Low Water Availability Increases Necrosis in Picea abies after Artificial Inoculation with Fungal Root Rot Pathogens Heterobasidion parviporum and Heterobasidion annosum

Beyond the surface: exploring the mycobiome of Norway spruce under drought stress and with Heterobasidion parviporum

The mycobiome, comprising fungi inhabiting plants, potentially plays a crucial role in tree health and survival amidst environmental stressors like climate change and pathogenic fungi. Understanding the intricate relationships between trees and their microbial communities is essential for developing effective strategies to bolster the resilience and well-being of forest ecosystems as we adopt more sustainable forest management practices. The mycobiome can be considered an integral aspect of a tree's biology, closely linked to its genotype. To explore the influence of host genetics and environmental factors on fungal composition, we examined the mycobiome associated with phloem and roots of Norway spruce (Picea abies (L.) Karst.) cuttings under varying watering conditions. To test the "mycobiome-associated-fitness" hypothesis, we compared seedlings artificially inoculated with Heterobasidion parviporum and control plants to evaluate mycobiome interaction on necrosis development. We aimed to 1) identify specific mycobiome species for the Norway spruce genotypes/families within the phloem and root tissues and their interactions with H. parviporum and 2) assess stability in the mycobiome species composition under abiotic disturbances (reduced water availability). The mycobiome was analyzed by sequencing the ribosomal ITS2 region. Our results revealed significant variations in the diversity and prevalence of the phloem mycobiome among different Norway spruce genotypes, highlighting the considerable impact of genetic variation on the composition and diversity of the phloem mycobiome. Additionally, specific mycobiome genera in the phloem showed variations in response to water availability, indicating the influence of environmental conditions on the relative proportion of certain fungal genera in Norway spruce trees. In the root mycobiome, key fungi such as Phialocephala fortinii and Paraphaeosphaeria neglecta were identified as conferring inhibitory effects against H. parviporum growth in Norway spruce genotypes. Furthermore, certain endophytes demonstrated greater stability in root ecosystems under low water conditions than ectomycorrhizal fungi. This knowledge can contribute to developing sustainable forest management practices that enhance the well-being of trees and their ecosystems, ultimately bolstering forest resilience.

Genetic variation of Picea abies in response to the artificial inoculation of Heterobasidion parviporum

Norway spruce Picea abies is one of Europe's most economically important tree species. However, it is highly susceptible to the root rot fungus Heterobasidion parviporum. Climate change will benefit the pathogen as the tree host is weakened by, e.g., extended drought. Breeding can improve forest health, and several root rot genetic markers are suggested to improve the resistance of Norway spruce. This study aimed to compare the resistance (here defined as necrosis length) of Norway spruce families and genotypes against two strains of H. parviporum under different water availabilities. Our results show that the family and the genotype within the family have an impact on the necrosis length that is related to the aggressiveness of the fungal strains. Under low water conditions, the necrosis increased only in horizontal directions in phloem and sapwood. Similarly, the growth (seedling height) was not disturbed by abiotic stress (less water), indicating that the stress level (drought) was too low in this setting. The knowledge gained in this study could improve forest health in the changing climate by understanding the response of Norway spruce to pathogenic attacks under additional stress at the family level. This knowledge could be strategically used in forest breeding to improve the resistance of Norway spruce trees against root rot.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10342-023-01534-3.

Interaction of drought- and pathogen-induced mortality in Norway spruce and Scots pine

Pathogenic diseases frequently occur in drought-stressed trees. However, their contribution to the process of drought-induced mortality is poorly understood. We combined drought and stem inoculation treatments to study the physiological processes leading to drought-induced mortality in Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) saplings infected with Heterobasidion annosum s.s. We analysed the saplings' water status, gas exchange, nonstructural carbohydrates (NSCs) and defence responses, and how they related to mortality. Saplings were followed for two growing seasons, including an artificially induced 3-month dormancy period. The combined drought and pathogen treatment significantly increased spruce mortality; however, no interaction between these stressors was observed in pine, although individually each stressor caused mortality. Our results suggest that pathogen infection decreased carbon reserves in spruce, reducing the capacity of saplings to cope with drought, resulting in increased mortality rates. Defoliation, relative water content and the starch concentration of needles were predictors of mortality in both species under drought and pathogen infection. Infection and drought stress create conflicting needs for carbon to compartmentalize the pathogen and to avoid turgor loss, respectively. Heterobasidion annosum reduces the functional sapwood area and shifts NSC allocation patterns, reducing the capacity of trees to cope with drought.

