Abstract: Nitric oxide (NO) is a key signaling molecule in plants. The potential sources of NO in plants include nitric oxide synthase, nitrate reductase, xanthine oxidoreductase and the nonenzymatic pathway. NO can enhance plant growth; delay senescence of leaves, flowers and fruits; stimulate germination of seeds that are dormancy and/or light-requiring seeds for germination;and interact with phytohormones to mediate stomatal movement. Programmed cell death and expression of defence-related genes can also be induced by NO. NO can serve an antioxidant function during various stresses. Intracellular signaling responses to NO involve generation of cyclic guanosine monophosphate, cyclic ADP-ribose and elevation of cytosolic calcium, but the knowledge of the NO signal transduction pathway and its biochemical and cellular nature is yet to be revealed.