## 3 Discussion
Here, we show that expression of _GmPgb1_ during salt stress attenuates the increase in H\({}_{2}\)O\({}_{2}\) and MDA, possibly by limiting the Na\({}_{2}\)SO\({}_{4}\) induction of _RBOHB_ and _RBOHD_, markers of ROS production in soybean [37]. The mitigation of ROS production in _GmPgb1_-overexpressing leaves is a contributing factor for a decreased leaf injury and a higher retention of the photosynthetic rate. Additionally, overexpression of _GmPgb1_ also attenuates the Na\({}_{2}\)SO\({}_{4}\) depression of the transcripts involved in the formation of photosystem I (PsaA and B), cytochrome B6F (cylB6F), and photosystem II (PsbB, PsbC,PsbD). Retention of these transcripts during conditions of stress has been associated to abiotic stress tolerance [37]. Reduced oxidative stress in leaves overexpressing \(GmPgb1\) can be attributed to an improved antioxidant system. Relative to the WT, overexpression of \(GmPgb1\) is associated to higher levels of the _CAT1_, \(2\) and _APX1_ transcripts, as well as the CAT and APX activity. This behavior might be accountable for the lower level of H\({}_{2}\)O\({}_{2}\) measured in the GmPgb1(S)17 leaves during salt stress. Elevated activity of CAT and APX are desirable tolerance traits for salinity stress [22]. While the direct mechanisms through which \(GmPgb1\) modulates the expression and activity of _CAT_ and APX1 are unknown, regulation of the APX1 activity might be the result of post-translational modifications mediated by NO. Nitration of APX by peroxnitrate, formed in environments enriched in NO, inhibits the enzyme [51], an observation consistent with the higher APX activity (Figure 7) in \(GmPgb1\) leaves which are depleted in NO (Supplemental Figure S4). Sustained activity of APX and removal of H\({}_{2}\)O\({}_{2}\) during conditions of stress is dependent upon the availability of the substrate ascorbate produced de novo and regenerated through the ascorbate-glutathione cycle [43]. Our work suggests that during salt stress, overexpression of \(GmPgb1\) might result in higher levels of ascorbate required for the operation of APX. First, the expression of the three ascorbate biosynthetic genes GDP-L-galactose phosphorylase GmGPP and L-galactose-dehydrogenase (GmGDH1 and 2), good indicators of ascorbate production in soybean [37; 52], is induced in leaves of Na\({}_{2}\)SO\({}_{4}\)-treated plants overexpressing \(GmPgb1\) (Supplemental Figure S5). Second, the same leaves exhibit a higher activity of DHAR, which is required for ascorbate recycling through the glutathione-dependent dehydroroscorbate reduction [23]. The relevance of this enzyme in replenishing the ascorbate pool during salinity stress was documented [53]. The slightly lower ascorbate level in the salt-stressed GmPgb1(S)17 leaves (Supplemental Figure S6) is difficult to interpret, but it might be a consequence of higher utilization by APX. While we suggest that removal of ROS by ascorbate is mediated by APX, the operation of APX-independent mechanisms, which also involve ascorbate [54], cannot be discounted. Activation of the antioxidant components and similar Na\({}^{+}\)/K\({}^{+}\) metabolic adaptations to those described here were associated to the ability of thymol to increase salinity tolerance in tobacco plants [15]. In the same study, it was observed that thymol elevates the level of NO, an observation which is, however, not consistent with our study. The relevance of high expression of \(GmPgb1\) as a mechanism to cope with salinity stress is not limited to transgenic material. Our study shows that this gene is preferentially induced in commercial cultivars exhibiting tolerance to Na\({}_{2}\)SO\({}_{4}\), relative to those susceptible to the same stress. This observation, consistent with a previous work linking induction of \(Pgbs\) to the ability of soybean plants to cope with hypoxic stress [37], could be exploited as a tool to screen plant material at early stages of development for salt stress tolerance.