Chelation in the cytoplasm is an intracellular buffer system to control PTE toxicity through reduction of the concentration of cytotoxic-free PTE ions. In this system, metallothioneins (MT), metal-binding polypeptides, and/or MT-like proteins may be involved. In contrast to ectomycorrhizal fungi, the participation of these compounds in metal detoxification has not been confirmed in AMF [69,70]. However, evidence suggesting the presence of Cd-binding thiols in AMF as well as high concentrations of N and S in mycorrhizal roots exposed to Cd have been reported [71,72]. Galli et al.  studied the effect of Cu on the uptake, amount, and composition of Cu-binding peptides (Cu-BPs) of mycorrhizal maize plants inoculated with Glomus intraradices. They observed increased concentrations of the thiols cysteine, y-glutamylcysteine, and glutathione up to an external Cu supply of 9 |g g-1. However, the amount of thiols in Cu-BPs was not increased by mycorrhizal colonization in Cu-treated plants and no differences in Cu-uptake were detected between nonmy-corrhizal and mycorrhizal plants.
Gonzalez  observed that the EM grown in a Cu-contaminated substrate presented higher tyrosinase activity (353 nmol mg-1 dry mycelium min-1), in contrast to the tyrosinase activity in the EM propagated in noncontaminated substrate (153 nM mg-1 dry mycelium min-1). The Cu substrate concentration (19 mg Cu g-1 substrate) induced a 2.3 times increase in tyrosinase activity in the EM of AMF tested. Glomus caledonium BEG-133 exhibited the highest tyrosinase activity, and G. mosseae BEG-132 and G. claroideum BEG-134 presented statistically similar activity. Comparisons using more species in different Cu levels are necessary in order to prove tyrosinase participation in Cu chelation and detoxification in AMF. This mechanism of chelation has been observed in other filamentous fungi, yeast, and ectomycorrhizal fungi [73-76].
An increase in PTE tolerance in plants has been suggested to be related to genes up-regulated in AMF-colonized roots. Rivera-Becerril et al.  reported that the expression of metallothionein genes increased in Cd-treated plants, but one gene (pcs) appeared to be activated specifically in Cd-treated mycorrhizal roots. In relation to these fungal PTE tolerance mechanisms, the use of visualization methods, such as transmission and scanning electron microscopy (TEM, SEM); energy-dispersive x-ray microanalysis (EDAX); and electron energy loss spectroscopy (EELS), to determine the extracellular and subcellular localization of elements has been very useful.
Gonzalez et al.  reported that crystal-like aggregates mainly comprising Fe were found on the mucilaginous outer hyphal wall layers of three AMF, when grown in Cu/As-contaminated soil. It was shown that, in the EM of G. mosseae BEG132 and G. claroideum BEG134, these aggregates contained significant amounts of Fe and Cu. This was not the case for G. caledonium BEG133; its aggregates only contained Fe.
In another comparative study, Turnau et al.  studied mycorrhizal roots of Pteridium aqui-linum collected from Cd-treated experimental plots. They showed that hyphae of AMF uniformly colonizing cortical cells contained a much higher amount of PTEs than the cytoplasm of the host cells. They suggested that the ability of this plant to detoxify PTEs was due to its association with AMF. Most of the Cd was located in phosphate-rich material fungal vacuoles, which contained S, N, Al, Fe, Ti, and B. Thus, intracellular sequestration of PTE by fungal polyphosphate intracellularly into the fungus may contribute to decreasing its transfer to the plant.
Polyphosphates are produced widely by microorganisms forming an important intracellular storage of phosphate. However, these compounds may also be involved in the regulation of concentrations of PTE ion in cells . Polyphosphate granules are maintaining ionic compartmen-talization of PTEs and it has been observed that their biosynthesis accompanies vacuolar accumulation .
Gonzalez et al.  reported that the EM of Glomus mosseae BEG-132 growing in a contaminated substrate (As/Cu), contained intracellular Cu-rich bodies. In addition, traces of arsenic were also observed on the EDAX spectra of these bodies. The presence of arsenic was explained by the elevated concentrations in the soil in which the AMF were grown. Arsenate and phosphate are chemically very analogous, so this result suggests that arsenic may sequester Cu in the form of Cu-arsenate complexes in the cytoplasm of the EM of G. mosseae BEG132. The arsenic and Cu accumulation in G. mosseae BEG132 is interesting to study because detoxification of PTE within cells of several yeast species has been shown to be linked with polyphosphate granules located in the cytoplasm  and vacuoles .
Was this article helpful?
Detoxification is something that is very important to the body, but it is something that isn't understood well. Centuries ago, health masters in the East understood the importance of balancing and detoxifying the body. It's something that Western medicine is only beginning to understand.