Hydrogen peroxide and nitric oxide as signalling molecules in plants

It is now clear that hydrogen peroxide (H2O2) and nitric oxide (NO) function as signalling molecules in plants. A wide range of abiotic and biotic stresses results in H2O2 generation, from a variety of sources. H2O2 is removed from cells via a number of antioxidant mechanisms, both enzymatic and non-enzymatic. Both biotic and abiotic stresses can induce NO synthesis, but the biosynthetic origins of NO in plants have not yet been resolved. Cellular responses to H2O2 and NO are complex, with considerable cross-talk between responses to several stimuli. In this review the potential roles of H2O2 and NO during various stresses and the signalling pathways they activate are discussed. Key signalling components that might provide targets for enhancing crop production are also identified.