Plasmids were constructed using golden-gate assembly (47 (link), 48 (link)), HiFi assembly (New England Biolabs), and yeast recombination (49 (link)), as described in SI Appendix, Supplementary Materials and Methods. All inserts were confirmed by Sanger sequencing. Plasmids were chemically transformed into E. coli strains (50 (link)) and mobilized into rhizobia via biparental conjugation with E. coli ST18 (51 (link)) or via triparental conjugation with a helper E. coli strain carrying plasmid pRK2013. Plasmids with R6K replicons were maintained at low copy number in E. coli EC100D Pir+ where possible.