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Regulation of Symbiotic Nitrogen Fixation in Legume Root Nodules
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Molecular Analyses of the Distribution and Function of Diazotrophic Rhizobia and Methanotrophs in the Tissues and Rhizosphere of Non-Leguminous Plants

1
Department of Applied Biological Chemistry, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan
2
National Agriculture and Food Research Organization, Kannondai 3-1-1, Tsukuba, Ibaraki 305-8666, Japan
3
School of Ecology and Environment, Inner Mongolia University, 235 West University Blvd., Hohhot 010021, Inner Mongolia, China
4
Graduate School of Life Sciences, Tohoku University, Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan
*
Authors to whom correspondence should be addressed.
Plants 2019, 8(10), 408; https://doi.org/10.3390/plants8100408
Received: 3 August 2019 / Revised: 26 September 2019 / Accepted: 10 October 2019 / Published: 11 October 2019
(This article belongs to the Special Issue Nitrogen-Fixing Plants )
Biological nitrogen fixation (BNF) by plants and its bacterial associations represent an important natural system for capturing atmospheric dinitrogen (N2) and processing it into a reactive form of nitrogen through enzymatic reduction. The study of BNF in non-leguminous plants has been difficult compared to nodule-localized BNF in leguminous plants because of the diverse sites of N2 fixation in non-leguminous plants. Identification of the involved N2-fixing bacteria has also been difficult because the major nitrogen fixers were often lost during isolation attempts. The past 20 years of molecular analyses has led to the identification of N2 fixation sites and active nitrogen fixers in tissues and the rhizosphere of non-leguminous plants. Here, we examined BNF hotspots in six reported non-leguminous plants. Novel rhizobia and methanotrophs were found to be abundantly present in the free-living state at sites where carbon and energy sources were predominantly available. In the carbon-rich apoplasts of plant tissues, rhizobia such as Bradyrhizobium spp. microaerobically fix N2. In paddy rice fields, methane molecules generated under anoxia are oxidized by xylem aerenchyma-transported oxygen with the simultaneous fixation of N2 by methane-oxidizing methanotrophs. We discuss the effective functions of the rhizobia and methanotrophs in non-legumes for the acquisition of fixed nitrogen in addition to research perspectives.
Keywords: biological nitrogen fixation; endophytic diazotrophs; methanotrophs; non-leguminous plants; rhizobia biological nitrogen fixation; endophytic diazotrophs; methanotrophs; non-leguminous plants; rhizobia
MDPI and ACS Style

Yoneyama, T.; Terakado-Tonooka, J.; Bao, Z.; Minamisawa, K. Molecular Analyses of the Distribution and Function of Diazotrophic Rhizobia and Methanotrophs in the Tissues and Rhizosphere of Non-Leguminous Plants. Plants 2019, 8, 408.

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