Abstract.We used video microscopy techniques as a tool for live examination of the dynamic aspects of plant/fungus interactions. Early, dynamic responses of epidermal midrib cells of leaves from a potato cultivar (Solanum tuberosum L. cv. Datura) carrying resistance gene R1 to Phytophthora infestans (race 1: compatible interaction, race 4: incompatible interaction) were monitored. Similar responses were observed in both types of interaction, ranging from no visible reaction of invaded plant cells to hypersensitive cell death. The overall defense response of each individual cell exhibited a highly dynamic behavior that appeared to be tightly coordinated with the growth of the fungus. Initial localized reactions, including major rearrangements within the cytoplasm, occurred directly at the fungal penetration site, where rapid apposition of autofluorescent material and callose took place. If fungal invasion stopped at this stage, the host cell restored its normal cytoplasmic activity and survived. Hypersensitive cell death occurred only when fungal growth had proceeded to the formation of a clearly identifiable haustoriurn. 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 haustorium. Only small quantitative differences between the compatible and incompatible interactions of the two fungal races were observed for these early responses of epidermal cells. In the incompatible interaction, a slightly larger number of epidermal cells responded to fungal attack. More pronounced quantitative differences between compatible and incompatible interactions occurred upon fungal invasion of the mesophyll. These differences in the number of responding cells were not reflected 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.Dedicated to Professor Peter Sitte, Freiburg, Germany, on the occasion of his 65th birthday Abbreviations: PAL = phenylalanine ammonia-lyase; PR 1 = pathogenesis-related protein 1; FITC = fluorescein isothiocyanateCorrespondence to: Elmon Schmelzer; FAX: 49 (221) 5062313

Mycology 12, 275-283. The effect of six different substrates on basidiospore germination and differentiation of infection structures was investigated in three species of Uromyces:U. appendiculatus var. appendiculatus, U. viciae-fabae, and U. vignae. Light and fluorescent microscopic observations were made after 3 days of incubation on the following substrates: 2 or 5% water agar, glass slide, collodion membrane, nitrocellulose sheet, and isolated host cuticle. Infection structure development up to the formation of a primary hypha was observed on hard substrates, such as 5% water agar, nitrocellulose sheet, and on isolated host cuticle. Addition of oil to the collodion membrane did not significantly increase the differentiation rate. The morphology and nuclear condition of these basidiospore infection structures were similar to those observed in host tissue. For the three directly penetrating species we studied, it appears that basidiospores respond to an unspecific stimulus, such as the thickness of host cuticle or the hardness of an artificial substrate. o 1988 Academic Press, Inc.

The hypersensitive reaction represents one of the major means by which plants actively defend themselves against infection by pathogenic bacteria, fungi, viruses, and nematodes. This complex defense reaction, often associated with the synthesis of phytoalexins (antimicrobial secondary metabolites), involves at the cellular level highly dynamic cytoplasmic rearrangements, rapid metabolic changes, and finally cell death. It also correlates with the rapid and transient activation of various defense-related genes in a region of tissue surrounding infection sites and later, with the systemic increase in expression of a number of other genes. Examination of the reactions of individual living cells of potato leaves infected with Phytophthora infestans enabled the comprehensive description of the dynamic aspects of all stages of the defense response. Cytochemical investigations, employing cultured cells of parsley infected with P. infestans as a versatile model system, have contributed to a better understanding of cytoplasmic and metabolic processes occurring during the defense response, and suggest that hypersensitive cell death requires the preceding activation of respiration and specific metabolic pathways. Key words: defense responses, defense-related genes, hypersensitive reaction, programmed cell death.

SUMMARYNine fluorescein isothiocyanate-labeiled leclins with affinity towards different carbohydrates were used to probe the surface carbohydrates of infection structures in vitro, derived from urediniospores and basidiospores of an autoecious rust species. Uromyces viciae-fabae (Pers,) Schroet., and of a heteroecious rust species, Lhomyces rumicis (Schum,) Wint, Lectin binding was quantified by measuring fluorescence photometrically.All lectins bound in a characteristic pattern to the infection structures of the respective spore types of each rust fungus. Differences were especially obvious between those infection structures normally produced inside leaf tissue, namely the urediniospore-derived substomatal vesicles with infection hyphae, and the basidiospore-derived intraepidermal vesicles. The dikaryotic stage and the monokaryotic stage of the heteroecious fungus differed mainly in their affinity for the lectins from Bandeira simplicifolia and Lotus tetragonolobus.A statistical anaiysis comparing the binding oi the iectins to infection structures of two rust fungi suggested that cell surface composition is determined by nuclear condition. The monokaryotic stages of both rust fungi have a higher degree of similarity than the dikaryotic and monokaryotic stages of the same rust fungus.