Convergence of calcium signaling pathways of pathogenic elicitors and abscisic acid in Arabidopsis guard cells

A variety of stimuli, such as abscisic acid (ABA), reactive oxygen species (ROS), and elicitors of plant defense reactions, have been shown to induce stomatal closure. Our study addresses commonalities in the signaling pathways that these stimuli trigger. A recent report showed that both ABA and ROS stimulate an NADPH-dependent, hyperpolarization-activated Ca2+ influx current in Arabidopsis guard cells termed I-Ca (Z.M. Pei, Y. Murata, G. Benning, S. Thomine, B. Klusener, G.J. Allen, E. Grill, J.I. Schroeder, Nature [2002] 406: 731-734). We found that yeast (Saccharomyces cerevisiae) elicitor and chitosan, both elicitors of plant defense responses, also activate this current and activation requires cytosolic NAD(P)H. These elicitors also induced elevations in the concentration of free cytosolic calcium ([Ca2+](cyt)) and stomatal closure in guard cells. ABA and ROS elicited [Ca2+](cyt), oscillations in guard cells only when extracellular Ca2+ was present. In a 5 mM KCl extracellular buffer, 45% of guard cells exhibited spontaneous [Ca2+](cyt) oscillations that differed in their kinetic properties from ABA-induced Ca2+ increases. These spontaneous [Ca2+](cyt) oscillations also required the availability of extracellular Ca2+ and depended on the extracellular potassium concentration. Interestingly, when ABA was applied to spontaneously oscillating cells, ABA caused cessation of [Ca2+](cyt), elevations in 62 of 101 cells, revealing a new mode of ABA signaling. These data show that fungal elicitors activate a shared branch with ABA in the stress signal transduction pathway in guard cells that activates plasma membrane I-Ca channels and support a requirement for extracellular Ca2+ for elicitor and ABA signaling, as well as for cellular [Ca2+](cyt) oscillation maintenance.