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 

De Oliveira, Vinicius H.

Tibbett, Mark

PeerJ

Central Archive at the University of Reading

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

Vinicius H. De Oliveira

Mark Tibbett

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BackgroundMetal 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;Sclerodermasp.;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.MethodsA 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 toH. subsaponaceumandSclerodermasp. Dry weight was recorded after 30 days, and in case of solid medium treatments, radial growth was also measured.Results and DiscussionAll species were adversely affected by high levels of Cd and Zn, andA. occidentaliswas the most sensitive, with considerable biomass decrease at 1 mg L−1Cd, whileSclerodermasp. andH. subsaponaceumwere 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 inH. subsaponaceum.</jats:sec

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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; 2430 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.</jats:p

Vinicius De Oliveira

Cadmium (Cd) is one of the most hazardous contaminants in the environment and

it is often associated with zinc (Zn) in polluted soils, a nutrient that can also cause

toxicity at high concentrations. Among soil remediation techniques,

phytoremediation – the use of plants to immobilise and/or extract contaminants

from soils - is a promising technique, considered to be less harmful to the

environment. This thesis studies Cd and Zn fungi- and phytotoxicity, and the

biotechnological potential of different organisms (ecto- and arbuscular

mycorrhizal fungi, trees and yeast) in environmental remediation. The

experiments conducted in this project aimed to investigate the potential of poplar

trees (Populus trichocarpa) in Cd and Zn phytoremediation, and the use of

mycorrhizal symbiosis (Rhizophagus irregularis) to enhance metal extraction and

sequestration in the host plant. Another aim was to understand some of the

physiological and molecular processes by which poplar trees withstand Cd and Zn

toxicity, and to provide additional knowledge on the metal uptake process in

mycorrhizal poplars. Transgenic yeast carrying a poplar gene (PtMT2b) was also

studied for its potential in Cd bioremediation from contaminated solutions.

Results showed that P. trichocarpa is highly tolerant to Cd stress, and has a

considerable accumulation capacity of Cd and Zn; under both Cd and Zn exposure,

poplar shoots reached hyperaccumulator levels. Mycorrhizal symbiosis increased

Cd sequestration in roots, and Zn accumulation in leaves, supporting their use for

Cd phytostabilisation and Zn phytoextraction. Gene expression assessment

indicated mainly the involvement of PtHMA4 and PtZIP1 in Cd and Zn transport.

Expression of PtMT2b was associated with mycorrhizal colonisation and its role in

Cd tolerance was demonstrated in transgenic yeast assays. A mutated version of

the MT2b gene (PtMT2b ‘Y’) promoted high Cd tolerance and accumulation in

transgenic yeast showing promising results for bioremediation of Cdcontaminated

wastewater. This thesis offers new opportunities for this possibly

sustainable soil remediation technique; the knowledge gathered in this work may

serve as basis for the genetic engineering of poplars or other organisms for heavy

metal remediation or further research in refining and enhancing this technique

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

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