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No description available.

In the early 1900's the American chestnut (Castanea dentata (Marsh.) Borkh.) was nearly destroyed by the introduction of the orange-pigmented, chestnut blight fungus (Cryphonectria parasitica (Murr.) Barr). Chestnut blight is less severe in Europe, where hypovirulent (= reduced virulence) strains of the fungus are found to be associated with healing cankers. These European hypovirulent strains are infected with a dsRNA virus, Cryphonectria hypovirus 1 (CHV1), and have a white phenotype when grown in culture. Transmission of CHV1 in C. parasitica is limited by incompatibility between isolates in different vegetative compatibility (vc) types. In 1982-83, naturally formed blight cankers on American chestnut grafts, derived from large survivors, were inoculated with a mixture of four European (white) hypovirulent strains of C. parasitica. After 14 years the white strains were recovered throughout the inoculated grafts, which had low levels of blight damage. CHV1 had infected at least 45 new vc types, and was present in four different fungal colony morphology groups, including one type that had intermediate or partial pigmentation. However, CHV1 was unable to move throughout a single vc type within a natural canker. The objectives of this study were: 1) to determine the frequency and phenotypic diversity of CHV1-infected C. parasitica isolates recovered from stromata and canker tissue from natural cankers on the grafted American chestnut trees and artificially established cankers on forest American chestnuts; 2) to determine the presence or absence of CHV1 in intermediate-pigmented isolates recovered from the American chestnut research plots; 3) to investigate the roles of colony age, resistance to hypovirus infection, and functional mycelial units in the failure of CHV1 to move throughout a vc type of C. parasitica in vitro, and; 4) to examine the role of low temperatures and a high elevation topographic site on CHV1 survival within C. parasitica colonies in vivo and in vitro. The results indicated that there was no direct correlation between the amount of colony pigmentation and the presence of dsRNA. Within each of the three colony phenotype categories (pigmented, intermediate and white), several C. parasitica isolates tested positive for the presence of CHV1. This presence of CHV1 in intermediate isolates, coupled with the relatively large number of intermediate isolates collected from stromata on cankers, indicates that intermediate isolates may perform an important, and previously overlooked, function in biological control of chestnut blight. In this study, all CHV1 movement trials indicated that the age of the C. parasitica colony limited the movement of CHV1 throughout the colony. The majority of the CHV1 movement through a C. parasitica colony occurred between 0 and 7 days following challenge with an isogenic CHV1-infected strain. Isolation data using a lattice grid did not indicate a consistent pattern of CHV1 movement throughout a C. parasitica colony. Low temperatures associated with high altitude had no effect on hypovirus survival in vivo or in vitro. Additionally, no long-term C. parasitica resistance to CHV1 infection or movement was identified in this study. This research has identified new insights into CHV1 spread and survival that may be important in understanding the role of CHV1 in the biological control of chestnut blight. / Ph. D.

Chez les plantes, le RET (Root Extracellular Trap) est une structure cellulo-moléculaire jouant un rôle central dans la défense racinaire face aux stress abiotiques et biotiques. De nombreuses similitudes de composition ont été observées entre le RET et le NET (Neutrophil Extracellular Trap) du système immunitaire des mammifères, connu pour capturer et tuer certains microorganismes bactériens et fongiques. Le RET est composé de cellules frontières et de leurs sécrétions (composés de haut et de bas poids moléculaire) comprenant des polysaccharides de la paroi cellulaire, des protéoglycannes et des métabolites secondaires. Il contient également des protéines antimicrobiennes et de l'ADN extracellulaire, tout comme le NET. Dans le cadre de mon projet de thèse, nous avons caractérisé la composition moléculaire et la structuration de cette entité de défense chez deux légumineuses, le soja (Glycine max (Merr) L.) et le pois (Pisum sativum L.), par des approches d’imagerie cellulaire photonique et électronique. Nous avons également étudié l’impact du RET du soja sur des pathogènes telluriques, à savoir Phytophthora parasitica et Aphanomyces euteiches. Nous avons ainsi pu mettre en évidence la présence de différents morphotypes de cellules frontières et de mucilage au sein du RET de soja et de pois. Pour la première fois, nous avons montré la présence d’hétéromannanes, de xyloglucane et de cellulose dans le RET, formant une ossature stabilisant le mucilage et reliant les cellules frontières entre elles. Ces polysaccharides structuraux semblent être essentiels à l’intégrité structurale et fonctionnelle du RET. Enfin, nos résultats ont montré que le RET de soja était impliqué dans la défense précoce de la racine contre P. parasitica. Cette étude apporte de nouvelles connaissances relatives à la composition moléculaire et la structure du RET, nous amenant ainsi à comparer le RET à d’autres modèles que le NET des mammifères, tels que les biofilms bactériens et les mucilages de graines. En effet, de nombreuses similitudes existent entre ces différents complexes en termes de composition et de fonctionnement, qui méritent d’être explorer plus en détail dans l’avenir. / In higher plants, the RET (Root Extracellular Trap) is a complex made up of border cells and secretions, released by root tips and believed to play a central role in biotic and abiotic stress tolerance. This structure is quite similar to the Neutrophil Extracellular Trap (NET) known as one of the first lines of defense in mammals, able to trap and kill microbial pathogens. RET secretions consist of high and low-molecular weight compounds including cell wall polysaccharides, proteoglycans and secondary metabolites. They also contain a variety of anti-microbial proteins and extracellular DNA much like the NET. During my thesis work, we investigated the release and morphology of root border cells in soybean (Glycine max (Merr) L.) using light and scanning electron microscopy. The molecular composition of the mucilage was also investigated using immunocytochemistry, anti-cell wall glycan antibodies and confocal microscopy. Immunocytochemistry was also applied to pea (Pisum sativum L.) border cells and secretions to examine the occurrence of specific polysaccharides. We also studied the impact of soybean RET on the soilborne pathogens, Phytophthora parasitica and Aphanomyces euteiches. Our findings showed that root tips of soybean released three border cell morphotypes all of which secreted substantial amounts of mucilage. Immunocytochemical data showed that mucilage was enriched in pectin and the two hemicellulosic polysaccharides xyloglucan and heteromannan. Mucilage also contained cellulose, histone and extracellular DNA. Interestingly, the structural polysaccharides formed a fibrous network surrounding the cells and holding them together, supporting their role in maintaining mucilage architecture and integrity. In addition, we found that xyloglucan and cellulose were also secreted into the mucilage of pea, connecting border cells together. Finally, our findings revealed that RET prevented P. parasitica zoospores from colonizing soybean root tip, by stopping their penetration and inducing their death. Overall the study revealed novel insights into the composition, structure and function of plant RETs. Currently, the RET is much less studied than its mammal counterpart, the NET, but structural and functional similarities exist between these two traps. Interestingly, similarities do also exist between the RET and other important biological complexes, including bacterial biofilms and seed mucilage, that deserve to be further investigated and compared in the context of immunity.