Ultrastructure and cytochemistry of early stages of colonization by Gremmeniella abietina in Pinus resinosa seedlings

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Abstract
This paper provides details on the infection processes at the ultrastructural level in Pinus resinosa Ait. seedlings during early stages of colonization by Gremmeniella abietina (Lagerb.) Morelot. Different gold-conjugated enzymes and antibodies were used to cytochemically localize cellulose, pectin, fungal laccase, and the pathogen cells in host tissues. Gremmeniella abietina penetrated into the host through stomata of the short shoot bracts and sparsely colonized both intercellular and intracellular areas of the bract tissues. The colonizing hyphae usually had a thick wall surrounded by an extracellular sheath composed of fibrillar material. Microhyphaelike cells were observed as having penetrated host cell walls. The fungal cells (except the extracellular sheath), even when embedded in cellulosic or pectic material of host tissues, did not appear to contain cellulose or pectin. We suggest that G. abietina is able to degrade cellulose and pectin and that phenoloxidases secreted by the pathogen could be involved in host cell wall degradation. The results indicate that the extracellular sheath of G. abietina is implicated in host–pathogen interactions such as attachment of hyphae to the host surface and cell wall degradation during colonization of host tissues. Key words: Gremmeniella, Pinus, infection processes, cell wall degradation, extracellular fungal sheath, gold labelling.

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