Effects of metal and salt stress on mycorrhizal interaction

Mycorrhiza is the mutualistic symbiosis of fungi and plant and known to stabilize plant health. Here, we focus on ectomycorrhiza of shrubs and trees, which is important in times of climate change and forest dieback, especially for reforestation of disturbed and/or contaminated area.

We investigated the ectomycorrhizal interaction of the basidiomycete Tricholoma vaccinum with spruce (Picea abies) adding different metals and seepage water from a former uranium mining site in Ronneburg, Thuringia, Germany. These treatments were used to mimick metal and salt stress of contaminated habitats, and to study its effect on the exchange of nutrients and signals between both mycorrhizal partners.

Our study was based on growth experiments, followed by expression analyses using RNAseq experiments, the analyses of volatiles by SPME fibers and GC-MS, the investigation phytohormones and their biosynthesis.

Cultivation on metal containing media showed reduced growth of mycorrhizal partners followed by slower mycorrhiza formation. Further the volatile profile of mycorrhized plants grown on medium treated with metal changed, those with mycorrhiza emitted tricyclene, a- and b-pinene, camphene, myrcene, limonene, terpinolene, camphor and bornyl acetate. Expression analyses comparing pure culture of the fungus, mycorrhiza and metal and salt stressed fungus and mycorrhiza revealed strong gene expression changes, e.g., an upregulation of transporters. These changes indicate a reduced plant defense, a rearranged secondary metabolite production, an upregulated water supply and energy production, as well as an increased detoxification in metal and salt stress conditions.

Mycorrhiza known to be beneficial for plant and fungi can be strongly affected by metal and salt stress. This might influence also other interacting organisms. In reforestation strategies with mycorrhizal fungi it has to be considered to select fungal strains showing abilities to manage different stresses occurring in the habitat.