## 3 Discussion  
Overexpression of _GmPgb1_ alleviated salinity stress in soybean by mitigating the salt-induced damage and the reduction of several morphophysiological parameters. In both WT and _GmPgb1_-downregulating plants, exposure to Na\({}_{2}\)SO\({}_{4}\) caused a reduction in biomass and a depression of the photosynthetic rate, the stomatal conductance, the transpiration, and the water use efficiency (Figure 2). These effects, typical of salt-treated soybean plants [5; 6], may be the consequence of the combinatorial effects of salinity resulting from the osmotic stress reducing the plant's hydraulic conductivity and, most importantly, ionic stress caused by the toxic accumulation of ions, especially of Na\({}^{+}\)[6; 7; 22; 44]. Noxious levels of Na\({}^{+}\) depress photosynthesis [13] by disrupting the function of the photosystem II reaction center and the oxygen-evolving complex [3]. In soybean, this can be further aggravated by the concomitant reduction in foliar K\({}^{+}\) (Figure 4) attributable to the Na\({}^{+}\) interference with its translocation [17; 45]. Many photosynthetic reactions, in fact, require K\({}^{+}\)[16]. Salt-treated WT plants also experience oxidative stress, as evidenced by the increase in H\({}_{2}\)O\({}_{2}\) and MDA, a marker of lipid peroxidation (Figure 5A), and these effects can be linked to excessive levels of Na\({}^{+}\)[46; 47]. Consistent with this observation is the transcriptional induction of _RBOHs_ (Figure 5B), components of the NADPH oxidative multicomplex system [41] and markers of ROS production [37], and the activation of several ROS-detoxifying enzymes, including APX, CAT, and SOD (Figure 5A). Thus, the responses observed in Na\({}_{2}\)SO\({}_{4}\) in the WT leaves are typical in many respects to those observed in many species [46], including soybean [5]. Overexpression of _GmPgb1_ ameliorates plant response to salinity through the retention of photosynthetic gas exchange parameters and mitigates foliar tissue damage (Figures 2 and 3). These effects are associated to the ability of these plants to limit accumulation of Na\({}^{+}\) (and retain a low Na\({}^{+}\)/K\({}^{+}\) ratio) and mitigate oxidative stress. A plausible reason for the reduced Na\({}^{+}\) levels in tissue overexpressing _GmPgb1_ is the increase in _SOS1_ (Figure 4D), a member of the SOS pathway contributing to salinity tolerance [48]. Strategically located in epidermal, cortical, and pericycle cells, SOS1 limits the Na\({}^{+}\) intake in root cells, restricts the radial movement of the ion across cells layers, thereby reducing its long-distance transport through the xylem, and excludes Na\({}^{+}\) in leaf cells [16]. The induction of _GmSOS1_ in both roots and leaves of the Na\({}_{2}\)SO\({}_{4}\)-treated GmPgb1(S)17 plants, consistent with the higher expression levels observed in species tolerant to salt stress [49] and the acquired tolerance of transgenic plants overexpressing SOS1 [20], suggests limited Na\({}^{+}\) intake in root cells and transport to the aerial organs. The increased expression of _GmATK1_ in the roots of the salt stress-exposed GmPgb1(S)17 plants (Figure 4D) is also an indication of increased tolerance [45; 50]. As an inward-rectifying K\({}^{+}\) channel, ATK1 is required for the maintenance of the Na\({}^{+}\)/K\({}^{+}\) balance by ensuring sustained K\({}^{+}\) intake, especially in high salt environments. The crucial function of this transporter during salinity stress was demonstrated by [PERSON] et al. [45] documenting enhanced tolerance and retention of the Na\({}^{+}\)/K\({}^{+}\) balance in _Arabidopsis_ plants overexpressing the soybean _GmATK1_. Besides restricting Na\({}^{+}\) from leaves, overexpression of _GmPgb1_ alleviates salt damage by relieving oxidative stress, specifically reducing the level of ROS and ROS-induced damage. This protective role, documented under developmental and stress events [26; 29; 30], is linked to the function of the protein to scavenge NO [34], as also observed for GmPgb1 (Supplemental Figure S4).