Paraburkholderia phymatum STM815^T Pectate Lyase Has a Negative Impact on Nitrogen-Fixing Symbiosis with Common Bean

In the face of global challenges such as food insecurity, environmental degradation, and climate change, biological nitrogen fixation by rhizobia has become increasingly crucial for supporting sustainable agriculture and reducing reliance on synthetic fertilizers. Paraburkholderia phymatum STM815^T is a beta-proteobacterial rhizobium notable for its exceptionally broad host range, forming nitrogen-fixing symbioses with over 50 legume species. In this study, we identified pelB on the P. phymatum STM815^T symbiotic plasmid, which codes for a pectate lyase, whose expression is activated by the presence of pectin in the medium and during symbiosis with common bean. In the absence of pelB, P. phymatum STM815^T shows improved symbiotic performance with common bean. Plants infected with the pelB mutant developed fewer but larger nodules and exhibited a 43% increase in nitrogenase activity, suggesting that pelB in P. phymatum STM815^T may negatively affect nodulation efficiency and nitrogen fixation in common bean.