Spatially and temporally distinct Ca2+ changes in Lotus japonicus roots orient fungal-triggered signalling pathways towards symbiosis or immunity.

Plants activate an immune or symbiotic response depending on the detection of distinct signals from root-interacting microbes. Both signalling cascades involve Ca²⁺ as a central mediator of early signal transduction. In this study, we combined aequorin- and cameleon-based methods to dissect the changes in cytosolic and nuclear Ca²⁺ concentration caused by different chitin-derived fungal elicitors in Lotus japonicus roots. Our quantitative analyses highlighted the dual character of the evoked Ca²⁺ responses taking advantage of the comparison between different genetic backgrounds: an initial Ca²⁺ influx, dependent on the LysM receptor CERK6 and independent of the common symbiotic signalling pathway (CSSP), is followed by a second CSSP-dependent and CERK6-independent phase, that corresponds to the well-known perinuclear/nuclear Ca²⁺ spiking. We show that the expression of immunity marker genes correlates with the amplitude of the first Ca²⁺ change, depends on elicitor concentration, and is controlled by Ca²⁺ storage in the vacuole. Our findings provide an insight into the Ca²⁺-mediated signalling mechanisms discriminating plant immunity- and symbiosis-related pathways in the context of their simultaneous activation by single fungal elicitors. [ABSTRACT FROM AUTHOR]