Genetic Characterization and Symbiotic Performance of Soybean Rhizobia Under Cold and Water-Deficient Conditions in Poland

Soybeans have been cultivated in Poland for more than 140 years. However, Poland’s cold and water-deficient climatic conditions hinder soybean cultivation. Although the availability of suitable soybean varieties in Poland contributes to meeting the demand for soybean production, it is important to identify rhizobial inoculants in Polish soils suitable for soybean cultivation. In this study, we cultivated soybean varieties (Abelina, Merlin, and Sultana) grown in soils taken from four regions in Poland and isolated 330 strains from soybean root nodules. 16S rRNA gene sequencing identified 49 strains of highly stress-tolerant nodule-associated bacteria, including <i>Bradyrhizobium</i>, <i>Rhizobium</i>, <i>Ensifer</i>, <i>Tardiphaga</i>, and <i>Ralstonia</i> spp. Several isolates exhibited positive effects on soybean growth under cold and water-deficient conditions. In particular, the isolate <i>Bradyrhizobium japonicum</i> PSN49, which is phylogenetically similar to <i>B</i>. <i>japonicum</i> USDA 123, increased plant biomass and nodule formation in the soybean cultivar Abelina under abiotic stress conditions due to its high nitrogen-fixing activity. Whole-genome comparisons between PSN49 and other <i>Bradyrhizobium</i> strains revealed that trehalose biosynthesis genes and cold shock proteins contributed to cold stress tolerance. These findings and the strains identified in this study will enhance soybean production and deepen the understanding of the soybean–rhizobium relationship in Poland.