Year: 2013
Type: dissertation
Abstract: The basidiomycete white-rot fungus Heterobasidion annosum sensu lato (s.l.) is a species complex comprising five species considered to be the most economically important pathogens of conifer trees in the northern hemisphere. The infection of new wood substrate is mediated by basidiospores, which land on the stump surface of a felled tree. After spore germination, the fungal mycelia actively colonise the stump and spread to new healthy trees by root-to-root contact. To start a new infection cycle, H. annosum s.l. must counteract the adverse environmental factors (abiotic stresses) at the stump surface. Moreover, active wood degradation requires the ability to detoxify the high levels of fungistatic and fungitoxic compounds (such as phenolics) that naturally accumulate in the tree wood tissue as a defence against pathogen attack. The availability of the genome sequence of the H. irregulare species allowed us to investigate the conservation of the intracellular pathways that are responsible for the abiotic stress response and cellular adaptation and proliferation. Using Saccharomyces cerevisiae as a model organism in which many of these pathways have been well characterised, we annotated all the conserved components of the mitogen activated protein kinase (MAPK) pathways in H. irregulare, namely, those involving the pheromone FUS3/KSS1, the high osmolarity gene HOG1, the cell integrity gene MPK1, calcium/calcineurin signalling, and the cAMP pathway. To better understand the H. annosum sensu stricto (s.s.) adaptation during abiotic stress and wood degradation, we investigated the general transcriptional profiles under several abiotic stresses (osmotic, oxidative, temperature, and nutrient starvation) and during growth on different pine woody materials (pine bark, sapwood, and heartwood). The results for abiotic stresses indicated the activation of genes involved in signalling (for example, protein kinase and transcription factors during starvation) but also genes involved in toxic substance detoxification and membrane transporters (cytochrome P450 and Major Facilitator Superfamily, MFS-1, respectively, in cold stress). During saprotrophic growth on different pine wood materials, a dramatic induction of several glycosyl hydrolase (GH) genes was observed. Some of these genes (for example, GH61) were specifically induced, mainly in pine heartwood, while others demonstrated less tissue specificity and were generally expressed during saprotrophic growth in all woody materials. During saprotrophic growth on pine lignocellulose material, several genes involved in lignin degradation, such as multi-copper oxidases (MCOs) and oxidoreductases, were also strongly induced. The central MAPK of one of the pathways involved in adaptation to abiotic stress (the HOG pathway) was further characterised. The H. annosum s.s. HaHOG1 gene was cloned and functionally studied to investigate its role in osmotic and oxidative stress response in this fungus. The HaHOG1 gene restored the function of the homologous HOG1, and the protein translocated to the cell nucleus under osmotic conditions in the S. cerevisiae heterologous host. Furthermore, HaHog1p was strongly phosphorylated in the presence of high concentrations of NaCl, KCl, and H2O2. These results suggest that the HOG pathway is activated when H. annosum s.s. is challenged with osmotic and oxidative stressors. This study sheds light on some adaptive mechanisms that characterise the growth of H. annosum s.s. under several conditions. Finally, this work provides new data at the transcriptome level to help identify genes that are activated during wood degradation and response to abiotic stresses.
Source: Työväentutkimus Vuosikirja
Author: Tommaso Raffaello
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
Funder: Helsingin Yliopisto
OpenAlex ID: W1585695842