Exploration of bacterial N₂-fixation systems in association with soil-grown sugarcane, sweet potato, and paddy rice: a review and synthesis

N₂ fixation systems in the nonleguminous crops and bacteria associations have been intensively studied over the last 50 years. Their structure and regulation have been investigated to explore the enhancement of N acquisition in these ecosystems leading to crop-growth with minimum chemical fertilizers. Several lines of important evidence have been accumulated indicating that the magnitudes of associative (nonsymbiotic) N₂ fixation in sugarcane (Saccharum spp.), sweet potato (Ipomoea batatas L.), and paddy rice (Oryza sativa L.) are agronomically significant. In these three crops, unique bacterial N₂-fixation systems may function in addition to the low-level activity (due to the competition in carbon/energy use) of the commonly occurring rhizosphere-associated system by free-living bacteria such as Beijerinckia, Azotobacter, and Klebsiella. Active expressions of the dinitrogenase reductase-encoded gene (nifH) phylogenetically similar to those of Bradyrhizobium spp. and Azorhizobium sp. were abundantly found in the N₂-fixing sugarcane stems, sweet potato stems, and storage tubers. These rhizobia micro-aerobically fix N₂ in the carbon compounds-rich apoplasts. Gluconacetobacter diazotrophicus and Herbaspirillum spp. were previously isolated from inside the sugarcane stems, as the candidates of endophytic N₂ fixers. However, the current molecular and physiological investigations suggest that their major role is production of phytohormonal substances. In paddy rice fields, methane is produced from organic compounds in anoxia and oxidized by contacting with oxygen gas. An active N₂-fixation by methane-oxidizing methanotrophs such as Methylosinus sp. takes place in the root tissues (aerenchyma) and also in the surface soil. This methanotrophic N₂-fixation supports the sustainability of soil fertility although the N₂-fixation and soil fertility are affected by chemical fertilizers. Finally, we discuss the ecological implications of the newly identified rhizobia and methanotroph systems in the N nutrition in nonlegumes and N reservation in field environments.