## 2 Results
### Na\({}^{+}\) and K\({}^{+}\) Accumulation during Salinity Stress Is Influenced by GmPgb1  
Depression of the photosynthetic rate and leaf injury during salinity stress are often related to the toxicity of the Na\({}^{+}\)[13; 14] accumulating in leaves and disturbing the homeostasis of K\({}^{+}\) needed in several photosynthetic functions [16]. Salt stress augmented root and leaf Na\({}^{+}\) in all the genotypes, but the increment in leaves was reduced in plants overexpressing _GmPgb1_ (Figure 4A). The level of K\({}^{+}\) was generally reduced by Na\({}_{2}\)SO\({}_{4}\) in both organs, and the reduction was abolished when _GmPgb1_ was overexpressed (Figure 4B). Because of these changes, the foliar Na\({}^{+}\)/K\({}^{+}\) ratio after 24 h of salt stress was lower in the leaves overexpressing _GmPgb1_ relative to those of WT and GmPgb1(RNAi)23 plants (Figure 4C)  
Figure 3: Retention of the water content (WC) and dry weight and leaf injury of soybean plants exposed to Na\({}_{2}\)SO\({}_{4}\) for seven days. Measurements were taken in the WT line and lines overexpressing (GmPgb1(S)17) or downregulated (GmPgb1(RNAi)23) _GmPgb1_. Values are the means \(\pm\) SE of five biological replicates, each consisting of at least five plants. Letters indicate statistically significant differences (\(p<0.05\)). Micrographs of leaves of waterlogged plants are also shown. To further understand the mechanisms for the differential accumulation of Na\({}^{+}\) and K\({}^{+}\), we measured the expression of two key genes involved in Na\({}^{+}\) and K\({}^{+}\) transport: the  
Figure 4: Levels of Na\({}^{+}\) (**A**), K\({}^{+}\) (**B**), and Na\({}^{+}\)/K\({}^{+}\) ratio (**C**) in WT plants and plants overexpressing (GmPgb1(S)17) or downregulated (GmPgb1(RNAi)23) _GmPgb1_. Values are the means \(\pm\) SE of three biological replicates, each consisting of at least five plants. Letters indicate statistically significant differences (\(p\) < 0.05). (**D**) Relative transcript levels of _GmSOS1_ and _GmAKT1_ in the same plants. Na+ efflux transporter SALT-OVERLY SENSITIVE 1 (SOS1) limiting the Na+ intake in roots and its long-distance movement [16] and the inward-rectifying K+ channel AKT1 required for the maintenance of the Na+/K+ ratio [40]. The expression of \(SOS1\), which remained unchanged in GmPgb1(RNAi)23 roots and leaves following imposition of Na2 SO4 stress, increased in both organs of the plants overexpressing _GmPgb1_ (Figure 4D). A small significant increase also occurred in WT leaves. Salinity also elevated the expression of _AKT1_ in the roots of GmPgb1(S)17 plants. Na2 SO4 Accumulation of Foliar H2O2 and Lipid Peroxidation Are Mitigated by the Overexpression of GmPgb1  
A consequence of Na+ toxicity is the overproduction of ROS causing severe cellular damage including peroxidation of lipid membranes, which can be estimated by the level of MDA [41]. In both the WT and GmPgb1(RNAi)23 leaves, exposure to salinity for 24 h elevated foliar H2O2 and MDA (Figure 5A). Salinity increased both H2O2 and MDA, whereas in the stressed plants overexpressing _GmPgb1_, the increase in H2O2 was limited, and the level of MDA was not different from the control (Figure 5). Figure 5: ROS and ROS-induced damage in leaves of soybean plants exposed to Na2 SO4. (**A**) Level of H2O2 and malondialdehydedehyde (MDA) in leaves of WT plants and plants overexpressing (GmPgb1(S)17) or downregulated (GmPgb1(RNAi)23) _GmPgb1_. Values are the means \(\pm\) SE of three biological replicates, each consisting of at least three plants. Letters indicate statistically significant differences (\(p<0.05\)). (**B**) Relative expression levels of respiratory burst oxidase homologs (RBOHB, D and G) in leaves of plants indicated in (**A**). Values are means \(\pm\) SE of three biological replicates, each consisting of at least three plants. A similar pattern was observed for the expression of two respiratory burst oxidase homologs (RBOHB and D), components of the ROS generator NADPH oxidase enzyme [42] and indicators of ROS production in soybean [37].