Comparative Efficacy of State-of-the-Art and New Biological Stump Treatments in Forests Infested by the Native and the Alien Invasive Heterobasidion Species Present in Europe

The Heterobasidion annosum species complex includes major fungal pathogens of conifers worldwide. State-of-the-art preventative stump treatments with urea or with commercial formulations of the fungal biological control agent Phlebiopsis gigantea (i.e., Rotstop^®) may become no longer available or are not approved for use in many areas of Europe infested by the three native Heterobasidion species and by the North American invasive H. irregulare, making the development of new treatments timely. The efficacy of Proradix^® (based on Pseudomonas protegens strain DSMZ 13134), the cell-free filtrate (CFF) of the same bacterium, a strain of P. gigantea (MUT 6212) collected in the invasion area of H. irregulare in Italy, Rotstop^®, and urea was comparatively investigated on a total of 542 stumps of Abies alba, Picea abies, Pinus pinea, and P. sylvestris in forest stands infested by the host-associated Heterobasidion species. Additionally, 139 logs of P. pinea were also treated. Results support the good performances of Rotstop^®, and especially of urea against the native Heterobasidion species on stumps of their preferential hosts and, for the first time, towards the invasive North American H. irregulare on stumps of P. pinea. In some experiments, the effectiveness of Proradix^® and of the strain of P. gigantea was weak, whereas the CFF of P. protegens strain DSMZ 13134 performed as a valid alternative to urea and Rotstop^®. The mechanism of action of this treatment hinges on antibiosis; therefore, further improvements could be possible by identifying the active molecules and/or by optimizing their production. Generally, the performance of the tested treatments is not correlated with the stump size.

Safe DNA-extraction Protocol Suitable for Studying Tree-fungus Interactions

We present a safe and low-cost method suitable for DNA extraction from mycelium and tree tissue samples. After sample preparation, the extraction takes about 60 min. Method performance was tested by extracting DNA from various tree tissue samples and from mycelium grown on solid and liquid media. DNA was extracted from juvenile and mature host material (Picea abies, Populus trichocarpa, Pseudotsuga menziesii) infected with different pathogens (Heterobasidion annosum, Heterobasidion parviporum, Leptographium wagenerii, Sphaerulina musiva). Additionally, DNA was extracted from pure cultures of the pathogens and several endophytic fungi. PCR success rate was 100% for young poplar material and fungal samples, and 48-72% for conifer and mature broadleaved plant samples. We recommend using 10-50 mg of fresh sample for the best results. The method offers a safe and low-cost DNA extraction alternative to study tree-fungus interactions, and is a potential resource for teaching purposes.

Physiological Responses to Abiotic and Biotic Stress in Forest Trees

Forests fulfill important ecological functions by sustaining nutrient cycles and providing habitats for a multitude of organisms. They further deliver ecosystem services such as carbon storage, protection from erosion, and wood as an important commodity. Trees have to cope in their environment with a multitude of natural and anthropogenic forms of stress. Resilience and resistance mechanisms to biotic and abiotic stresses are of special importance for long-lived tree species. Since trees exist for many decades or even centuries on the same spot, they have to acclimate their growth and reproduction to constantly changing atmospheric and pedospheric conditions. In this special issue, we invited contributions addressing the physiological responses of forest trees to a wide array of different stress factors. Among the eighteen papers published, seventeen covered drought or salt stress as major environmental cues, highlighting the relevance of this topic in times of climate change. Only one paper studied cold stress [1]. The dominance of drought and salt stress studies underpins the need to understand tree responses to these environmental threats from the molecular to the ecophysiological level. The papers contributing to this Special Issue cover these scientific aspects in different areas of the globe and encompass conifers as well as broadleaf tree species. In addition, two studies deal with bamboo (Phyllostachys sp., [1,2]). Bamboo, although botanically belonging to grasses, was included because its ecological functions and applications are similar to those of trees.