Nitric oxide (NO) has emerged as a key signaling molecule in plants. This review discusses the sources of endogenous NO, the biological processes it mediates, and the downstream signaling pathways by which it exerts its effects. NO can be synthesized from nitrite via nitrate reductase (NR) and, although similar enzymes to mammalian nitric oxide synthase (NOS) have been identified, no NOS genes have been found in plants. NO induces various processes in plants, including defense-related gene expression, programmed cell death, stomatal closure, seed germination, and root development. Intracellular signaling responses to NO involve the generation of cGMP, cADPR, and elevation of cytosolic calcium, but the precise biochemical and cellular nature of these responses is not fully understood. Research priorities include the reliable quantification of downstream signaling molecules in NO-responsive cells and the cloning and manipulation of enzymes responsible for their synthesis and degradation. NO is a gaseous free radical that can exist in three forms: the radical (NO), the nitrosonium cation (NO⁺), and the nitroxyl radical (NO⁻). It is sparingly soluble in water and can move through aqueous and lipid phases of cells. Once produced, it can move between cells or within a cell, but has a short half-life. NO can react with oxygen to form nitrogen dioxide and degrade to nitrite and nitrate in aqueous solution. The formation of nitrites and nitrates is often used to measure the presence of NO. NO can also react with superoxide and hydrogen peroxide to produce peroxynitrite, a reactive and destructive compound. NO can react with proteins, particularly with thiol side groups, or low molecular weight thiols. NO is produced in plants from several sources, including nitric oxide synthase (NOS), nitrate reductase (NR), xanthine oxidoreductase, and nonenzymatic sources. NOS is a family of enzymes that catalyze the formation of NO from L-arginine. NR is a key enzyme in nitrate assimilation and can generate NO from nitrite. Other enzymes, such as xanthine oxidoreductase, can also produce NO. Nonenzymatic sources of NO include chemical reduction of nitrite under acidic or reducing conditions and light-mediated conversion of nitrogen dioxide to nitric oxide. NO has various biological functions in plants, including plant growth and development, hormone signaling, and responses to abiotic and biotic stresses. NO can stimulate seed germination and root development. It is involved in the regulation of stomatal closure in response to abscisic acid (ABA). NO also plays a role in plant defense against pathogens and in the response to various stresses, such as drought, heat, and chilling. NO can interact with reactive oxygen species (ROS) to produce peroxynitrite, which can have both beneficial andNitric oxide (NO) has emerged as a key signaling molecule in plants. This review discusses the sources of endogenous NO, the biological processes it mediates, and the downstream signaling pathways by which it exerts its effects. NO can be synthesized from nitrite via nitrate reductase (NR) and, although similar enzymes to mammalian nitric oxide synthase (NOS) have been identified, no NOS genes have been found in plants. NO induces various processes in plants, including defense-related gene expression, programmed cell death, stomatal closure, seed germination, and root development. Intracellular signaling responses to NO involve the generation of cGMP, cADPR, and elevation of cytosolic calcium, but the precise biochemical and cellular nature of these responses is not fully understood. Research priorities include the reliable quantification of downstream signaling molecules in NO-responsive cells and the cloning and manipulation of enzymes responsible for their synthesis and degradation. NO is a gaseous free radical that can exist in three forms: the radical (NO), the nitrosonium cation (NO⁺), and the nitroxyl radical (NO⁻). It is sparingly soluble in water and can move through aqueous and lipid phases of cells. Once produced, it can move between cells or within a cell, but has a short half-life. NO can react with oxygen to form nitrogen dioxide and degrade to nitrite and nitrate in aqueous solution. The formation of nitrites and nitrates is often used to measure the presence of NO. NO can also react with superoxide and hydrogen peroxide to produce peroxynitrite, a reactive and destructive compound. NO can react with proteins, particularly with thiol side groups, or low molecular weight thiols. NO is produced in plants from several sources, including nitric oxide synthase (NOS), nitrate reductase (NR), xanthine oxidoreductase, and nonenzymatic sources. NOS is a family of enzymes that catalyze the formation of NO from L-arginine. NR is a key enzyme in nitrate assimilation and can generate NO from nitrite. Other enzymes, such as xanthine oxidoreductase, can also produce NO. Nonenzymatic sources of NO include chemical reduction of nitrite under acidic or reducing conditions and light-mediated conversion of nitrogen dioxide to nitric oxide. NO has various biological functions in plants, including plant growth and development, hormone signaling, and responses to abiotic and biotic stresses. NO can stimulate seed germination and root development. It is involved in the regulation of stomatal closure in response to abscisic acid (ABA). NO also plays a role in plant defense against pathogens and in the response to various stresses, such as drought, heat, and chilling. NO can interact with reactive oxygen species (ROS) to produce peroxynitrite, which can have both beneficial and