REVERSIBLE CYTOPLASMIC REARRANGEMENTS PRECEDE WALL APPOSITION, HYPERSENSITIVE CELL-DEATH AND DEFENSE-RELATED GENE ACTIVATION IN POTATO PHYTOPHTHORA-INFESTANS INTERACTIONS

We used video microscopy techniques as a tool for live examination of the dynamic aspects of plant/fungus interactions. Early, dynamic respo nses of epidermal midrib cells of leaves from a potato cultivar (Solan um tuberosum L. cv. Datura) carrying resistance gene R1 to Phytophthor a infestans (race 1: compatible interaction, race 4: incompatible inte raction) were monitored. Similar responses were observed in both types of interaction, ranging from no visible reaction of invaded plant cel ls to hypersensitive cell death. The overall defense response of each individual cell exhibited a highly dynamic behavior that appeared to b e tightly coordinated with the growth of the fungus. Initial localized reactions, including major rearrangements within the cytoplasm, occur red directly at the fungal penetration site, where rapid apposition of autofluorescent material and callose took place. If fungal invasion s topped at this stage, the host cell restored its normal cytoplasmic ac tivity and survived. Hypersensitive cell death occurred only when fung al growth had proceeded to the formation of a clearly identifiable hau storium. In such cases, cytoplasm and nucleus conglomerated around the intracellular fungal structure, followed by a sudden collapse of the whole conglomerate and an instantaneous collapse of the fungal haustor ium. Only small quantitative differences between the compatible and in compatible interactions of the two fungal races were observed for thes e early responses of epidermal cells. In the incompatible interaction, a slightly larger number of epidermal cells responded to fungal attac k. More pronounced quantitative differences between compatible and inc ompatible interactions occurred upon fungal invasion of the mesophyll. These differences in the number of responding cells were not reflecte d at the level of gene expression: the spatial and temporal activation patterns of two defense-related genes, encoding phenylalanine ammonia -lyase and pathogenesis-related protein 1, were similar in both types of interaction.