Symbiotic signalling is at the core of an endophytic Fusarium solani-legume association

Legumes interact with a wide range of microbes in their root system, ranging from beneficial symbionts to pathogens. Symbiotic rhizobia and arbuscular mycorrhizal glomeromycetes trigger a so-called common symbiotic signalling pathway (CSSP), including the induction of nuclear calcium spiking in the root epidermis. In our study, the recognition of an endophytic Fusarium solani strain K in Lotus japonicus induced the expression of LysM receptors for chitin-based molecules, CSSP members and CSSP-dependent genes in L. japonicus. In LysM and CSSP mutant/RNAi lines, root penetration and fungal intraradical progression was either stimulated or limited while FsK exudates are perceived in a CSSP-dependent manner, triggering nuclear calcium spiking in epidermal cells of Medicago truncatula Root Organ Cultures. Our results corroborate that the CSSP is a more common pathway than previously envisaged, involved in the perception of signals from other microbes beyond the restricted group of symbiotic interactions sensu stricto.

Epidermal LysM receptor ensures robust symbiotic signalling in Lotus japonicus

Recognition of Nod factors by LysM receptors is crucial for nitrogen-fixing symbiosis in most legumes. The large families of LysM receptors in legumes suggest concerted functions, yet only NFR1 and NFR5 and their closest homologs are known to be required. Here we show that an epidermal LysM receptor (NFRe), ensures robust signalling in L. japonicus. Mutants of Nfre react to Nod factors with increased calcium spiking interval, reduced transcriptional response and fewer nodules in the presence of rhizobia. NFRe has an active kinase capable of phosphorylating NFR5, which in turn, controls NFRe downstream signalling. Our findings provide evidence for a more complex Nod factor signalling mechanism than previously anticipated. The spatio-temporal interplay between Nfre and Nfr1, and their divergent signalling through distinct kinases suggests the presence of an NFRe-mediated idling state keeping the epidermal cells of the expanding root system attuned to rhizobia.