Multiphasic calcium signatures in the cytosol and nucleus of Lotus japonicus roots unlock the early discrimination of fungal signals

Changes in intracellular calcium concentration are known to mediate plant responses to both beneficial and detrimental microorganisms. Increasing evidence suggests that boundaries between early signalling events in plant pathogenesis and symbiosis are less defined than previously thought, given the occurrence of Ca2+ signals partially sharing subcellular location and dynamics. In this study we used differentially targeted aequorin-based Ca2+ reporters to measure cytosolic and nuclear Ca2+ signatures triggered in Lotus japonicus roots by putative symbiotic and pathogenic fungal molecules. By using mutants impaired in the Common Symbiotic Signalling Pathway, we dissected the multiphasic nature of the stimulus-specific Ca2+ transients, highlighting significant differences in the genetic components underlying the cytosolic and nuclear Ca2+ response to short-chain (CO4), long-chain (CO8) and lipidated (mycLCO) chitin oligomers. Moreover, a pharmacological approach shed light on the origin, modulation and interdependence of the observed Ca2+ fluxes. Our analysis of the overall plant root response in terms of cytosolic and nuclear Ca2+ changes helps disentangling the complex communication circuits in the pre-contact signal exchange phase and enhances our quantitative understanding of how plants can perceive and discriminate different fungal molecules. ### Competing Interest Statement The authors have declared no competing interest.