Thesis (MSc (Plant Biotechnology))--University of Stellenbosch, 2010. / ABSTRACT: Nitric Oxide (NO) is an essential bioregulatory molecule in plant growth, development, and tolerance against biotic and abiotic stresses. In legume root nodules, abiotic stresses impose restraint on metabolic capacity of bacteria and cause oxidative damage to cellular macromolecules, leading to inhibition of nitrogenase activity. In this study, the primary aim was to determine the influence of NO signaling on cysteine protease activity in soybean (Glycine. max [L] Merr) root nodules. Intact plants were treated with a NO donor, diethylenetriamine/nitric oxide adjunct (DETA/NO), 8-(4-chlorophenylthio)-[CPT]- cGMP, sorbitol and sodium chloride (NaCl). The root nodule cysteine protease activity was measured using the chromogenic substrate N-benzoil-L-p-nitroanilide (L-BAPNA). The results demonstrated NO as acting both as a protection against programmed cell death (PCD) at low exogenously applied NO concentrations, or as inducing PCD through regulating the cysteine proteases activity in root nodules when NO is applied at elevated concentrations. In the root nodules, the activity of cysteine protease is regulated either through cyclic guanosine monophosphate (cGMP)-dependent during abiotic stress or cGMP-independent pathways during normal root nodule development. The purpose of this research was to highlight the importance of NO in cell signaling and cysteine protease activity in legume root nodules. We also focused on the effect of abiotic stress on two maize genotypes as well as the influence of abiotic stress on cysteine protease activity in the abiotic stress-sensitive maize genotype than the tolerant genotype. The study suggests that cysteine protease activity can be used as early screen to identify abiotic stress-sensitive/tolerant maize genotype upon exposure to abiotic stress. / AFRIKAANSE OPSOMMING: Geen Afrikaanse opsomming beskikbaar. / National Research Foundation