The rhizosphere contains many types of microbes interacting with plant roots, creating a complex symbiotic system. Microbial processes occurring in the rhizosphere are essential to the productivity of terrestrial ecosystems. In particular, exopolysaccharide produced by soil microbes allows dynamic regulation of soil moisture by modulating water transport. We have demonstrated that purified microbial exopolysaccharide (EPS) impacts soil water retention through enhancing the variability of water distributions in the soil microstructure. Here we report genetic engineering of soil bacterium Sinorhizobium meliloti to enable in situ control of EPS production within synthetic soil microstructures. We show that the photo-sensitive transcription factor EL222, derived from Erythrobacter litoralis, allows robust control of gene expression in S. meliloti. Essential genes in the type II EPS (a major component of EPS from S. meliloti in the soil) production pathway were identified, and deletion strains were generated. Complementation of the essential gene using a synthetic promoter controlled by EL222 led to robust light-activated production of EPS. Optimization of the engineered genetic construct was performed by varying promoters, ribosome binding sites, and using alternative start codons. Using the engineered EPS production strain, we observed rapid settlement of EPS producing S. meliloti in liquid culture, and selective biofilm formation quantified by a crystal violet staining assay. This approach enables spatially regulated EPS production in a synthetic soil microsystem that emulates aggregated sandy loam soil. The impact of EPS production on the transport of cells and water drying rate in the synthetic soil microsystem will be presented. We anticipate that the engineered genetic constructs will be broadly applicable for dissecting gene function in a defined population of microbes in the rhizosphere.

Optical control of exopolysaccharide production in Sinorhizobium meliloti in a synthetic soil microsystem

2019 Spring National Meeting

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