1.2.32
HYPERSENSITIVE RESPONSE (HR) ASSOCIATED CELL DEATH IN CONIFERS IS NOT A SUCCESSFUL RESISTANCE BARRIER AGAINST NECROTROPHIC PARASITES

FO ASIEGBU, M JOHANSSON and J STENLID

Department of Forest Mycology and Pathology, Swedish University of Agriculture, Box 7026, 750 07 Uppsala, Sweden

Background and objectives
Studies with crop plants have shown that host resistance to biotrophic fungal pathogens is usually accompanied by some detectable reaction of the plant such as HR [1, 2]. The question is, how might a plant react following challenge with necrotrophic parasites that rely on dead cells for their nutrition? The present study therefore is aimed at understanding the kind of strategy used by living tree tissues in their defence reactions during interaction with saprotrophic and necrotrophic microbes.

Materials and methods
Using Scots pine seedlings as an experimental model, we investigated general resistance and recognition patterns of living conifer tissues challenged in vitro with either parasitic, saprotrophic, epiphytic or mutualistic microbial species.

Results and conclusions
Three distinct resistance and recognition patterns were observed. In one set, a positive hypersensitive type of browning reaction (HR) that restricted vascular penetration in the seedlings when inoculated with the saprotroph Marasimus androsaceus or the mycorrhiza-associated bacterium Pseudomonas fluorescens was observed. The second pattern was a negative recognition, a kind of HR in which the host defence system was ultimately ineffective, and was typically observed on 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 neutral recognition, observed on seedlings showing no observable HR symptoms, 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 (Bacillus sp.). Since HR is often linked to rapid accumulation of H2O2, increases in peroxidase activity were determined. Only seedlings challenged with the S-type of H. annosum caused a systemic increase in peroxidase levels of needles, shoot and roots. Except for S. granulatus, P. croceum, C. versicolor, T. aureoviride, M. androsaceus and Bacillus sp., all other fungal species and also Pseudomonas bacteria caused a 2-4-fold increase in peroxidase level of inoculated roots, suggesting that a relationship exists between degree of HR and the ratio of peroxidase induction. But neither HR nor peroxidase activity was correlated with increased resistance against disease development.

References
1. Heath MC, 1980. Annual Review of Phytopathology 18, 211-236.
2. Keen NTK, 1990. Annual Review of Genetics 24, 447-463.