Nitric Oxide and Reactive Oxygen Species Interaction for Stress Signaling

Both reactive nitrogen species (RNS) and reactive oxygen species (ROS), among which nitric oxide (NO) is included, can behave as key signal molecules in plants able to regulate several aspects of plant metabolism including development, growth, reactivity and tolerance to environmental stress, the transport of solutes, and apoptotic processes. After its formation in plant tissues, NO will bring about signaling together with several other reactive molecules possessing similar chemical behavior (e.g. ROS, H 2 S, glutathione, and other antioxidants), depending on the redox state of the reaction milieu. By reacting with such reaction partners, NO will form new molecules prone to behaving as signal molecules, such as peroxynitrite and nitrosothiols, giving rise to chemical competitions that will determine the final signal and extent of the signaling pathway. In plants, responses to environmental stresses (biotic or abiotic) triggered by ROS and NO may give rise to general outcomes, such as an increase in the antioxidant status, or may consist of stress-specific adaptations, according to the stress type, and require crosstalk with other signaling molecular entities, such as protein kinases, phytohormones, and secondary messengers such as calcium. The activity of ROS and NO in vivo often stems from their ability to bring about protein modifications at the posttranslational level, especially through S -glutathionylation and S -nitrosylation, respectively. Such changes will affect proteins in terms of their activity, stability, cellular localization, and reactions with other molecules, influencing the dynamics of the entire cell and helping in maintaining homeostasis. Despite the recent progress in understanding the signaling actions of ROS and of NO, several open questions remain, so that further research is required, especially as far as their molecular crosstalk is concerned.