Analysis of Norway Spruce Rust-Resistance

Norway spruce trees in the subalpine forests of the European Alps are increasingly attacked by the needle rust Chrysomyxa rhododendri. This biotrophic parasite undergoes a complex life cycle with a host shift between rhododendrons (Rhododendron spp.) and Norway spruce (Picea abies). It causes yellowing and defoliation of the current-year needles in summer and, in consequence, reduced growth or even dieback of infected trees. The susceptibility to infection varies between individual trees, and rarely resistant specimens can be found. Previous studies rebutted earlier hypotheses that this rust-resistance could be based on tree phenology (i.e. escaping infection by flushing after spore flight). However, there is evidence that plant defence compounds (i.e. fungicidal needle substances like phenols) or mechanical barriers (cuticula) may play a role. Main goals of this project are: (i) Development of new methods to quantify Chrysomyxa infections. Most important, drone flights will enable an improved quantification of infection rates at the tree and local canopy level. (ii) Comparison of the cuticula of susceptible and resistant trees via microscope imaging techniques. (iii) Analysis of phenolic needle compounds connected with resistance. This analysis will also focus on developmental changes and heritability of phenolic patterns. (iv) Genetic analyses. The phenotypic variation of resistance and the expression and variation of candidate resistance genes will be studied. Furthermore, the occurrence of the pathogen will be verified with molecular markers. Resistant and control plants will be tested as mature trees, rooted cuttings and as seedlings derived from the mature trees. The project is based on cooperation of research teams of the Department of Botany, University of Innsbruck, Federal Research and Training Centre for Forests, Natural Hazards and Landscape, BFW Vienna and University of Natural Resources and Life Sciences (BOKU) Vienna. It is also supported by the Forest Administration Tyrol, which provides help with search for resistant trees, seed collection and growth of cuttings. Project outcomes are important to better understand host- pathogen interactions and underlying resistance mechanisms. Genetic analyses and diagnosis tools will be important for future strategies to obtain and use resistant plant material for afforestation in Tyrol and other European regions. Sustainable management of mountain ecosystems is essentially important to society, especially in the face of the looming challenges predicted by climate change scenarios.

Analysis of Norway spruce rust resistance Norway spruce trees in the subalpine forests of the European Alps are increasingly attacked by the needle rust Chrysomyxa rhododendri. This biotrophic parasite undergoes a complex life cycle with a host shift between rhododendrons (Rhododendron spp.) and Norway spruce (Picea abies). It causes yellowing and defoliation of the current-year needles in summer and, in consequence, reduced growth or even dieback of infected trees. The susceptibility to infection varies between individual trees, and rarely resistant specimens can be found. Previous studies rebutted earlier hypotheses that this rust-resistance could be based on tree phenology (i.e., escaping infection by flushing after spore flight) or enhanced mechanical barriers (epidermis and cuticula). The project thus focussed on plant defence compounds (i.e., fungicidal needle substances like phenols) and underlying gene regulation mechanisms. Main goals of the project included: (i) Development of new methods to quantify Chrysomyxa infections. Drone flights enabled the acquisition of aerial photos of individual tree crowns and groups of trees, which were evaluated with a newly developed image analysing method to quantify the infection degree in an accurate and objective way. Coupled analyses of stem radial increment showed strong growth declines in infected trees. (ii) Analysis of phenolic needle compounds. Measurements revealed strong concentration changes of specific compounds during needle development and as a response to infection. Phenolic profiles were associated with the rust resistance. (iii) Genetic analyses. Gene expression patterns of infected needles showed a specific, locally restricted, response of the tree to infection two to three weeks following spore exposure. Identified candidate resistance genes were confirmed in adult trees at high-alpine sites. Combined RNA-Seq and biochemical data suggest that Norway spruce response to infection by C. rhododendri is restricted locally and primarily activated between 9 and 21 days after infestation, involving a potential isolation of the fungus by a hypersensitive response (HR) associated with an activation of phenolic pathways. Furthermore, we showed that drought stress, in contrast to fertilization, does not increase spruce susceptibility to needle rust and that infections do not impair wood anatomy. The project was based on cooperation of research teams of the Department of Botany, University of Innsbruck, and the Federal Research Centre for Forests, BFW Vienna and was supported by the Forest Administration Tyrol. Project outcomes improve our understanding of host-pathogen interactions and resistance mechanisms in forest trees and facilitate the monitoring of infection intensities. They will also be important for future strategies to obtain and use resistant plant material for afforestation of high-alpine sites and therefore contribute to the sustainable management of mountain ecosystems.