Plant volatiles regulate the activities of Ca2+-permeable channels and promote cytoplasmic calcium transients in Arabidopsis leaf cells

A variety of plant species emit volatile compounds in response to mechanical stresses such as herbivore attack. Although these volatile compounds promote gene expression leading to antiherbivore responses, the underlying transduction mechanisms are largely unknown. While indirect evidence suggests that the cytoplasmic free Ca2+ concentration ([Ca2+](c)) plays a crucial role in the volatile-sensing mechanisms in plants, these roles have not been directly demonstrated. In the present study, we used Arabidopsis leaves expressing apoaequorin, a Ca2+-sensitive luminescent protein, in combination with a luminometer, to monitor [Ca2+](c) transients that occur in response to a variety of volatile compounds and to characterized the pharmacological properties of the increase in [Ca2+]c. When leaves were exposed to volatiles, [Ca2+](c) was transiently raised. The [Ca2+](c) increases induced by acyclic compounds were disrupted by Ruthenium Red, a potential plasma-membrane and endo-membrane Ca2+-permeable channel inhibitor, but not by 1,2-bis(2-aminophenoxy) ethane-N, N, N', N'-tetraacetic acid (BAPTA), an extracellular Ca2+-chelator, suggesting that acyclic compounds promote Ca2+-release from intracellular stores. On the other hand, the electrophilic compound (E)-2-hexenal promoted Ca2+-influx via ROS production by natural oxidation at the aquarius phase. In a gpa1-2 mutant lacking a canonical Ga subunit, the [Ca2+](c) transients induced by all tested volatiles were not attenuated, suggesting that G-protein coupled receptors are not involved in the volatile-induced [Ca2+](c) transients in Arabidopsis leaves.