Cyclic di-GMP Modulation of Quorum Sensing and Its Impact on Type VI Secretion System Function in <em>Sinorhizobium fredii</em>

The establishment of effective rhizobium-legume symbiosis relies on a complex interplay of molecular signaling pathways, encompassing not only classical nodulation factors and surface polysaccharides, but also a range of protein secretion systems. Among these, the Type VI Secretion System (T6SS) has attracted attention due to its dual roles in interbacterial competition and interactions with eukaryotic hosts, though its precise function in symbiosis remains unclear. Key regulatory messengers, including the main autoinducer of the quorum sensing (QS) systems, the N-acyl homoserine lactones (AHLs), and the second messenger cyclic di-GMP (c-di-GMP), modulate the transition between motility and biofilm formation, especially in the context of bacteria interacting with eukaryotes, including rhizobia. While c-di-GMP’s impact on exopolysaccharide production in these organisms is well established, its influence on protein secretion systems, particularly in conjunction with QS, is largely unexplored. In this study, we investigate the regulatory interplay between AHLs, c-di-GMP, and their influence in the T6SS activity in three Sinorhizobium fredii strains of agronomic relevance. By leveraging genetic and biochemical tools, we aim to elucidate how these interconnected networks could contribute to the symbiotic performance and competitive fitness of S. fredii, providing new insights into the layered regulatory landscapes governing beneficial plant-microbe interactions.