Immune modulation by the host's peptidyl-prolyl cis/trans isomerase in root nodule symbiosis

In most legume-rhizobium symbioses, rhizobial colonization occurs through host-derived intracellular infection threads, which enable recruitment of compatible rhizobia while modulating the host immune system to prevent rejection. To investigate how legumes regulate immune responses through post-translational mechanisms during symbiosis, we focused on Cyclophilin A (CyPA), a peptidyl-prolyl cis/trans isomerase. Previous studies have shown that Arabidopsis CyPA, especially its gain-of-function variant, suppresses immune responses against pathogens, an effect that may seem counterproductive for the host in plant-pathogen interactions. Here, we hypothesized that native CyPA might fulfill a beneficial function in symbiotic interactions, such as facilitating the suppression of immune responses against compatible rhizobia. We analyzed CyPAs in the context of root nodule symbiosis using the model legume Lotus japonicus and found that one of CyPAs (named CyPA1) is required for intracellular infection in legumes. CyPA1 was associated with the immune hub RIN4, whose cis and trans conformations respectively promoted and inhibited symbiont acceptance. These results illuminate our understanding of mechanisms for acceptance of symbiotic partners.