Modulation of ascorbate peroxidase activity by nitric oxide in soybean
Thesis (PhD)--Stellenbosch University, 2012.
Salinity stress is one of the major environmental factors that lead to poor crop yield. This is due to overproduction of reactive oxygen species (ROS) which consequently lead to oxidative stress. Although these ROS may be required for normal physiological functions, their accumulation acts as a double edge sword, as they also cause oxidative damage to nucleic acids, lipids and proteins of plant cell membranes. Plants have evolved with an efficient antioxidant defensive system in order to protect and detoxify harmful effects of ROS. Ascorbate peroxidase (APX) is regarded as one of the major scavengers of H2O2. Although some studies have described the role of nitric oxide (NO) in diverse physiological processes in plants, there is still much to know as regards to modulation of APX activity by nitric oxide in salinity-induced stressed plants. For the purposes of this study, the effect of salt and exogenously applied NO on APX, dehydroascorbate reductase and antioxidant metabolite content was determined. This study investigated the use of NO donor 2,2'-(hydroxynitrosohydrazono) bis-ethanimine (DETA/NO) and diethylenetriamine (DETA) on soybean. The data obtained from this study shows that application of DETA/NO resulted in an increase of NO nodular content and also regulated APX activity. The NO-induced changes in APX enzymatic activity were coupled to altered nodule H2O2 content. Further analysis of APX enzymatic activity identified three APX isoforms for which augmented enzymatic activity occurred in response to NO. By supplementing salinity-induced stress soybeans with NO, this study shows that tolerance to salt stress is improved. The underlying mechanism of the NO-mediated tolerance to salt is shown to be its role in modulating the plant antioxidant defense system thus maintaining redox status under salinity-induced stress. Here, although there was increased APX activity in salt stressed plant, supplementing the salinity-induce stressed plants with NO resulted to even higher APX activity which was sufficient to detoxify ROS. Furthermore, this study shows that the NO-mediated effect is not limited in antioxidant enzymes but also involves regulating antioxidant metabolite ratio through modulating the antioxidant enzymes that are involved in the ascorbate -glutathione cycle.