Abstract
Using a Non-invasive Micro-test Technique, flux profiles of Cd2+, Ca2+, and H+ were investigated in axenically grown cultures of two strains of Paxillus involutus (MAJ and NAU), ectomycorrhizae formed by these fungi with the woody Cd2+-hyperaccumulator, Populus × canescens, and non-mycorrhizal (NM) roots. The influx of Cd2+ increased in fungal mycelia, NM and ectomycorrhizal (EM) roots upon a 40-min shock, after short-term (ST, 24 h), or long-term (LT, 7 days) exposure to a hydroponic environment of 50 μM CdCl2. Cd2+ treatments (shock, ST, and LT) decreased Ca2+ influx in NM and EM roots but led to an enhanced influx of Ca2+ in axenically grown EM cultures of the two P. involutus isolates. The susceptibility of Cd2+ flux to typical Ca2+ channel blockers (LaCl3, GdCl3, verapamil, and TEA) in fungal mycelia and poplar roots indicated that the Cd2+ entry occurred mainly through Ca2+-permeable channels in the plasma membrane (PM). Cd2+ treatment resulted in H2O2 production. H2O2 exposure accelerated the entry of Cd2+ and Ca2+ in NM and EM roots. Cd2+ further stimulated H+ pumping activity benefiting NM and EM roots to maintain an acidic environment, which favored the entry of Cd2+ across the PM. A scavenger of reactive oxygen species, DMTU, and an inhibitor of PM H+-ATPase, orthovanadate, decreased Ca2+ and Cd2+ influx in NM and EM roots, suggesting that the entry of Cd2+ through Ca2+-permeable channels is stimulated by H2O2 and H+ pumps. Compared to NM roots, EM roots exhibited higher Cd2+-fluxes under shock, ST, and LT Cd2+ treatments. We conclude that ectomycorrhizal P. × canescens roots retained a pronounced H2O2 production and a high H+-pumping activity, which activated PM Ca2+ channels and thus facilitated a high influx of Cd2+ under Cd2+ stress.
Highlights
The presence of highly toxic cadmium (Cd2+) in the environment is a serious threat to human health as heavy metals can be enriched in plants and eventually enter the human body through the food chain (Nawrot et al, 2006; Kaplan et al, 2011)
Similar to the Cd2+ kinetics in EM roots, an instantaneous increase in the Cd2+ influx was detected in pure P. involutus mycelia after CdCl2 exposure (50 μM; Figure 1A)
The woody Cd2+-hyperaccumulator P. × canescens exhibited a vigorous Cd2+ uptake after a 50 μM CdCl2 shock (40 min), ST (24 h), and LT (7 days) treatment (Figures 1A and 2)
Summary
The presence of highly toxic cadmium (Cd2+) in the environment is a serious threat to human health as heavy metals can be enriched in plants and eventually enter the human body through the food chain (Nawrot et al, 2006; Kaplan et al, 2011). × canescens roots with the ectomycorrhizal fungus Paxillus involutus improves growth, primes for increased stress tolerance, increases nutrition, and regulates the ion balance under salt stress (Schützendübel and Polle, 2002; Gafur et al, 2004; LangenfeldHeyser et al, 2007; Luo et al, 2009, 2011; Li J. et al, 2012; Ma X. et al, 2014). Ectomycorrhizal poplar plants offer a great potential for phytoremediation of Cd2+polluted soils (Sell et al, 2005; Krpata et al, 2008, 2009; Luo et al, 2014; Ma Y. et al, 2014)
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