Metaproteomic Identification of Diazotrophic Methanotrophs and Their Localization in Root Tissues of Field-Grown Rice Plants
ABSTRACTIn a previous study by our group, CH4oxidation and N2fixation were simultaneously activated in the roots of wild-type rice plants in a paddy field with no N input; both processes are likely controlled by a rice gene for microbial symbiosis. The present study examined which microorganisms in rice roots were responsible for CH4oxidation and N2fixation under the field conditions. Metaproteomic analysis of root-associated bacteria from field-grown rice (Oryza sativaNipponbare) revealed that nitrogenase complex-containing nitrogenase reductase (NifH) and the alpha subunit (NifD) and beta subunit (NifK) of dinitrogenase were mainly derived from type II methanotrophic bacteria of the familyMethylocystaceae, includingMethylosinusspp. Minor nitrogenase proteins such asMethylocella,Bradyrhizobium,Rhodopseudomonas, andAnaeromyxobacterwere also detected. Methane monooxygenase proteins (PmoCBA and MmoXYZCBG) were detected in the same bacterial group of theMethylocystaceae. Because these results indicated thatMethylocystaceaemembers mediate both CH4oxidation and N2fixation, we examined their localization in rice tissues by using catalyzed reporter deposition-fluorescencein situhybridization (CARD-FISH). The methanotrophs were localized around the epidermal cells and vascular cylinder in the root tissues of the field-grown rice plants. Our metaproteomics and CARD-FISH results suggest that CH4oxidation and N2fixation are performed mainly by type II methanotrophs of theMethylocystaceae, includingMethylosinusspp., inhabiting the vascular bundles and epidermal cells of rice roots.