Studies on the infection of Norway spruce roots by Heterobasidion annosum

Abstract

The interaction between the root rot fungus (Heterobasidion annosum) and seedling roots of Norway spruce was studied using scanning and transmission electron microscopy, light microscopy, and immunohistochemistry. Results showed that attachment to root surface and penetration of root epidermal walls were facilitated using pregerminated conidiospores as inocula. High inoculum concentrations (107 spores/mL) were also required for rapid colonization and establishment of the pathogen in the cortex. At lower spore concentrations, fewer hyphae were detected within the cortical region. The induction of hypersensitive response was a function of both time and spore concentration. Formation of infective structures on the root surface was evident 24 h after inoculation. Internal colonization of cortical tissues started 24–48 h postinoculation and reached the endodermis within 72 h. Severe destruction of stelar cells was observed 12 – 15 d postinfection. Cell invasion by the pathogen was correlated with formation of papillae and lignification in cortical cells. Increases in the enzymatic activity of defence-related proteins (chitinase and peroxidase) of infected roots were also evident. When vascular tissues were severely damaged disruption of endodermal cells was also recorded using light microscopy. The presence of H. annosum antigenic materials within cell wall boundaries suggests intercellular penetration by the pathogen. Key words: Norway spruce, Heterobasidion annosum, histochemistry, immunofluorescence, chitinase, peroxidase.