Calcium ions (Ca2+
) represent the most important intracellular second messengers in the signaling networks of plants. After herbivore damage the opening of specific ion channels achieve a rapid transient increase of the cytoplasmatic Ca2+
-level. The enhanced concentrations can be monitored in planta
after expression of the bioluminescent Aequorin, that emits light upon binding of Ca2+
-ions. The signal spreads with ca. 1-2 cm/min in the directly connected vascular system and corresponds with the speed of electrical signals triggered by herbivore damage.