Receptor-Mediated Plant Root Perception of Beneficial Microbes

Plant roots are naturally associated with beneficial microbes, such as symbiotic and commensal microbes. Well-studied symbiotic microbes include nitrogen-fixing rhizobial bacteria and arbuscular mycorrhizal fungi (AMF), which establish symbiotic relationships with host plants to enhance the acquisition of limited nutrients from the environment. These symbiotic associations require plant Lysin-motif-containing receptor-like kinases (LysM-RLKs) to recognize elicitors, such as chitooligosaccharides (COs) and lipochitooligosaccharides (LCOs), from rhizobia or AMF. These elicitors activate symbiotic signaling to promote root colonization and endosymbiotic infection. There are a total of 23 LysM-RLKs in Medicago truncatula, and the function of most of them remains unknown. We used a genetic approach to screen mutant lines of each LysM-RLK gene for their roles in symbiosis and identified new LysM-RLKs that positively regulate mycorrhizal symbiosis. The activation of symbiotic signaling must be balanced with immunity signaling, which is triggered by microbe-associated molecular patterns (MAMPs) from symbionts during early infection in plant roots. We demonstrate that different LysM-RLK complexes mediate the discrimination between plant immunity and symbiosis. Additionally, we show that LysM-RLKs influence the composition of the broader root microbiota and are required for commensal bacteria-induced plant growth promotion in M. truncatula. Our work reveals how plants utilize their cell-surface receptors to encourage beneficial microbial colonization.