ABSTRACT
Nitric oxide (NO), a free radical in living organisms, is considered a key signaling molecule functioning in various physiological processes of plants. These physiological processes include germination, growth, senescence, and photosynthesis as well as response mechanisms to specific environmental stresses. It is well known that various abiotic stresses such as drought, low and high temperatures, UV and ozone exposure induce the generation of reactive oxygen species (ROS). ROS not only initiate several oxidative destructive processes but also trigger various signaling pathways. Thus, maintenance of appropriate ROS levels might represent a survival response. In fact, NO interacts with ROS in various ways and might have an antioxidant function during various stresses. Because of its chemistry and reactivity, NO can have a number of important direct functions in plant defense in parallel with ROS. It can be directly cytotoxic to microbes, affect gene expression by altering the redox status of the cell, regulate protein function through direct posttranslational modifications, and cause damage to DNA and proteins. The aim of the present review is to provide a deeper understanding of the functional role of NO in plants with special emphasis on defense signaling under abiotic stress conditions.
