## 2 Results  
### Overexpression of GmPgb1 Augments Tolerance to Salinity Stress  
The transcript levels of _GmPgb1_ during Na2 SO4 stress were measured in the WT line and in previously characterized lines [37] overexpressing (GmPgb1(S)17) or downregulating (GmPgb1(RNAi)23) _GmPgb1_. Salinity gradually augmented the _GmPgb1_ transcripts in WT roots and leaves, with a marked increment observed in the latter between 6 and 12 h (Figure 1). The increase in _GmPgb1_ in leaves trailed the increase in roots by approximately 6 h. The _GmPgb1_ overexpression resulted in an approximately six-fold increase in root transcripts and an approximately twelve-fold increase in leaf transcripts, likely reflecting the relatively low base levels of transcripts in leaves compared to roots of non-transformed plant materials [38]. The RNAi line did not completely repress the induction of _GmPgb1_ in roots whereas it did repress the leaf _GmPgb1_ expression.  
Differences in photosynthetic parameters between the lines were clearly detectable after 24 h of Na2 SO4 treatment. In leaves of WT plants, salinity stress depressed the photosynthetic rate, the transpiration rate, the stomatal conductance, and the water use efficiency by more than 50% (Figure 2). With the exclusion of transpiration, leaves overexpressing _GmPgb1_ exhibited a higher retention of gas exchange parameters. The GmPgb1(RNAi)23 line had the lowest retention of the photosynthetic rate, the transpiration, and the stomatal conductance relative to the WT (Figure 2). Salinity also induced the closure of the stomata (Supplemental Figure S1), as evidenced by the reduced stomatal  
Figure 1: Expression level of _GmPgb1_ in roots and leaves of soybean plants exposed to Na2 SO4 for 24 h. Measurements were taken in the WT line and lines overexpressing (GmPgb1(S)17) or downregulating (GmPgb1(RNAi)23) _GmPgb1_. Values are the means + SE of three biological replicates, each consisting of at least three plants.  
conductance. This effect was mitigated in leaves of the GmPgb1(S)17 line. During stress, reduction in the photosynthetic rate is often concomitant to the degradation of thylakoid-associated protein complexes [39]. The transcript levels of the selected genes known to be sensitive to abiotic stress and involved in the formation of photosystem I (PsaA, PsaB, and LHCA), cytochrome B6F (cylB6F), and photosystem II (PsbA, PsbB, PsbC, and PsbD) complexes were measured at 24 h. Except for LHCA, the relative transcript levels of all the genes, comparable between genotypes under control conditions, decreased in leaves of the WT plants exposed to Na2 SO4 (Supplemental Figure S2). This decrease was mitigated by the overexpression of _GmPgb1_. Relative to the WT, GmPgb1(RNAi)23 leaves exhibited lower levels of PsaA, PsbB, and PsbC (Supplemental Figure S2).  
Figure 2: Retention, expressed as the percentage of control (Na2 SO4), of the photosynthetic rate, the transpiration rate, the stomatal conductance, and the water use efficiency of Na2 SO4-treated soybean plants overexpressing (GmPgb1(S)17) or downregulating (GmPgb1(RNAi)23) _GmPgb1_. Measurements were conducted after 24 h of treatment. Values are the means \(\pm\) SE of at least three biological replicates each consisting of at least five plants. Letters indicate statistically significant differences (\(p\) < 0.05).  
Morphological differences between the lines were visible after seven days of stress. Overexpression of _GmPgb1_ limited the Na\({}_{2}\)SO\({}_{4}\) depression of the shoot water content (WC) and the root and shoot dry weight, which were more severe in the WT and GmPgb1(RNAi)23 lines (Figure 3). Leaf injury, not noticeable in leaves of the GmPgb1(S)17 line exposed to salinity for seven days, was apparent in the other two lines, and especially in leaves of the GmPgb1(RNAi)23 plants (Figure 3).