Drought-Induced Stress Priming in Two Distinct Filamentous Saprotrophic Fungi.

Sessile organisms constantly face environmental fluctuations and especially drought is a common stressor. One adaptive mechanism is "stress priming," the ability to cope with a severe stress ("triggering") by retaining information from a previous mild stress event ("priming"). While plants have been extensively investigated for drought-induced stress priming, no information is available for saprotrophic filamentous fungi, which are highly important for nutrient cycles. Here, we investigated the potential for drought-induced stress priming of one strain each of two ubiquitous species, Neurospora crassa and Penicillium chrysogenum. A batch experiment with 4 treatments was conducted on a sandy soil: exposure to priming and/or triggering as well as non-stressed controls. A priming stress was caused by desiccation to pF 4. The samples were then rewetted and after 1-, 7-, or 14-days of recovery triggered (pF 6). After triggering, fungal biomass, respiration, and ß-glucosidase activity were quantified. P. chrysogenum showed positive stress priming effects. After 1 day of recovery, biomass as well as ß-glucosidase activity and respiration were 0.5 to 5 times higher during triggering. Effects on biomass and activity decreased with prolonged recovery but lasted for 7 days and minor effects were still detectable after 14 days. Without triggering, stress priming had a temporary negative impact on biomass but this reversed after 14 days. For N. crassa, no stress priming effect was observed on the tested variables. The potential for drought-induced stress priming seems to be species specific with potentially high impact on composition and activity of fungal communities considering the expected increase of drought events.

Assessment of prokaryote to eukaryote ratios in environmental samples by SSU rDNA length polymorphism.

Microbial communities are important regulators of many processes in all ecosystems. Understanding of ecosystem processes requires at least an overview of the involved microorganisms. While in-depth identification of microbial species in environmental samples can be achieved by next generation sequencing, profiling of whole microbial communities can be accomplished via less labour-intensive approaches. Especially automated ribosomal intergenic spacer analysis (ARISA) are of interest as they are highly specific even at fine scales and widely applicable for environmental samples. Yet, established protocols lack the possibility to compare prokaryotic and eukaryotic communities as different primer sets are necessary. However, shifts in the eukaryote to prokaryote ratio can be a useful indicator for ecosystem processes like decomposition or nutrient cycling. We propose a protocol to analyse prokaryotic and eukaryotic communities using a single primer pair based reaction based on a region with variable length (V4, which is about 180 bp shorter in prokaryotes compared to eukaryotes) in the small ribosomal subunit flanked by two highly conservative regions. Shifts in the prokaryotic and eukaryotic ratio between samples can be reliably detected by fragment length polymorphism analysis as well as sequencing of this region. Together with established approaches such as ARISA or 16S and ITS rDNA sequencing, this can provide a more complex insight into microbial community shifts and ecosystem processes.

Redistribution of soil water by a saprotrophic fungus enhances carbon mineralization.

The desiccation of upper soil horizons is a common phenomenon, leading to a decrease in soil microbial activity and mineralization. Recent studies have shown that fungal communities and fungal-based food webs are less sensitive and better adapted to soil desiccation than bacterial-based food webs. One reason for a better fungal adaptation to soil desiccation may be hydraulic redistribution of water by mycelia networks. Here we show that a saprotrophic fungus (Agaricus bisporus) redistributes water from moist (-0.03 MPa) into dry (-9.5 MPa) soil at about 0.3 cm ⋅ min(-1) in single hyphae, resulting in an increase in soil water potential after 72 h. The increase in soil moisture by hydraulic redistribution significantly enhanced carbon mineralization by 2,800% and enzymatic activity by 250-350% in the previously dry soil compartment within 168 h. Our results demonstrate that hydraulic redistribution can partly compensate water deficiency if water is available in other zones of the mycelia network. Hydraulic redistribution is likely one of the mechanisms behind higher drought resistance of soil fungi compared with bacteria. Moreover, hydraulic redistribution by saprotrophic fungi is an underrated pathway of water transport in soils and may lead to a transfer of water to zones of high fungal activity.

Vitamin B2 (riboflavin) increases drought tolerance of Agaricus bisporus.

Drought is a stressor for many soil-inhabiting organisms. Although plants have been extensively investigated for drought-adaptive mechanisms, little information is available for fungi. Antioxidants are especially relevant, since desiccation is accompanied by an excessive intracellular production of reactive oxygen species. Riboflavin (vitamin B2) is one antioxidant regulating drought tolerance in plants. A similar function may exist in fungi. Here, we examined the respiratory and transcriptional responses of Agaricus bisporus to drought and the impact of riboflavin. Mesocosm experiments with four groups were established: hyphae were treated with or without 50 µM riboflavin under drought or no drought conditions. Drought increased riboflavin content in hyphae about 5 times with, but also without, addition of riboflavin. Without addition of riboflavin, fungal respiration decreased by more than 50% at water potentials of about -20 MPa. With addition of riboflavin, respiration remained about 2-3 times higher. The transcriptional responses to only drought or only riboflavin strongly overlapped and were mainly based on factors regulating transcription and translation. This was even stronger in combined treatments. Riboflavin induced protective mechanisms in drought-stressed hyphae. Most pronounced was the methylglyoxal (cytotoxic by-product of glycolysis) detoxifying of lactoylglutathione lyase. Thus, our data suggest a stress-priming function and a role of riboflavin in drought responses of A. bisporus.