[Show abstract][Hide abstract] ABSTRACT: During stomatal closure, nitric oxide (NO) operates as one of the key intermediates in the complex, abscisic acid (ABA)-mediated, guard cell signaling network that regulates this process. However, data concerning the role of NO in stomatal closure that occurs in turgid vs. dehydrated plants is limited. The data presented demonstrate that, while there is a requirement for NO during the ABA-induced stomatal closure of turgid leaves, such a requirement does not exist for ABA-enhanced stomatal closure observed to occur during conditions of rapid dehydration. The data also indicate that the ABA signaling pathway must be both functional and to some degree activated for guard cell NO signaling to occur. These observations are in line with the idea that the effects of NO in guard cells are mediated via a Ca(2+)-dependent rather than a Ca(2+)-independent ABA signaling pathway. It appears that there is a role for NO in the fine tuning of the stomatal apertures of turgid leaves that occurs in response to fluctuations in the prevailing environment.
[Show abstract][Hide abstract] ABSTRACT: Abscisic acid (ABA)-induced stomatal closure is mediated by a complex, guard cell signalling network involving nitric oxide (NO) as a key intermediate. However, there is a lack of information concerning the role of NO in the ABA-enhanced stomatal closure seen in dehydrated plants. The data herein demonstrate that, while nitrate reductase (NR)1-mediated NO generation is required for the ABA-induced closure of stomata in turgid leaves, it is not required for ABA-enhanced stomatal closure under conditions leading to rapid dehydration. The results also show that NO signalling in the guard cells of turgid leaves requires the ABA-signalling pathway to be both capable of function and active. The alignment of this NO signalling with guard cell Ca(2+)-dependent/independent ABA signalling is discussed. The data also highlight a physiological role for NO signalling in turgid leaves and show that stomatal closure during the light-to-dark transition requires NR1-mediated NO generation and signalling.
[Show abstract][Hide abstract] ABSTRACT: Various experimental data indicate signalling roles for nitric oxide (NO) in processes such as xylogenesis, programmed cell death, pathogen defence, flowering, stomatal closure, and gravitropism. However, it still remains unclear how NO is synthesized. Nitric oxide synthase-like activity has been measured in various plant extracts, NO can be generated from nitrite via nitrate reductase and other mechanisms of NO generation are also likely to exist. NO removal mechanisms, for example, by reaction with haemoglobins, have also been identified. NO is a gas emitted by plants, with the rate of evolution increasing under conditions such as pathogen challenge or hypoxia. However, exactly how NO evolution relates to its bioactivity in planta remains to be established. NO has both aqueous and lipid solubility, but is relatively reactive and easily oxidized to other nitrogen oxides. It reacts with superoxide to form peroxynitrite, with other cellular components such as transition metals and haem-containing proteins and with thiol groups to form S-nitrosothiols. Thus, diffusion of NO within the plant may be relatively restricted and there might exist 'NO hot-spots' depending on the sites of NO generation and the local biochemical micro-environment. Alternatively, it is possible that NO is transported as chemical precursors such as nitrite or as nitrosothiols that might function as NO reservoirs. Cellular perception of NO may occur through its reaction with biologically active molecules that could function as 'NO-sensors'. These might include either haem-containing proteins such as guanylyl cyclase which generates the second messenger cGMP or other proteins containing exposed reactive thiol groups. Protein S-nitrosylation alters protein conformation, is reversible and thus, is likely to be of biological significance.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: A look back at the early literature on reactive oxygen species (ROS) gives the impression that these small inorganic molecules had a singular defined role, that of host defence in mammalian systems. However, it is now known that their roles also include a major part in cell signalling, in a broad range of organisms from mammals to plants. Similarly, a look back at papers on the proteins now thought to be involved in the perception of hydrogen peroxide (H(2)O(2)) will show that they too had defined functions assigned to them, completely independent to H(2)O(2) signalling. These proteins have disparate roles, in ethylene perception or even major metabolic pathways such as glycolysis. However, the chemistry of H(2)O(2) sensing dictates that the proteins have a commonality, with active thiol groups being potential ROS targets. The challenge now is to determine the full range of proteins which may partake in the role of H(2)O(2) perception, and to determine the mechanisms by which the signal is transmitted to the next players in the signal transduction pathways.
[Show abstract][Hide abstract] ABSTRACT: Hydrogen peroxide (H2O2) is now recognised as a key signalling molecule in eukaryotes. In plants, H2O2 is involved in regulating stomatal closure, gravitropic responses, gene expression and programmed cell death. Although several kinases, such as oxidative signal-inducible 1 (OXI1) kinase and mitogen-activated protein kinases are known to be activated by exogenous H2O2, little is known about the proteins that directly react with H2O2. Here, we utilised a proteomic approach, using iodoacetamide-based fluorescence tagging of proteins in conjunction with mass spectrometric analysis, to identify several proteins that might be potential targets of H2O2 in the cytosolic fraction of Arabidopsis thaliana, the most prominent of which was cytosolic glyceraldehyde 3-phosphate dehydrogenase (cGAPDH; EC 22.214.171.124). cGAPDH from Arabidopsis is inactivated by H2O2 in vitro, and this inhibition is reversible by the subsequent addition of reductants such as reduced glutathione (GSH). It has been suggested recently that Arabidopsis GAPDH has roles outside of its catalysis as part of glycolysis, while in other systems this includes that of mediating reactive oxygen species (ROS) signalling. Here, we suggest that cGAPDH in Arabidopsis might also have such a role in mediating ROS signalling in plants.