The objective of this study was to relate the activation of enzymatic antioxidant system to the production of reactive oxygen species induced by salt stress. Rice (Oryza sativa L.) genotypes BRS Bojuru and BRS Pampa, tolerant and sensitive to salinity, respectively, were subjected to 150 mM NaCl for 0, 6, 24, 48, and 72 h. A significant increase of superoxide anion and H₂O₂ and a decrease in malondialdehyde (MDA) content were observed in the tolerant genotype, whereas in the sensitive genotype, there was no change in superoxide anion content, reduced H₂O₂ content, and increased MDA content. The superoxide dismutase (SOD) activity increased significantly in both genotypes, and increases in amounts of transcript were observed for OsSOD3Cu/Zn and OsSODA1-Mn in the tolerant genotype and for OsSOD4-Cu/Zn, OsSOD3-Cu/Zn, OsSODCc1-Cu/Zn, OsSOD-Fe, and OsSODA1-Mn in the sensitive genotype. The activities of catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) were not significantly and consistently changed, but OsCATA, OsAPX2 and OsGR1 were induced in both genotypes. OsCATB transcription was increased in the tolerant genotype and OsCATC and OsAPX3 in the sensitive genotype under salinity. It is concluded that OsAPX3, OsGR2, OsGR3, and OsSOD3-Cu/Zn genes are the most suitable to distinguish tolerant from sensitive genotypes under salt stress.

To obtain accurate and reliable results for the expression of genes of interest using quantitative real-time polymerase chain reaction (RT-qPCR) techniques, it is necessary to normalize the data by comparing them to constitutive genes that exhibit uniform expression levels under experimental conditions. In this study, the stability of expression was evaluated for the following ten candidate reference genes in rice leaves (Oryza sativa L.) from the BRS Bojuru and BRS Ligeirinho genotypes that were subjected to salt stress (150 mM): actin 11 (ACT11), beta-tubulin (β-TUB), eukaryote elongation factor 1-α (Eef-1), eukaryotic initiation factor 4-α (eIF-4-α), E2 ubiquitin-conjugating enzyme (UBC-E2), ubiquitin 5 (UBQ5), ubiquitin 10 (UBQ10), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), TIP41-like, and cyclophilin. The stability of expression for the aforementioned genes was then compared to that of three LTP genes using UBQ10, Eef-1, and eIF-4-α as references. After analyzing the expression levels using analysis of variance tests, the results indicated that UBQ10 was the most stable in all treatments (M = 0.404 and SV = 0.327). Furthermore, the eIF-4-α, TIP41-like, and cyclophilin genes exhibited the highest total coefficient of variation (CV = 269, 169.2, 179.2, respectively), which signifies that they exhibited the least stable expression. The expression levels of each candidate gene (LTP7, LTP10, and LTP13) were in contrast to the reference genes. Therefore, we concluded that UBQ10 is the best reference gene for RT-qPCR reactions under the experimental conditions. The expression analysis of LTP7, LTP10, and LTP13 confirmed the importance of validating reference genes to achieve accurate RT-qPCR results.

Different rice (Oryza sativa L.) genotypes were subjected to high salinity and low temperature (150 mM NaCl and 13°C, respectively) for 0, 6, 24, 48, or 72 h. We evaluated the simultaneous expression of the genes OsCATA, OsCATB, and OsCATC, correlated gene expression with enzyme activity, and verified the regulation of these genes through identification of cis-elements in the promoter region. The hydrogen peroxide content increased in a tolerant genotype and decreased in a sensitive genotype under both stress conditions. Lipid peroxidation increased in the tolerant genotype when exposed to cold, and in the sensitive genotype when exposed to high salinity. Catalase activity significantly increased in both genotypes when subjected to 13°C. In the tolerant genotype, OsCATA and OsCATB were the most responsive to high salinity and cold, while in the sensitive genotype, OsCATA and OsCATC responded positively to saline stress, as did OsCATA and OsCATB to low temperature. Cis-element analysis identified different regulatory sequences in the catalase promoter region of each genotype. The sensitive genotype maintained a better balance between hydrogen oxyacid levels, catalase activity, and lipid peroxidation under low temperature than the resistant genotype. OsCATA and OsCATB were the most responsive in the salt-tolerant genotype to cold, OsCATA and OsCATC were the most responsive to saline stress, and OsCATA and OsCATB were the most responsive to chilling stress in the sensitive genotype. There were positive correlations between catalase activity and OsCATB expression in the tolerant genotype under saline stress and in the sensitive genotype under cold stress.