Abstract
Biological nitrogen fixation catalyzed by Mo-nitrogenase of symbiotic diazotrophs has attracted interest because its potential to supply plant-available nitrogen offers an alternative way of using chemical fertilizers for sustainable agriculture. Phototrophic purple nonsulfur bacteria (PNSB) diazotrophically grow under light anaerobic conditions and can be isolated from photic and microaerobic zones of rice fields. Therefore, PNSB as asymbiotic diazotrophs contribute to nitrogen fixation in rice fields. An attempt to measure nitrogen in the oxidized surface layer of paddy soil estimates that approximately 6–8 kg N/ha/year might be accumulated by phototrophic microorganisms. Species of PNSB possess one of or both alternative nitrogenases, V-nitrogenase and Fe-nitrogenase, which are found in asymbiotic diazotrophs, in addition to Mo-nitrogenase. The regulatory networks control nitrogenase activity in response to ammonium, molecular oxygen, and light irradiation. Laboratory and field studies have revealed effectiveness of PNSB inoculation to rice cultures on increases of nitrogen gain, plant growth, and/or grain yield. In this review, properties of the nitrogenase isozymes and regulation of nitrogenase activities in PNSB are described, and research challenges and potential of PNSB inoculation to rice cultures are discussed from a viewpoint of their applications as nitrogen biofertilizer.
Highlights
Global energy demand has increased partly due to the rise of fertilizer consumption and the shift toward more energy-intensive fertilizers [1]
Besides the symbiotic nitrogen fixation in legume root–nodule symbiosis, it has been estimated in the surface of rice paddy soil that approximately 6–8 kg N/ha/year might be accumulated as a result of phototrophic nitrogen fixation [5]
Among R. palustris strains isolated from peat swamp forests, a strain that is effective in providing high-affinity Fe (III) ligands siderophores, ammonium, and phosphate; strains that are effective in production of plant growth-promoting substances; and a strain showing the highest inhibition against rice pathogens are potentially beneficial purple nonsulfur bacteria (PNSB) for rice cultivation in acidic soil
Summary
Global energy demand has increased partly due to the rise of fertilizer consumption and the shift toward more energy-intensive fertilizers [1]. As biosynthesis of ammonia in nitrogen fixation proceeds using nitrogen gas under natural environmental conditions [2], use of diazotrophic bacteria as biofertilizer would reduce chemical fertilizer consumption and contribute to sustainable agriculture. Representatives of such diazotrophic bacteria are root nodule bacteria, which provide the nitrogen source to host legume [3]. Purple sulfur and purple nonsulfur bacteria (PNSB) have nitrogen-fixing ability in the light and can grow using nitrogen gas as the sole nitrogen source [7,8] In these bacterial groups, activity of nitrogenase that catalyzes nitrogen fixation is highly regulated by light, oxygen, and ammonia [9,10]. This review describes the current knowledge regarding nitrogenase isozymes and regulation of nitrogenase activities in PNSB and focuses on achievements and future directions in their applications as biofertilizer toward the establishment of sustainable agriculture
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