NO and H2O2 crosstalk in plant adaptation to stress condition

Nitric oxide (NO) is a redox-active molecule that acts as a signal and plays a key role in plant responses to changed environmental conditions. In a number of plant stress responses, NO generation occurs in parallel with H 2 O 2 , and both molecules can operate synergistically or independently. Plants react to biotic as well as abiotic factors by enhanced accumulation of reactive oxygen species (ROS), resulting in imbalance of redox homeostasis. This activates both signaling events and damaging processes. Under stress conditions, much attention is focused on plasma membrane and apoplast compartments, in which increases in H 2 O 2 could be associated with the activities of NADPH oxidases (NOX) and amine oxidases (AO), such as diamine oxidase (DAO) and polyamine oxidase (PAO). It has been shown that both NOX and AO are involved in plant growth and development as well as in plant responses to pathogens and abiotic stress factors. The recent findings that polyamines (PA) induce NO biosynthesis provide a new insight into PA-mediated signaling, in which NO acts as a potential mediator of PA action. NO biosynthesis that occurs as a result of DAO and PAO-dependent catabolism of PA could explain many functions of PA-mediated stress responses. NO can affect enzyme activity by posttranslational modifications including S-nitrosylation of cysteine residues and irreversible nitration of tyrosine residues. S-nitrosylated glutathione (GSNO), which serves as an NO reservoir and transport form, can be degraded in both nonenzymatic way and enzymatic reaction, catalyzed by GSNO reductase (GSNR). Inhibition of GSNOR by H 2 O 2 has been demonstrated, suggesting that this could be one of the mechanisms of crosstalk between ROS and NO signaling. Moreover, it was shown that NADPH-oxidase activity is inhibited by S-nitrosylation, resulting in the reduction of ROS biosynthesis during immune response. Other studies suggest the role of the mitogen-activated protein kinase (MAPK) cascade in NO production induced by H 2 O 2 . Research using mutants deficient in ROS or NO generation confirmed the strong association between both signaling pathways. Crosstalk between ROS and NO is a common phenomenon in adaptation of plants to stress conditions.