Study of the mechanisms of iron homeostasis in the arbuscular mycorrhizal fungus Rhizophagus irregularis

Abstract

Arbuscular mycorrhizal (AM) symbioses that involve most plants and Glomeromycota (AM) fungi are integral and functional parts of plant roots. In these associations, the fungi not only colonize the root cortex but also maintain an extensive network of hyphae that extend out of the root into the surrounding environment. These external hyphae contribute to plant uptake of low mobility nutrients, such as P, Fe and Zn. Besides improving plant mineral nutrition, AM fungi can alleviate heavy metal (HM) toxicity to their host plants. The HM Fe plays essential roles in many biological processes but is toxic when present in excess, since it can produce toxic free radicals via the Fenton reaction. This makes its transport and homeostatic control of particular importance to all living organisms. AM fungi play an important role in modulating plant HM acquisition in a wide range of soil metal concentrations and have been considered to be a key element in the improvement of micronutrient concentrations in crops and in the phytoremediation of polluted soils. Although the main benefit of the AM association is an improved P status of the mycorrhizal plant, AM fungi also play a role in Fe nutrition of their host plants, and direct evidence of the capability of the extraradical mycelium (ERM) to take up Fe from the soil and to transfer it to the host plant has been found. Conversely, other studies have shown that AM fungi play a role in reducing Fe uptake when the soil concentration is high. Nevertheless, little is known about the mechanisms of Fe uptake and homeostasis in arbuscular mycorrhizas. Within this PhD thesis, several methods have been used to analyze the mechanisms of Fe homeostasis in the model AM fungus Rhizophagus irregularis, which is easily grown in monoxenic cultures and whose genome has been recently sequenced. Genome-wide analyses were undertaken in order to identify R. irregularis genes involved in Fe transport and homeostasis, by making use of transport databases, genome organism websites and in silico bioinformatics tools for sequence analyses, such as software for protein structure predictions and methods for phylogenetic analyses. For experimental studies, R. irregularis monoxenic cultures were established in order to obtain exclusively fungal material for the subsequent gene isolation, gene expression analyses or enzymatic assays. Since it is not still possible to genetically manipulate AM fungi, functional and localization analyses of the newly identified R. irregularis genes were performed in a heterologous system: the budding yeast Saccharomyces cerevisiae.