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.