Unearthing Optimal Symbiotic Rhizobia Partners from the Main Production Area of Phaseolus vulgaris in Yunnan

is a globally important legume cash crop, which can carry out symbiotic nitrogen fixation with rhizobia. The presence of suitable rhizobia in cultivating soils is crucial for legume cropping, especially in areas beyond the plant-host native range, where soils may lack efficient symbiotic partners. We analyzed the distribution patterns and traits of native rhizobia associated with in soils of Yunnan, where the common bean experienced a recent expansion. A total of 608 rhizobial isolates were tracked from soils of fifteen sampling sites using two local varieties of . The isolates were discriminated into 43 genotypes as defined by IGS PCR-RFLP. Multiple locus sequence analysis based on , and of representative strains placed them into 11 rhizobial species of involving , , , , , , , , , and , and five unknown species; genosp. I~V. and were the dominant species (28.0% and 28.8%) most widely distributed, followed by (14.8%). The other rhizobial species were less numerous or site-specific. Phylogenies of and H markers, were divided into two specific symbiovars, sv. phaseoli regardless of the species affiliation and sv. viciae associated with . Through symbiotic effect assessment, all the tested strains nodulated both varieties, often resulting with a significant greenness index (91-98%). However, about half of them exhibited better plant biomass performance, at least on one common bean variety, and two isolates (CYAH-6 and BLYH-15) showed a better symbiotic efficiency score. Representative strains revealed diverse abiotic stress tolerance to NaCl, acidity, alkalinity, temperature, drought and glyphosate. One strain efficient on both varieties and exhibiting stress abiotic tolerance (BLYH-15) belonged to genosp. IV sv. phaseoli, a species first found as a legume symbiont.