Abstract
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.
Highlights
Biological nitrogen fixation (BNF) by plant–rhizobia symbiotic systems is mediated by a specialized plant organ known as a nodule (Figure 1A,B)
Other N2 -fixing bacteria found in the rhizosphere soil were from diverse genera such as Methylocystis, Beijerinckia, Geobacter, Rhodovulum, Methylobacterium, Gluconacetobacter, Methylocella, and Delftia, while those in the stem and root corresponded to less diverse genera such as Methylosinus, Rhizobium, and Dechloromonas
The data corresponding to nifH DNA and nifH cDNA from the root tissues showed that nifH was expressed in Burkholderia spp., Methylobacterium nodulans, Rhizobium helanshanense, and Geobacter sp., whereas nifH was not expressed in Bradyrhizobium spp., Sphingomonas azotifigens, and Methylocystis sp. in the root tissues (Table 3)
Summary
Biological nitrogen fixation (BNF) by plant–rhizobia symbiotic systems is mediated by a specialized plant organ known as a nodule (Figure 1A,B). A greenish signal corresponding to type II methanotrophs was detected in the intercellular spaces around the stele of root tissue of paddy rice [4]. In a nonsymbiotic system of associative N2 fixation, which commonly occurs in the rhizosphere of higher plants, N2 -fixing bacteria fix N2 by using carbon and energy sources supplied from the environment [5]. Was observed in the N2 -fixing sugarcane stems and sweet potato stems and tubers, called the “rhizobia system” (Figure 1C) These rhizobia microaerobically fix N2 in the carbon compound-rich apoplasts.
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