ULTRASTRUCTURE AND CYTOCHEMISTRY OF EARLY STAGES OF COLONIZATION BY GREMMENIELLA-ABIETINA IN PINUS-RESINOSA SEEDLINGS

This paper provides details on the infection processes at the ultrastr uctural level in Pinus resinosa Ait. seedlings during early stages of colonization by Gremmeniella abietina (Lagerb.) Morelot. Different gol d-conjugated enzymes and antibodies were used to cytochemically locali ze cellulose, pectin, fungal laccase, and the pathogen cells in host t issues. Gremmeniella abietina penetrated into the host through stomata of the short shoot bracts and sparsely colonized both intercellular a nd intracellular areas of the bract tissues. The colonizing hyphae usu ally had a thick wall surrounded by an extracellular sheath composed o f fibrillar material. Microhyphaelike cells were observed as having pe netrated host cell walls. The fungal cells (except the extracellular s heath), even when embedded in cellulosic or pectic material of host ti ssues, did not appear to contain cellulose or pectin. We suggest that G. abietina is able to degrade cellulose and pectin and that phenoloxi dases secreted by the pathogen could be involved in host cell wall deg radation. The results indicate that the extracellular sheath of G. abi etina is implicated in host-pathogen interactions such as attachment o f hyphae to the host surface and cell wall degradation during coloniza tion of host tissues.