ROLE OF NITRIC OXIDE IN PLANT RESPONSE TO ABIOTIC STRESS

Nitric oxide (NO) is a small gaseous radical molecule previously studied primarily as an air pollutant and metabolic product of certain bacteria. NO's uptake into leaves.. as well as its metabolism and phytotoxicity are well documented. It was subsequently demonstrated that plants not only respond to atmospheric NO but also produce substantial amount of nitric oxide. After such discovery our appreciation of NO dramatically changed. Nowadays there is no doubt that nitric oxide has emerged as an important molecule in plant signal transduction pathways, where NO may directly or indirectly interact with other signaling molecules such as cyclic nucleotides (cAMP. cGMP), H2O2. salicylic acid, and cytosolic Ca2+. It is likely that concentration of biologically active molecule such as nitric oxide must be precisely regulated by its synthesis and removal. There are many possible sources of nitric oxide. It can be generated by nitric oxide synthase from L-arginine or from nitrite via nitrate reductase. Moreover, NO can be generated non-enzymaticly as a byproduct of denitrification, nitrogen fixation and respiration. Simple chemical reactions and some compounds such as superoxide anions, glutathione, transition metals and non-symbiotic haemoglobins are responsible for quick NO removal from the solution. Various experimental data indicate that in plants nitric oxide plays important signaling role in diverse (patho)physiological processes from reduction of seed dormancy and promotion of seed germination, regulation of plant senescence, suppression of floral transition, stomatal movement as an intermediate downstream of abscisic acid signaling to programmed cell death and xylogenesis. Moreover, nitric oxide mediates a multiple plant responses toward a variety of biotic (pathogen infection) and abiotic (drought, salt, heat, UV light irradiation, heavy metals, herbivores, mechanical wounding) stresses. All stresses mostly induce NO production in plants. NO alleviates the harmfulness of reactive oxygen species and reacts with other target molecules such as salicylic acid, calcium ions and cyclic GMP. It also regulates the expression of stress responsive genes. In the present review. we introduce how NO is produced and removed in plants and highlight the recent progress that provides novel insights into the functions of NO under abiotic stresses. Moreover, interactions of NO signaling with other signaling molecules in regulation of stomatal closure in responses to dehydratation were also discussed.