Reactions of Pinus sylvestris (Scots Pine) Root Tissues to the Presence of Mutualistic, Saprotrophic and Necrotrophic Micro-organisms

Using Scots pine seedlings as experimental hosts, the general reactions and recognition patterns of living conifer tissues challenged in vitro with either parasitic, saprotrophic, epiphytic or mutualistic microbial species were investigated. Three distinct reaction and recognition patterns were observed. In one, a necrosis type of browning reaction occurred that restricted vascular penetration of the seedlings when inoculated with a saprotroph (Marasmius androsaceus) or a mycorrhizal-associated bacterium (Pseudomonas fluorescens). The second pattern, a kind of necrosis in which the host defence system was ultimately ineffective, was typically observed in seedlings challenged with the S or P types of the root rot fungus (Heterobasidion annosum), the fine root parasite (Fusarium avenaceum) or the saprotrophic wood decay fungus (Phlebia gigantea). The third type, described as non-recognition, was observed on seedlings showing no necrosis and was particularly common with seedlings inoculated with either mycorrhizal fungi (Suillus granulatus, Piloderma croceum) or saprotrophs (Trichoderma aureoviride, Coriolus versicolor), or an unidentified rod-shaped bacterial isolate. Since necrosis is often linked to rapid accumulation of H2O2, increases in peroxidase activity were determined. Only seedlings challenged with the S-type of H. annosum had a systemic increase in peroxidase levels of needles, shoot and roots. Similarly, only H. annosum, F. avenaceum, P. gigantea and the Pseudomonas bacterium caused a two- to four-fold increase in the peroxidase level of inoculated roots, results suggesting that a relationship exists between the degree of necrosis and the ratio of peroxidase induction. But the higher levels of peroxidase activity was not correlated with increased resistance to disease development.