Systemic cytosolic Ca2+ elevation is activated upon wounding and herbivory in Arabidopsis

Calcium ion (Ca2+) signalling triggered by insect herbivory is an intricate network with multiple components, involving positive and negative regulators. Real-time, noninvasive imaging of entire Arabidopsis thaliana rosettes was employed to monitor cytosolic free calcium ([Ca2+](cyt)) elevations in local and systemic leaves in response to wounding and Spodoptera littoralis feeding. Luminescence emitted by the cytosol-localized Ca2+ reporter aequorin was imaged using a high-resolution photon-counting camera system.Spodoptera littoralis feeding on Arabidopsis induced both local and systemic [Ca2+](cyt) elevations. Systemic [Ca2+](cyt) signals were found predominantly in adjacent leaves with direct vascular connections to the treated leaf and appeared with a delay of 1 to 2min. Simulated herbivory by wounding always induced a local [Ca2+](cyt) response, but a systemic one only when the midrib was wounded. This systemic [Ca2+](cyt) response was suppressed by the presence of insect-derived oral secretions as well as in a mutant of the vacuolar cation channel, Two Pore Channel 1 (TPC1). Our results provide evidence that in Arabidopsis insect herbivory induces both local and systemic [Ca2+](cyt) signals that distribute within the vascular system. The systemic [Ca2+](cyt) signal could play an important signalling role in systemic plant defence.