Extended: Decoding Ca2+ signatures and signaling to the actin cytoskeleton during the plant immune response

Plants have evolved conserved molecular mechanisms to recognize microbe-associated or damage-associated molecular patterns (MAMPs or DAMPs), mechanical perturbations, and/or changes in cell wall integrity to initiate the first layer of defense responses termed pattern-triggered immunity (PTI). Cytosolic Ca2+ signals are recognized as a key early PTI signaling element as they are highly specific in duration, amplitude and frequency, and therefore encode specific information to trigger distinct downstream signaling pathways in response to different stimuli. However, Ca2+ signatures and their spread at the spatial level in response to PTI signals have been largely uncharacterized. In this work, by developing a high spatiotemporal quantitative imaging pipeline, we characterized the single-cell Ca2+ signatures as well as the spatial spread of Ca2+ waves induced by bacterial or fungal elicitors. We demonstrated that MAMP-induced Ca2+ signal spread exhibits an Initiator vs. Standby cell pattern, with intercellular waves propagate at a low but constant speed. This is distinct from wound-induced local waves, indicating a different molecular mechanism underlying Ca2+ signal propagation at the tissue level. Our findings contribute to the understanding of early signaling networks and signal propagation at both cellular and multicellular levels during plant immunity.