DEFENSE-RELATED REACTIONS OF SEEDLING ROOTS OF NORWAY SPRUCE TO INFECTION BY HETEROBASIDION-ANNOSUM (FR.) BREF

Norway spruce seedlings were cultivated in sterile conditions. Roots were infected with a concentration series of germinating conidiospores of Heterobasidion annosum (10(1)-10(6) ml(-1)). In other experiments, roots were treated with either mycelial preparations of H. annosum, other wood inhabiting fungi, with protein fractions of culture filtrates of H. annosum, or with chemical elicitors. Successive steps observed during infection were necrosis, formation of phenolics and increasing lignification of cortex and endodermis, colonization of meristem and finally of the stele. High spore concentrations caused necrosis and invasion within 48-72 h; these processes were delayed at low spore concentrations. Lyophilized culture filtrates of H. annosum caused a greater hypersensitive response than protein fractions but less than NaCl, Polygalacturonic acid or ethephon. Mycelial homogenate from nine other wood inhabiting fungi (saprophytes/parasites) induced a hypersensitive response to various extents but this was not correlated to their degree of cross-reactivity with a polyclonal antibody to H. annosum [enzyme-linked immunosorbent assay (ELISA)]. Peroxidase activity increased (two-threefold) in roots infected with H. annosum and one acidic isozyme was considered responsible for the increase in peroxidase. Using immunohistochemical and enzyme staining, peroxidase was found mainly in the cortical/endodermal regions of roots. Cytochemical labelling using anti-peroxidase and immunogold demonstrated increased peroxidase activity in cell walls, papillae and uninvaded middle lamellar cell corners of infected roots.