Epsilon Archive for Student Projects

Abstract

As the most important species in Europe’s forest industry, Norway spruce [Picea abies (L.) Karst.] has a very high economical impact. Unfortunately, the forest industry has yearly losses of approximately 800 million euro, because many Norway spruce stands suffer infection by one of the tree’s major pathogens Heterobasidion parviporum (Fr.) Niemelä & Korhonen.

H. parviporum is a basidiomycete fungus. It belongs to a species complex called Heterobasidion annosum sensu lato, which includes four other Heterobasidion species besides H. parviporum, with other but slightly overlapping host ranges. To infect Norway spruce, H. parviporum spores need a surface presenting fresh wood, e. g. freshly occurring wounds in stems and roots or stump surface of a tree, to germinate and begin growing into the tree. Once H. parviporum has reached the inner column (heartwood) of the tree, it spreads vertically in the tree’s heartwood towards the crown and root system. When it reaches the root system, H. parviporum can use root-to- root contacts between spruce trees to spread from tree to tree. During its infection process it decays the heartwood as well as the roots. Therefore, the disease highly reduces timber quality and enhances the changes of storm breakage of infected Norway spruce trees.

If the infection progresses H. parviporum will come into contact and interact with the sapwood, consisting not only of dead cells (tracheids) but also of living ray and parenchyma cells. Norway spruce reacts to this penetration by forming a defense barrier, called the reaction zone, which is highly compact and enriched in secondary metabolites that have the ability to restrict the invaders growth. Thus, the secondary metabolites encased in the reaction zone were shown to hinder fungal growth. Interestingly, recent research could correlate the presence of a reaction zone with reduced diameter growth of the trees. The authors suggested that formation of the reaction zone goes along with an enhanced investment in secondary metabolism instead of biomass production, with the reaction zone acting as a carbon sink, allocating carbon from neighboring tissue in H. parviporum challenged trees. In addition, it was suggested that the increase in secondary metabolite production will be due to changes in the expression of genes that encode proteins which function in the production of the enriched metabolites. Previous studies thus analysed the expression levels of these genes by in various Picea species which had been inoculated with H. parviporum in bark. It could be shown that various genes involved in secondary metabolism respond to H. parviporum attack in respect to their expression profiles. Interestingly, wounding tissue without adding H. parviporum revealed the same but weaker response.

In this study, we investigated the difference of Norway spruce’s defense response in a radial direction analysing i) bark, primary xylem and sapwood tissue bordering the reaction zone in trees inoculated with H. parviporum into the sapwood compared to trees wounded in the sapwood and ii) sapwood adjacent to the reaction zone compared to sapwood distal of the reaction zone in naturally infected and healthy trees. We choose to determine the expression of genes encoding enzymes representative for several secondary metabolite pathways and production of precursors. Further we studied gene expression of genes associated with the plant hormone synthesis of the pathogen defense response related plant hormones jasmonic acid (JA) biosynthesis and its signaling and the ethylene biosynthesis.

In comparison with previous studies, our results indicate that Norway spruce defense shows an up-regulation of the secondary metabolism and the hormonal response at the position of the H. annosum s. l. challenge, but the response is similar regardless of the tissue type. Comparison of H. parviporum to wounding treatment suggests that carbon distribution within the secondary metabolite pathways rather than the amount of carbon allocated from the energy metabolism is important. Further, we propose that distal tissues react to the challenge with H. parviporum by decreasing their own secondary metabolite production and providing carbon at the inoculation site. However, our findings need further testing using chemical analysis in Norway spruce inoculated with H. parviporum at different time points with a high spatial resolution.