Biological Function of Lanthanide in Plant-Symbiotic Bacteria: Lanthanide-Dependent Methanol Oxidation System

In plant-symbiotic bacteria, such as some mehylotrophic bacteria and rhizobia, a novel type of pyrroloquinoline quinone (PQQ)-dependent methanol dehydrogenase (MDH) was recently identified. This MDH, named XoxF encoded by the xox cluster, requires lanthanide (Ln) as a cofactor. Moreover, there is steady indication that these plant symbiotic bacteria strains possess some Ln-dependent cell functions: the strains are able to recognize Ln species under growth conditions, to uptake Ln species into the cell, and to regulate their Ln-dependent methanol metabolisms based on the particular Ln species present. In this review, we focus on the molecular mechanisms involved in Ln-dependent methanol metabolism and Ln-utilizing systems in the plant-symbiotic bacteria, and discuss the physiological roles of these Ln-dependent systems for the plant-symbiotic bacteria in the phyllosphere and rhizosphere.