Quantification of host and pathogen DNA and RNA transcripts in the interaction of Norway spruce with Heterobasidion parviporum

Colonization levels of the pathogenic fungus Heterobasidion parviporum in tissue cultures and bark lesions of 33-year-old Norway spruce trees was monitored using quantitative PCR. Expression of selected host and fungal transcripts was also measured in the same material using quantitative reverse transcriptase (RT)-PCR. The host genes phenylalanine ammonia lyase, peroxidase, glutathione-S-transferase, germin-like protein and class IV chitinase were all induced in tissue cultures in response to infection. Higher transcript levels of phenylalanine ammonia lyase and peroxidase were detected in the more resistant clone 589 than in the less resistant clone 409. Expression of a class IV chitinase was higher in clone 409 than in 589. Several of the fungal genes assayed were differentially expressed during colonization, including putative glutathione-S-transferases, laccase, cellulase, cytochrome P450 and superoxide dismutase genes. The transcriptional responses suggest an important role for the antioxidant systems of both organisms. The changes in the levels of all the examined host and pathogen transcripts were similar between tissue cultures and bark over their overlapping colonization range. This suggests that tissue cultures can be used to select candidate host defense and fungal pathogenicity genes operative in tissues composed of primarily living cells, such as phloem, in the Norway spruce/H. parviporum pathosystem.