1.3.2
CHESTNUT BLIGHT: MONITORING THE HOST RESPONSE WITH HETEROLOGOUS cDNA PROBES

R. SCHAFLEITNER and E. WILHELM

Austrian Research Centers Seibersdorf, A-2444 Seibersdorf, Austria

Background and objectives
Cryphonectria parasitica, the chestnut blight pathogen, exists in two forms: virulent and hypovirulent. Virulent C. parasitica is a potent pathogen of chestnut, invading the bark through wounds, causing lesions and destroying the cambium. Infested trees decline due to dysfunction of water and nutrient transport. C. parasitica harbouring a double-stranded RNA virus in the cytoplasm, called hypovirus, represents the second type of this fungus. It is a weak pathogen and, because of its capacity to transmit the hypovirus to vegetatively compatible, virulent C. parasitica, thereby converting the virulent into the hypovirulent phenotype, it can be used as a biocontrol against chestnut blight [1]. Chestnut defence responses were investigated at the protein level, and activities of pathogenesis-related proteins were shown to be increased by hypovirulent C. parasitica [2]. To understand more about the defence mechanisms of the tree, inoculation experiments on in vitro chestnut plantlets were performed and the transcription of pathogenesis-related genes was monitored.

Results and conclusions
Northern blots, probing RNA of fungus-treated chestnut with cDNA probes of tobacco pathogenesis-related genes, were performed 3-7 days post-inoculation. Generally, pathogenesis-related transcripts were found to be more strongly accumulated in plants treated with hypovirulent than with virulent fungus. Virulent C. parasitica induced the transcription of pathogenesis-related genes, if at all, later than the hypovirulent fungus. Acid class III chitinase was found to be expressed most strongly in wounded plants on day 3.

These results show that the host response is different for the interactions with the virulent and hypovirulent forms. Therefore we can conclude that the weaker pathogenicity of the hypovirulent fungus also involves the recognition and defence mechanisms of the tree.

References
1. Anagnostakis SL, 1995. Advances in Botanical Research 21, 126-145.
2. Schafleitner R, Wilhelm E, 1998. Physiolical and Molecular Plant Pathology, in press.