Ultrastructure of compatible and incompatible interactions of soybean roots infected with the plant pathogenic oomycete Phytophthora sojae

Compatible and incompatible interactions of two soybean isolines containing either Rpsla or Rpslb resistance genes with races 2 and 8 of Phytophthora sojae were examined by light and electron microscopy. Phytophthora sojae race 2 is virulent on Rpslb plants and avirulent on Rpsla plants. Race 8 shows the reverse reaction; it is avirulent on Rpslb plants, but virulent on Rpsla plants. All combinations of races and cultivars were examined at times ranging from 30 min to 20 h postinoculation. Zoospore encystment, germination, and infection occurred within 30 min in all interactions. No evidence of appressorium formation was found. Wall appositions in epidermal cells adjacent to hyphae were very frequent by 30 min postinoculation. Differences between compatible and incompatible interactions became evident as early as 4 h postinoculation. The major difference appeared to relate to timing of host responses, which lead to two different types of relationships. In compatible interactions, P. sojae exhibited a short biotrophic phase with the establishment of many haustoria without triggering visible plant responses in cortical cells until approximately 10 h postinoculation. By 15 h postinoculation, almost the entire root was necrotic, wall appositions were abundant, and vascular tissue was colonized. The incompatible interaction was characterized by a nearly complete absence of haustoria, rapid host cell necrosis, and formation of many wall appositions by 4 h postinoculation. The pathogen rarely penetrated beyond the endodermis of the resistant host and colonization of vascular tissue was rare. Overall there were clear ultrastructural differences between compatible and incompatible interactions of soybean with P. sojae. These data support a strong correlation of resistance with host cell death, formation of wall appositions. and absence of root stele colonization.