1 Nitric Oxide: Chemistry, Biosynthesis, and Physiological Role

NOis recognized as abiologicalmessenger inplants. It is a highly reactive gaseous free radical, soluble in water and lipid. It can be synthesized in plants via different enzymaticandnonenzymatic sourcessuchasNOS,NR,XOR,andNi-NOR.Due to its high lipophilic nature, it can easily diffuse throughmembrane and can act as a inter- and intracellular messenger and regulate diverse physiological and bio-chemical processes in plants in a concentration-dependent manner, such as seed dormancy, growth and development, senescence, respiration, photosynthesis, programmed cell death, antioxidant defense system, and so on. Moreover, NO alsohas an ability to act simultaneouslywith othermolecules and signals in plants. This chapter covers the advances in chemical properties and mechanism of its biosynthesis with special emphasis on the role of NO in the physiological and biochemical changes that occur in plants under normal conditions due to the exogenously applied or endogenously produced NO, along with the cross talk between NO and other phytohormones. 1.