Zn pollution counteracts Cd toxicity in metal-tolerant ectomycorrhizal fungi and their host plant, Pinus sylvestris

Suillus: an emerging model for the study of ectomycorrhizal ecology and evolution

Research on mycorrhizal symbiosis has been slowed by a lack of established study systems. To address this challenge, we have been developing Suillus, a widespread ecologically and economically relevant fungal genus primarily associated with the plant family Pinaceae, into a model system for studying ectomycorrhizal (ECM) associations. Over the last decade, we have compiled extensive genomic resources, culture libraries, a phenotype database, and protocols for manipulating Suillus fungi with and without their tree partners. Our efforts have already resulted in a large number of publicly available genomes, transcriptomes, and respective annotations, as well as advances in our understanding of mycorrhizal partner specificity and host communication, fungal and plant nutrition, environmental adaptation, soil nutrient cycling, interspecific competition, and biological invasions. Here, we highlight the most significant recent findings enabled by Suillus, present a suite of protocols for working with the genus, and discuss how Suillus is emerging as an important model to elucidate the ecology and evolution of ECM interactions.

Mechanisms of stress tolerance and their effects on the ecology and evolution of mycorrhizal fungi

Stress is ubiquitous and disrupts homeostasis, leading to damage, decreased fitness, and even death. Like other organisms, mycorrhizal fungi evolved mechanisms for stress tolerance that allow them to persist or even thrive under environmental stress. Such mechanisms can also protect their obligate plant partners, contributing to their health and survival under hostile conditions. Here we review the effects of stress and mechanisms of stress response in mycorrhizal fungi. We cover molecular and cellular aspects of stress and how stress impacts individual fitness, physiology, growth, reproduction, and interactions with plant partners, along with how some fungi evolved to tolerate hostile environmental conditions. We also address how stress and stress tolerance can lead to adaptation and have cascading effects on population- and community-level diversity. We argue that mycorrhizal fungal stress tolerance can strongly shape not only fungal and plant physiology, but also their ecology and evolution. We conclude by pointing out knowledge gaps and important future research directions required for both fully understanding stress tolerance in the mycorrhizal context and addressing ongoing environmental change.

Acute biotoxicity assessment of heavy metals in sewage sludge based on the glucose consumption of Escherichia coli

As a simple and feasible method for acute biotoxicity assessment, personal glucose meter (PGM) can be successfully applied in the early warning of environmental pollutants in sewage. In this paper, the acute biotoxicity of single and joint heavy metals in sewage and real sludge samples was systematically described based on the glucose metabolism of Escherichia coli (E. coli). Results indicated that the biotoxicity order of five single heavy metals in sewage was Hg2+ > As3+ > Cu2+ > Zn2+ > Cd2+. The joint heavy metals of Cu2+ + Zn2+, Cu2+ + Cd2+, and Cu2+ + Hg2+ produced synergistic effects, while Cu2+ + As3+ and Cd2+ + Zn2+ possessed antagonistic effects for the combined biotoxicity. In spiked sludge, Cd2+ and Zn2+ owned higher biotoxicity than Cu2+ and As3+. Notably, the electroplate factory and housing estate sludge respectively showed the highest and lowest inhibition rates as 57.4% and 17.7% under the real sludge biotoxicity assessment. These results demonstrated that PGM was a sensitive and portable method, which could be widely used for acute biotoxicity assessment of heavy metals in sewage sludge.

Cd and Zn interactions and toxicity in ectomycorrhizal basidiomycetes in axenic culture

Background

Metal contamination in soils affects both above- and belowground communities, including soil microorganisms. Ectomycorrhizal (ECM) fungi are an important component in belowground community and tolerant strains have great potential in enhancing plant-based remediation techniques. We assessed cadmium and zinc toxicity in five ECM species in liquid media (Hebeloma subsaponaceum; H. cylindrosporum; H. crustuliniforme; Scleroderma sp.; Austroboletus occidentalis) and investigated the potential of Zn to alleviate Cd toxicity. Due to highly divergent results reported in the literature, liquid and solid media were compared experimentally for the first time in terms of differential toxicity thresholds in Cd and Zn interactions.

Methods

A wide range of Cd and Zn concentrations were applied to ectomycorrhizal fungi in axenic cultures (in mg L⁻¹): 0; 1; 3; 9; 27; 81; 243 for the Cd treatments, and 0; 1; 30; 90; 270; 810; 2,430 for Zn. Combined Zn and Cd treatments were also applied to H. subsaponaceum and Scleroderma sp. Dry weight was recorded after 30 days, and in case of solid medium treatments, radial growth was also measured.

Results and Discussion

All species were adversely affected by high levels of Cd and Zn, and A. occidentalis was the most sensitive, with considerable biomass decrease at 1 mg L⁻¹ Cd, while Scleroderma sp. and H. subsaponaceum were the most tolerant, which are species commonly found in highly contaminated sites. Cd was generally 10 times more toxic than Zn, which may explain why Zn had little impact in alleviating Cd effects. In some cases, Cd and Zn interactions led to a synergistic toxicity, depending on the concentrations applied and type of media used. Increased tolerance patterns were detected in fungi grown in solid medium and may be the cause of divergent toxicity thresholds found in the literature. Furthermore, solid medium allows measuring radial growth/mycelial density as endpoints which are informative and in this case appeared be related to the high tolerance indices found in H. subsaponaceum.