Nitric Oxide (NO) and Physio-biochemical Adaptation in Plants Against Stress

Abstract

Nitric oxide synthase (NOS) enzyme is responsible for NO generation by catalyzing the oxidation of l-arginine to l-citrulline. Presence of this enzyme is reported from unicellular prokaryotes to multicellular higher plants and animals. Plants employ diverse enzymes to generate NO that can be grouped into animal-like NOS or alternative enzymes. Alternative enzymes that are structurally unlike animal NOS but that generate NO include NR, PM NiNOR, XOR, and polyamine oxidase. NO is also produced by nonenzymatic reactions in plants. Both in natural and agriculture conditions, plants are frequently exposed to stress. NO has been found to be involved in an array of plant physiological processes including pollen germination, seed germination, root development, stomatal movements, flowering, senescence, and programmed cell death along with abiotic and biotic stress conditions such as temperature, salinity, drought, heavy metal stress, and pathogen attack or herbivory. Some environmental factors such as temperature, cold or heat, can become stressful in just a few minutes; others may take days to weeks (soil water) or even months (mineral nutrients) to become stressful. All these conditions induce the generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS). These species not only initiate several oxidatively destructive processes, but also trigger various signaling pathways that maintain appropriate cellular ROS levels. It has been found that NO generated during various stress conditions interacts with ROS in various ways and serves as an antioxidant molecule that helps plants adapt in adverse environmental condition.