Enhancing nitrogen fixation by cultivated crops reduces the need for nitrogenous fertilizers, thus lowering the production costs and environmental pollution. A recent article in Plant Biotechnology Journal describes how genetic manipulation of rice enhances the biological nitrogen fixation by soil bacteria.

Plant cells cannot fix atmospheric nitrogen, but some soil bacteria can. Nitrogen-fixing bacteria (diazotrophs) produce nitrogenase, the enzyme complex that mediates the reduction of N₂ to ammonia(NH₃). Diazotrophs can interact with plant roots symbiotically or not symbiotically. In symbiotic interactions, such as those between legumes and rhizobia, bacteria are located in differentiated root cells, called nodules, where nitrogen fixation occurs. There have been trials for expression of bacterial nitrogenase genes in plants or developing a legume-like root nodule symbiosis in cereal crop plants. Nevertheless, stable expression of the nitrogenase or the generation of nodules in cereals has not been achieved yet.

Enhancing nitrogen fixation by CRISPR-modified rice

The interactions between plant roots and the microbe-rich soil environment are critical for plant fitness. Plants extrude metabolites through their routes, which affects the root–bacteria interactions. These interactions are supported by the formation of biofilms, a bacteria self-produced matrix of extracellular polymeric substances. The formation of biofilms is essential for successful root colonization.

The researchers found that apigenin (a flavonoid, phytochemical) has the highest capacity to induce bacterial biofilm synthesis and biological nitrogen fixation. They then used CRISPR-based gene editing to knockout apigenin catabolizing enzymes in rice, generating apigenin-enriched rice plants that extruded apigenin into the root environment. Biofilm production modified the root microbiome structure with enrichment of diazotrophic bacteria recruitment inside the roots and in the rhizosphere (root environment). At limiting soil nitrogen conditions, the researchers got increased biofilm production, increased biological nitrogen fixation, and increased grain yield.

The success of such crop cultivation can lead to less dependency on nitrogenous fertilizers. This will result in lowering the cost of food production, as well as lowering the environmental pollution.

Reference: Yan D. et al. Genetic modification of flavone biosynthesis in rice enhances biofilm formation of soil diazotrophic bacteria and biological nitrogen fixation. https://doi.org/10.1111/pbi.13894

See also: Vitamin D from Tomatoes by Gene Editing

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