New Insights into the Functional Role of Nitric Oxide and Reactive Oxygen Species in Plant Response to Biotic and Abiotic Stress Conditions

In the natural environment, plants are continuously exposed to unfavourable stress factors during their life cycle, which negatively affect their growth and development, and also crop productivity and, consequently, vast economic losses. These adverse conditions are usually accompanied by a rise in the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS), chiefly hydrogen peroxide (H2O2) and nitric oxide (NO), respectively. This leads to an imbalance between their production and scavenging to trigger nitro-oxidative damage that can hinder plant cell survival. Despite their deleterious and harmful effects, reactive oxygen and nitrogen species can also play many beneficial roles at low concentrations, in several physiological processes. NO and ROS are important signalling molecules in plants that regulate different metabolic pathways in the proximity of damaged tissues, and in tissues and organs not yet exposed to these stimuli. Hence, their production and accumulation in plant cells should be controlled due to their higher reactivity and toxicity at elevated concentrations. This chapter summarises recent progress made in understanding the role of NO and ROS in plant responses to biotic and abiotic stress situations by placing special emphasis on the specific impact of these reactive oxygen and nitrogen species on the biochemical and molecular alterations that happen in plants under stress conditions.