Arbuscular mycorrhizal fungi (AMF) have repeatedly been demonstrated to alleviate heavy metal stress of plants. The current manuscript
summarizes results obtained to date on the colonization of plants by AMF in heavy metal soils, the depositions of heavy metals in
plant and fungal structures and the potential to use AMF-plant combinations in phytoremediation, with emphasis on pennycresses
(Thlaspi ssp.). The focus of this manuscript is to describe and discuss studies on the expression of genes in plants and fungi under heavy
metal stress. The summary is followed by data on differential gene expression in extraradical mycelia (ERM) of in vitro cultured Glomus
intraradices Sy167 supplemented with different heavy metals (Cd, Cu or Zn). The expression of several genes encoding proteins potentially
involved in heavy metal tolerance varied in their response to different heavy metals. Such proteins included a Zn transporter, a
metallothionein, a 90 kD heat shock protein and a glutathione S-transferase (all assignments of protein function are putative). Studies
on the expression of the selected genes were also performed with roots of Medicago truncatula grown in either a natural, Zn-rich heavy
metal ‘‘Breinigerberg’’ soil or in a non-polluted soil supplemented with 100 lM ZnSO4. The transcript levels of the genes analyzed were
enhanced up to eight fold in roots grown in the heavy metal-containing soils. The data obtained demonstrate the heavy metal-dependent
expression of different AMF genes in the intra- and extraradical mycelium. The distinct induction of genes coding for proteins possibly
involved in the alleviation of damage caused by reactive oxygen species (a 90 kD heat shock protein and a glutathione S-transferase)
might indicate that heavy metal-derived oxidative stress is the primary concern of the fungal partner in the symbiosis.
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