Nitric oxide is a novel component of abscisic acid signaling in stomatal guard cells.

Stomatal closure in response to the hormone abscisic acid (ABA) is mediated by a complex signaling network involving both calcium-dependent and calciumindependent pathways (Assmann and Shimazaki, 1999; Webb et al., 2001), activated by several signaling intermediates (Schroeder et al., 2001) that include hydrogen peroxide (Miao et al., 2000; Pei et al., 2000; Zhang et al., 2001) and lipids such as sphingosine-1phosphate (Ng et al., 2001). Here, we provide evidence that nitric oxide (NO) is also a signaling component of ABA-induced stomatal closure. Our data show that NO synthesis is required for ABA-induced closure and that ABA enhances NO synthesis in guard cells. Exogenous NO induces stomatal closure, and ABA and NO-induced closure require the synthesis and action of cGMP and cyclic ADP Rib (cADPR).