A Specific <i>Sinorhizobium</i> Flagellin Suppresses Legume Nodulation Through Immune Activation

Bacterial flagellin-activated immunity plays a crucial role in shaping plant-microbe interactions, leading to either parasitism, mutualism, or commensalism. In the legume-rhizobium symbiosis, while it has been hypothesized that rhizobial infection involves avoidance of plant immunity following flagellin perception, direct evidence supporting this regulation remains unclear. Here, we conducted bioinformatic analyses of flagellin variations across the genus Sinorhizobium and identified a specific variant of the flagellin-derived peptide, flg22^Sin-II (clade II flg22 from Sinorhizobium genus), which acts as an immunity elicitor during nodulation. Flg22^Sin-II, but not flg22^Sin-I or flg22^Sin-III, activates immune responses, including reactive oxygen species production, MPK phosphorylation, and immunity-related gene expression in soybean, with Tyr-7 being critical for the immune activation. Three different Sinorhizobium mutants knocking out the flagellin that produces flg22^Sin-II enhanced nodulation across three diverse legume species, highlighting how beneficial microbes modulate host immunity to optimize symbiotic interactions. Soybean gmfls2a gmfls2b double mutant lacking both flagellin receptors, GmFLS2a and GmFLS2b, exhibited an increased nodule number following S. fredii HH103 inoculation and showed reduced expression of immune-related genes in nodules. Rather than complete immune evasion, the retention of an immune-activating flagellin epitope by Sinorhizobium likely represents a sophisticated coevolutionary strategy to actively modulate host responses, ensuring symbiotic homeostasis and preventing detrimental over-colonisation.