Physiological and molecular analysis of the interaction between the conifer pathogen,Heterobasidion annosum s.l. and Phlebiopsis gigantea

Prophylactic stump treatment with Phlebiopsis gigantea has been used for a long time in the biological control of the conifer pathogen, Heterobasidion annosum sensu lato. However, the mechanisms underlying the biocontrol process are poorly understood. A total of 64 isolates of P. gigantea from different geographical regions were screened for fungal traits (growth rate, enzyme production, wood decay capabilities and antagonism) important for the biocontrol properties. The genomic sequence and the expression profile of P. gigantea hydrophobin encoding genes 1 and 2 in a selected set of isolates of the biocontrol fungus was analysed. Additionally, the distribution and the regulatory patterns of different hydrophobin encoding genes in the ecologically important fungi, P. gigantea and H. irregulare were investigated alongside the evolutionary forces driving hydrophobin gene evolution. The results show that different growth media had a huge influence on the outcome of the interaction between P. gigantea and H. annosum s.s. and the ability to degrade the different structural components of wood could partly explain the higher competitive advantage over H. annosum s.s. The antagonistic ability of P. gigantea also correlated positively with the transcript levels of hydrophobin 1 and 2 encoding genes (Pgh1 and Pgh2) but there was no relationship between the antagonistic ability and the sequence variants of either Pgh1 or Pgh2. Furthermore, the regulatory patterns of Pgh1 and Pgh2 suggest a role in the formation of aerial hyphea during the growth and development of P. gigantea. Hydrophobin encoding genes are redundant in both fungal species studied in the thesis research. However, in H. annosum s.s., each gene coding for hydrophobins seemed to be regulated by different environmental factors. Hydrophobin encoding genes were found to have witnessed a considerable expansion in both P. gigantea and H. annosum s.l. as well as in other basidiomycetes while a massive contraction of the hydrophobin encoding genes has occurred in the ascomycetes. Evidence of positive selection was also observed in P. gigantea and H. annosum s.l. hydrophobins and the hydrophobins from other fungal species. Finally, to understand the effect of secondary metabolites produced by P. gigantea on the gene expression profile of H. annosum s.s., transcriptomics analysis was carried out using microarray expression method. Also macroarray analysis was used to compare the transcriptomics profile of different strains of P. gigantea (competitively effective wild type, less competitively effective wild type and their progeny) against the commercial isolate, Rotstop F when cultivated on artificial growth medium. The metabolites from P. gigantea effectively repressed some genes involved in diverse metabolic pathways in H. annosum s.s. Aditionally, several genes were found to be uniquely expressed in the progeny strain of P. gigantea, an indication that breeding could serve as an alternative for improving the P. gigantea isolates for a better biocontrol property. Generally, the results reported in this study have highlighted some of the physiological, biochemical and molecular mechanisms governing the biological control of the conifer pathogen, H. annosum s.l., by P. gigantea.