Plasma membrane potential depolarization and cytosolic calcium flux are early events involved in tomato (Solanum lycopersicon) plant-to-plant communication

Tomato plants respond to herbivory by emitting volatile organic compounds (VOCs), which are released into the surrounding atmosphere. We analyzed the tomato herbivore-induced VOCs and tested the ability of tomato receiver plants to detect tomato donor volatiles by analyzing early responses, including plasma membrane potential (V-m) variations and cytosolic calcium ([Ca2+](cyt)) fluxes. Receiver tomato plants responded within seconds to herbivore-induced VOCs with a strong V-m depolarization, which was only partly recovered by fluxing receiver plants with clean air. Among emitted volatiles, we identified by GC-MS some green leaf volatiles (GLVs) such as (E)-2-hexenal, (Z)-3-hexenal, (Z)-3-hexenyl acetate, the monoterpene alpha-pinene, and the sesquiterpene beta-caryophyllene. GLVs were found to exert the stronger V-m depolarization, when compared to alpha-pinene and beta-caryophyllene. Furthermore, V-m depolarization was found to increase with increasing GLVs concentration. GLVs were also found to induce a strong [Ca2+](cyt) increase, particularly when (Z)-3-hexenyl acetate was tested both in solution and with a gas. On the other hand, alpha-pinene and beta-caryophyllene, which also induced a significant V-m depolarization with respect to controls, did not exert any significant effect on [Ca2+](cyt) homeostasis. Our results show for the first time that plant perception of volatile cues (especially GLVs) from the surrounding environment is mediated by early events, occurring within seconds and involving the alteration of the plasma membrane potential and the [Ca2+](cyt) flux.