Nitric oxide synthase in the plant kingdom

After the discovery of nitric oxide (NO) as an important signaling molecule in plants, its involvement has been reported in several key physiological processes. At the cellular level, slight alterations in the quantity of NO or its various adducts, also known as reactive nitrogen intermediates (RNIs), have phenomenal implications. In plants this highly reactive, diatomic gaseous molecule regulates a plethora of physiological processes ranging from development, to reproduction, and defense against biotic and abiotic stresses. In animals, NO is produced enzymatically via the nitric oxide synthase (NOS) enzyme. However, after decades of research, it is now clear that in plants there is not one but several routes for NO production. Interestingly the discovery of a NOS enzyme in plants has remained an attractive topic of research for plant scientists over the years; the enzyme still remains elusive. In this chapter we briefly discuss the different pathways responsible for NO production in plants with special emphasis on the enzymatic production. We also discuss the NOS enzyme and its presence in lower and higher plants.