RHD6LA regulates root hair responses to both symbionts and commensals

While intracellular symbiosis with rhizobia relies on Nod factor signaling through the conserved common symbiosis signaling pathway (CSSP), it remains unclear how legumes simultaneously manage interactions with commensal soil microbes. Using single cell RNA-sequencing, we show that commensal soil bacteria induce a Nod factor-independent transcriptional response in specific root hairs. This response is similar to the rhizobium response in the CSSP-deficient cyclops mutant, which is unable to accommodate rhizobia in root hair infection threads. Both responses include the nodulation gene NODULATION SIGNALING PATHWAY 2 (NSP2) and a transcription factor, which we name ROOT HAIR DEFECTIVE 6 LIKE A (RHD6LA). We show that RHD6LA is required for facilitating infection thread formation in response to rhizobia and for preventing exaggerated root hair responses to commensal soil bacteria. The overlap between commensal and symbiotic signaling highlights the complexity of legume-microbe interactions at the root hair interface and suggests additional mechanisms for microbial discrimination in rhizobium-responsive root hairs.