Characterization of the Rhizophagus irregularis multicopper oxidase family indicates that the iron transporter RiFTR1 does not require a ferroxidase partner

The contribution of arbuscular mycorrhizal fungi (AM fungi) to plant iron (Fe) acquisition has been demonstrated in several studies. Recently, it has been shown that AM fungi use a high-affinity reductive pathway for Fe uptake. In the AM fungus Rhizophagus irregularis the ferric reductase RiFRE1 and the Fe permeases RiFTR1 and RiFTR2 have already been characterized. In an attempt to identify the third component of the reductive iron uptake pathway, a genome-wide approach has been used in R. irregularis to find genes encoding ferroxidases of the multicopper oxidase (MCO) gene family. Nine genes putatively encoding MCOs (RiMCO1-9) were identified. A phylogenetic analysis of MCO sequences of fungi from different taxonomic groups revealed that all RiMCOs clustered together in the ferroxidase/laccase group, and none with the Fet3-type ferroxidases. RiMCO1 and RiMCO3 were the only MCO genes displaying a detectable gene expression pattern typical of a high-affinity Fe transport system, indicating that RiMCO1 and RiMCO3 might have a role in the reductive high-affinity Fe uptake system. Moreover, yeast mutant complementation assays showed that the iron permease RiFTR1 can operate without the presence of a ferroxidase, indicating that it is able to transport also ferrous (II) iron.