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Nitric oxide, stomatal closure, and abiotic stress
1Centre for Research in Plant Science, Faculty of Health and Life Sciences, University of the West of England, Bristol BS16 1QY, UK
2Departemento de Biologia Vegetal, Universidade Federale de Viçosa, Viçosa 36571-000, Minas Gerais, Brazil
3Division of Biology, Imperial College, London SW7 2AZ, UK
4School of Life Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
* To whom correspondence should be addressed. E-mail: Steven.Neill{at}uwe.ac.uk
Various data indicate that nitric oxide (NO) is an endogenous signal in plants that mediates responses to several stimuli. Experimental evidence in support of such signalling roles for NO has been obtained via the application of NO, usually in the form of NO donors, via the measurement of endogenous NO, and through the manipulation of endogenous NO content by chemical and genetic means. Stomatal closure, initiated by abscisic acid (ABA), is effected through a complex symphony of intracellular signalling in which NO appears to be one component. Exogenous NO induces stomatal closure, ABA triggers NO generation, removal of NO by scavengers inhibits stomatal closure in response to ABA, and ABA-induced stomatal closure is reduced in mutants that are impaired in NO generation. The data indicate that ABA-induced guard cell NO generation requires both nitric oxide synthase-like activity and, in Arabidopsis, the NIA1 isoform of nitrate reductase (NR). NO stimulates mitogen-activated protein kinase (MAPK) activity and cGMP production. Both these NO-stimulated events are required for ABA-induced stomatal closure. ABA also stimulates the generation of H2O2 in guard cells, and pharmacological and genetic data demonstrate that NO accumulation in these cells is dependent on such production. Recent data have extended this model to maize mesophyll cells where the induction of antioxidant defences by water stress and ABA required the generation of H2O2 and NO and the activation of a MAPK. Published data suggest that drought and salinity induce NO generation which activates cellular processes that afford some protection against the oxidative stress associated with these conditions. Exogenous NO can also protect cells against oxidative stress. Thus, the data suggest an emerging model of stress responses in which ABA has several ameliorative functions. These include the rapid induction of stomatal closure to reduce transpirational water loss and the activation of antioxidant defences to combat oxidative stress. These are two processes that both involve NO as a key signalling intermediate.
Key words: Abscisic acid, antioxidants, guard cells, hydrogen peroxide, nitric oxide, oxidative stress, stomata, water stress
Received 19 June 2007; Revised 21 September 2007 Accepted 5 November 2007
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