible) was examined by light and electron microscopy in hypocotyl tissues fixed 2, 3, 5 and 7 h after inoculation. The extent and rate of colonization was similar in both incompatible and compatible interactions. Hyphae reached the first layer of the cortex by 2 h and the third layer by 3 h. However, there were major differences in host-cell responses. In the incompatible interaction (Harosoy 63-race 1) virtually all epidermal cells at and surrounding infection sites were dead by 3 h, and throughout the cortex a majority of cells associated with hyphae were dead after each incubation period. Penetrated cells were almost always dead even when wall appositions were present. There were relatively few dead cells in the compatible interactions although more were associated with hyphae of race 1 than with those of race 4. The majority of penetrated cells in the compatible interactions were alive. There was ultrastructural evidence of damage to hyphae in the incompatible but not in the compatible interactions by 5 h after inoculation. Host-cell death in the temperature-induced susceptible response of cv. Altona to race 4 at 33 °C was only marginally less than in the resistance response at 25 °C, although the rate and extent of colonization was much greater at 33 °C. It is concluded that, whereas the observations are consistent with the operation of a recognition system and the triggering of host-cell death as an almost immediate response in incompatible interactions, it is unlikely that temperature-induced susceptibility in cv. Altona is due to changes in such a system. © 1989.