Engineering the Maize Root Microbiome: A Rapid MoClo Toolkit and Identification of Potential Bacterial Chassis for studying Plant-Microbe Interactions

Sustainably enhancing crop production is a necessity given the increasing demands for staple crops and their associated carbon/nitrogen inputs. Plant-associated microbiomes offer one avenue for addressing this demand; however, studying these communities and engineering them has remained a challenge due to limited genetic tools and methods. In this work, we detail the development of the Maize Root ToolKit (MRTK); a rapid Modular Cloning (MoClo) toolkit that only takes 2.5 hours to generate desired constructs (5400 potential plasmids) that replicate and express heterologous genes in Enterobacter ludwigii strain AA4 (Elu), Pseudomonas putida AA7 (Ppu), Herbaspirillum robiniae strain AA6 (Hro), Stenotrophomonas maltophilia strain AA1 (Sma) and Brucella pituitosa strain AA2 (Bpi) which comprise a model maize root synthetic community (SynCom). In addition to these genetic tools, we describe a highly efficient transformation protocol (10^7-10^9 transformants/{micro}g of DNA) for each of these strains. Utilizing this highly efficient transformation protocol, we identified endogenous expression sequences for each strain (ES; promoter and ribosomal binding sites) via genomic promoter trapping. Overall, the MRTK is a scalable platform that expands the genetic engineering toolbox while providing a standardized, high efficiency transformation method that can be implemented across a diverse group of root commensals. These results unlock the ability to elucidate and engineer plant-microbe interactions promoting plant growth for each of the 5 bacterial strains in this study.