The article reviews the role of nitric oxide (NO) as a signaling molecule in plants. NO, a small, water and lipid-soluble gas, has emerged as a key signaling molecule in plants. The review outlines the potential sources of endogenous NO, including nitric oxide synthase (NOS), nitrate reductase (NR), and nonenzymatic sources. It discusses the biological processes mediated by NO, such as defense gene expression, programmed cell death (PCD), stomatal closure, seed germination, and root development. The article also highlights the downstream signaling processes by which NO exerts its cellular effects, including the generation of cyclic guanosine monophosphate (cGMP), cyclic adenosine 5′-diphosphoribose (cADPR), and elevation of cytosolic calcium. The review emphasizes the need for reliable methods to quantify intracellular NO synthesis and release and the classification of biosynthetic origins of NO. The article concludes by discussing the potential roles of NO in plant growth, development, and interactions with abiotic and biotic stresses.The article reviews the role of nitric oxide (NO) as a signaling molecule in plants. NO, a small, water and lipid-soluble gas, has emerged as a key signaling molecule in plants. The review outlines the potential sources of endogenous NO, including nitric oxide synthase (NOS), nitrate reductase (NR), and nonenzymatic sources. It discusses the biological processes mediated by NO, such as defense gene expression, programmed cell death (PCD), stomatal closure, seed germination, and root development. The article also highlights the downstream signaling processes by which NO exerts its cellular effects, including the generation of cyclic guanosine monophosphate (cGMP), cyclic adenosine 5′-diphosphoribose (cADPR), and elevation of cytosolic calcium. The review emphasizes the need for reliable methods to quantify intracellular NO synthesis and release and the classification of biosynthetic origins of NO. The article concludes by discussing the potential roles of NO in plant growth, development, and interactions with abiotic and biotic stresses.