Abstract
Arbuscular mycorrhizal fungi (AMF), belonging to the Glomeromycota, are soil microorganisms that establish mutualistic symbioses with the majority of higher plants. The efficient uptake of low mobility mineral nutrients by the fungal symbiont and their further transfer to the plant is a major feature of this symbiosis. Besides improving plant mineral nutrition, AMF can alleviate heavy metal toxicity to their host plants and are able to tolerate high metal concentrations in the soil. Nevertheless, we are far from understanding the key molecular determinants of metal homeostasis in these organisms. To get some insights into these mechanisms, a genome-wide analysis of Cu, Fe and Zn transporters was undertaken, making use of the recently published whole genome of the AMF Rhizophagus irregularis. This in silico analysis allowed identification of 30 open reading frames in the R. irregularis genome, which potentially encode metal transporters. Phylogenetic comparisons with the genomes of a set of reference fungi showed an expansion of some metal transporter families. Analysis of the published transcriptomic profiles of R. irregularis revealed that a set of genes were up-regulated in mycorrhizal roots compared to germinated spores and extraradical mycelium, which suggests that metals are important for plant colonization.
Highlights
The transition metals Fe, Cu and Zn play essential and catalytic roles throughout the cell in various subcellular compartments
To get some clues about the expression profiles of these genes throughout the fungal life cycle, we explored the published transcriptomic profiles in the extraradical mycelium (ERM) and symbiotic roots obtained using the R. irregularis expression oligoarray (Tisserant et al, 2012) and the RNA-Seq reads obtained from germinated spores and Medicago-colonized roots (Tisserant et al, 2013)
The morphological alterations observed in the ERM of R. irregularis grown in vitro in association with root organ cultures in media without Cu or with Cu concentrations that are lethal to a majority of other organisms reflect its extremely adaptable character (Figure 1)
Summary
The transition metals Fe, Cu and Zn play essential and catalytic roles throughout the cell in various subcellular compartments These metal cofactors are critical for processes such as transcription, translation, the production of ATP in the mitochondria and the scavenging of toxic free radicals (van Ho et al, 2002; Schaible and Kaufmann, 2004; Kim et al, 2008). These metals are a highly reactive group of elements and are toxic at high concentrations (Valko et al, 2005). The activity and specificity of the transporters of physiologically important heavy metals control the lethality of the toxic metals
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