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.