Harnessing synthetic microbial communities with nitrogen-fixing activity to promote rice growth

Abstract Improving the biological nitrogen fixation of rhizosphere bacteria in rice is expected to improve plant growth and productivity. Most studies have focused on the application of a single N2-fixing bacterium on rice plants. Little is known about the beneficial impacts of a bacterial consortium, which is known as the microbial synthetic community (SynCom) on rice growth, and the extent of the interaction with the microbial community. The study investigated the effect of a SynCom on rice growth and productivity. A SynCom consisting of three bacteria Azosprillium sp. NL1, Bradyrhizobium sp. NL2, and Azotobacter sp. NL3 was designed to increase the survival and density of root microorganisms. The microbial culture collections used for the reconstitution experiment were established using bacterium-free soil. A profound increase in bacterial root microbiota survival and density was observed in plants inoculated with either mono- or multi-synthetic microbial consortia. SynCom is essential for rice plant growth by improving shoot ramification and root development. The results showed that SynCom could increase the total root nitrogen-fixing (%Ndfa) value by 51.6%, with Azotobacter sp. NL3 being the most dominant. High levels of indole acetic acid production in the range of 0.65–0.8 ng per g root were observed in Azosprillium sp. NL1 and Azotobacter sp. NL3. This indicates Azotobacter sp. NL3 is a nitrogen-fixing rhizosphere bacterium that is dominant in rice-amended soils. These results support the early application of the designed SynCom as a feasible strategy for the induction of nitrogen fixation, suggesting a role for earlier effects on rhizosphere colonization and tillering of plants.