H2S and rhizobia synergistically regulate osmoprotectants to adapt to water deficiency
Osmotic adaptation ability under water stress seems to be of particular importance to turgor pressure and plant growth (Yang et al., 2019). Soluble sugars and N-rich compounds such as GB and proline constitute part of the compounds accumulated in the cytoplasm of plant cells under drought conditions, which helped to maintain a low osmotic potential inside the cells (Ashraf & Foolad, 2007; Gomes et al., 2010; Shan et al., 2011). The increase in soluble saccharides concentration facilitates plants to resist environmental stress (Jha & Subramanian, 2018). In the present study, our results showed that the accumulation of sucrose, fructose, and glucose in the leaves of soybean treated with H2S and rhizobia was higher than control treatment under SW condition (Fig. 8A-C), suggesting that H2S and rhizobia enhanced water deficiency resistance by regulating the accumulation of soluble sugars, especially the content of fructose. Additionally, fructose-1, 6-bisphosphatase (FBP) is an important regulatory enzyme in the gluconeogenesis pathway. The FBP activity can regulate the gluconeogenesis pathway and is related to the amount of glucose released. Similarly, Rivero et al. (2014) and Chen et al. (2016) reported that salt and drought stress could specifically up-regulated the expression levels of the FBP gene in tomato and soybean. Our results showed the higher expression levels of GmFBP was found in in the Q8+NaHS treatment plants compared with those in the control treatment under SW condition, indicating FBPase played a very important for glucose regeneration, which is the main carbon skeleton in trehalose and starch (Rivero et al., 2014). Unlike the increase in sucrose content, the expression of GmSUS gene was down-regulated by Q8+NaHS treatment in soybean leaves under water deficiency condition (Fig. 9G). Therefore, we speculated that this phenomenon may be attributed to the enhancement of glucose and fructose as a result of increasing the hydrolysis of the sucrose.
GB and PRO are major organic osmolytes, which are involved in regulating the response to environmental stress in plants (Chen & Jiang, 2010; Zhang & Becker, 2015). GB mainly accumulates in the chloroplast and is involved in the maintenance of PSII efficiency under water stress conditions (Ben et al., 2008). We found that the GB content was significantly increased by Q8+NaHS in soybean leaves under the SW condition (Fig. 8E). A previous study showed that the GB accumulation in the chloroplast is more effective than that in other cellular compartments in protecting plants against oxidative stress (Park et al., 2010). In addition, exogenous GB treatment can prevent salt-induced excess ROS from damaging organelle structures, such as chloroplasts and mitochondria (Ashraf & Foolad, 2007). These results suggested that H2S and rhizobia prevented excess ROS production through promoting the increase of GB during water deficiency, and GB may have a prominent role in osmotic regulation. It is widely recognized that the accumulation of PRO decreases cell osmotic potential resulting in compel plants to absorb water from the outside to maintain the stability of cell membranes and adjust (Verbruggen & Hermans, 2008). PRO accumulation was an adaptation of plants to adversity and plays an important role in improving plant resistance (Chen et al., 2016; Zhang & Becker, 2015). H2S and rhizobia markably increased the accumulation of PRO in soybean plants exposed to SW condition (Fig. 8D), suggesting the proline accumulation may protect the stressed plant from dehydration and stabilize its subcellular structure under water deficiency condition. The H2S and rhizobia-mediated PRO accumulation might have improved the osmotic adjustment restoring soybean plants’ growth under water deficiency. In addition, maize seedlings pretreated with H2S manifested a more significant increase in proline contents by increasing the activities of pyrroline-5-carboxylate synthase and substantially reducing proline dehydrogenase activities (Li et al., 2013). Exogenous H2S induced a noteworthy increase in the foxtail millet’s endogenous proline levels under Cd toxicity (Tian et al., 2016). Similarly, Kolupaev et al. (2019) reported an increase of proline accumulation in wheat with H2S pretreatment under drought. These results showed that the interaction of H2S and rhizobia could improve the water-deficiency tolerance by influencing the accumulation of osmoprotective compounds in soybean.