Graduate Thesis Or Dissertation
 

Histological comparisons of needle tissues of four species of white pine infected by Cronartium ribicola

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  • Hypersensitive response-like (HR-like) needle reactions to infection by the white pine blister rust pathogen, Cronartium ribicola, have been reported for several species of five needle pines native to western North America. The best-studied examples are in Pinus monticola and P. lambertiana. In these species a "needle spot" phenotype has been identified in which HR-like needle reactions are related to disease resistance that is conditioned by a major gene. Conventionally it is believed that the HR-like needle reactions in resistant pines prevent spread of the pathogen to vascular stem tissue by HR mechanisms commonly seen in other plant-pathogen incompatibility interactions, i.e. a rapidly induced plant cell death and subsequent localized tissue necrosis. The dead cells present a barrier to colonization by biotrophic pathogens and cause degeneration of fungal hyphae preventing further pathogen colonization. Structural analyses of early C. ribicola needle colonization in resistant Pinus spp. have shown, however, that these symptoms and their underlying physiology are fundamentally different from the clearly defined HR described in other host-pathogen systems. Contrary to the pattern of HR responses seen in most incompatible host reactions, onset of needle lesions was first seen several weeks after initial entry of C. ribicola. We observed extensive proliferation of fungal hyphae in the host and penetration of the needle endodermis and vascular tissue by the pathogen prior to the onset of a discernable HR or cell necrosis. The amount of fungal tissue present and progress of needle colonization was similar for both resistant and susceptible Pinus spp. Therefore, typical HR does not appear to function in needles as the mechanism of disease resistance in the "needle reaction" phenotype.
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file details SweeneyKat2013.pdf 2017-08-17 Público Baixar