Nitric Oxide and Abscisic Acid Cross Talk in Guard Cells

Despite recent efforts to elucidate the regulation of stomatal movement, many components within the branched pathways of guard cell abscisic acid (ABA) signaling remain to be identified. Here, we show results supporting the involvement of nitric oxide (NO) as a new component of this signaling pathway. NO is a short life bioactive molecule first described as a toxic compound, but now recognized to be an important signal and effector molecule both in animal and plant cell physiology. Even though NO research in plants is not as advanced as in animals, in the last decade NO was proved to participate in many key physiological processes such as growth, pathogen defense reaction, development, programmed cell death, and stress tolerance (Foissner et al., 2000; Pedroso et al., 2000; Beligni and Lamattina, 2001a). In plants, as in animals, NO was proved to interact with other signaling elements such as cADPR, lipids, cGMP, ion channels, Ca2+, and others. In addition, much evidence is appearing lately about cross talk between NO and some plant hormones during adaptive responses to adverse conditions (Hausladen and Stamler, 1998; Durner and Klessig, 1999; Jacob et al., 1999; Beligni and Lamattina, 2001b; Wendehenne et al., 2001). Furthermore, it is now becoming clear that a network constituted at many levels is operating during plant responses to stress stimulus (Knight and Knight, 2001).