N-acyl-L-homoserine lactones (AHLs) are quorum sensing (QS) signal molecules that are commonly used in gram-negative bacteria. Recently, it has become evident that AHLs can influence the behavior of plant cells. However, little is known about the mechanism of the plants' response to these bacterial signals. Calcium ions (Ca2+), ubiquitous intracellular second messengers, play an essential role in numerous signal transduction pathways in plants. In this study, the cytosolic free Ca2+ concentration ([Ca2+](cyt)) was measured by a luminometric method in the excised root cells of Arabidopsis plants that were treated with N-butyryl-homoserine lactone (C4-HSL). There was a transient and immediate increase in [Ca2+](cyt), levels, and the highest level (0.4 mu M), approximately 2-fold higher than the basal level, was observed at the 6th second after the addition of 10 mu M C4-HSL. Pretreatment; with La3+, verapamil or ethylene glycol tetraacetic acid (EGTA) inhibited the increase in [Ca2+](cyt) caused by C4-HSL, whereas it remained unaffected by pretreatment with Li+, indicating that the Ca2+ contributing to the increase in [Ca2+](cyt) was mobilized from the extracellular medium via the plasma membrane Ca2+ channels but not from the intracellular Ca2+ stores. Furthermore, electrophysiological approaches showed that the transmembrane Ca2+ current was significantly increased with the addition of C4-HSL. Taken together, our observations suggest that C4-HSL may act as an elicitor from bacteria to plants and that Ca2+ signaling participates in the ability of plant cells to sense the bacterial QS signals. (C) 2011 Elsevier Inc. All rights reserved.