H2S and rhizobia synergistically increase
antioxidant defense capacity of soybean leaves under water deficiency
Plants are often subjected to continuous threat from toxic ROS and lipid
peroxidation. In order to cope with environment stress, plants increase
their resistance by activating their antioxidant defense system (Farooq
et al., 2019; Niu et al., 2012). ROS-generated oxidative injuries in
plants were reflected by the MDA content, a byproduct of lipid
peroxidation (Sato et al., 2011). We found that the over accumulation of
MDA was observed under SW condition. Compared with the control, the
Q8+NaHS-treated plants did not maintain lower levels of MDA content to
some extent under water deficiency (Fig. 5B). This is contrast with the
study conducted by Wang et al. (2016), who found that there is notably
lower MDA content in the leaves of alfalfa with rhizobia than control
seedlings. These results suggested that severe water deficiency could
damage the membrane structure and lead to excessive oxidative damage in
soybean leaves. Furthermore, it was reported that H2S
reduced membrane peroxidation by regulating the activities of
antioxidant enzymes in rice under low temperature stress (Mostofa et
al., 2015), and also regulated the antioxidant system of pepper to
relieve zinc toxicity (Kaya et al., 2018). In the present study,
H2S and rhizobia synergistically decrease in
H2O2 levels through increasing
activities of antioxidants in soybean leaves under water deficiency
condition (Fig. 5C). Further proof was provided by the assay of
antioxidant enzyme activities in soybean leaves tissues. Our results
showed that the activities of CAT and POD were markedly increased in
soybean treated with H2S and rhizobia compared with the
control treatment under water deficiency (Fig. 6B, C), indicating that
plants maintained high levels of antioxidants activities to remove water
stress induced-excess ROS. In other studies, it has been shown that the
balance among POD, CAT and SOD is crucial for determining the
steady-state level of H2O2 and
superoxide radicals (Dong et al., 2019; Li et al., 2020). Together,
these results suggested that the activities of antioxidants played a
vital role in alleviating the oxidative damage of soybean under water
deficiency. Although plants maintained high levels of CAT and POD
activities to some extent by Q8+NaHS treatment under water deficiency,
the activity of SOD did not markedly increase. Notably, this is little
decrease for the OFR content to some extent for H2S and
rhizobia treatment under water deficiency. Meanwhile, compared with NW
condition, the content of OFR was increased by H2S and
rhizobia under SW condition (Fig. 5 D). In addition, the transcript
abundances of several antioxidant defense-related genes were increased
by H2S and rhizobia under SW condition (Fig. 9).
Importantly, GmSOD2 , which is encoded the ron-SOD2, exhibited
higher expression levels in Q8+NaHS-treated plants than in the control
plants under SW condition. This may be due to the fact that SOD, as an
inducible enzyme, produces more superoxide anion and induces SODgene expression under water deficiency condition. These results revealed
that H2S and rhizobia are more efficient in mitigating
the damage caused by ROS in soybean under water deficiency.
The AsA-GSH cycle plays a vital role for plant to resist oxidative
damage (Avashthi et al., 2018; Nanda & Agrawal, 2016). It is reported
that Brassica napus plants can be protected from salt induced
oxidative stress by regulating ASA-GSH pathway. (Hasanuzzaman et al.,
2018). Furthermore, Chen et al. (2011) reported that H2S
may lead to the accumulation of GSH in plant tissues. Our results showed
that water deficiency altered the redox status of AsA and GSH in soybean
seedlings under water deficiency condition (Fig. 7). Moreover, the mRNA
abundance of 1-Cys peroxiredoxin and glutaredoxin, which are encoded byGmPrx and GmGrx , are higher in the Q8+NaHS treatment
plants than control plants under SW condition. This may corroborate why
the H2S and rhizobia show higher GR activity under water
deficiency (Fig. 7I). Thus, the decline in the redox status levels by
water deficiency is either due to the serious damage of plants. These
results clearly revealed that water deficiency caused the alteration in
the redox status of the cell by interfering with AsA and GSH pools in
legumes. Furthermore, we found that H2S and rhizobia
synergistically enhanced the AsA-GSH cycle by regulating the activities
of APX, GR, and DHAR in soybean under water deficiency (Fig. 6D, 7H, I).
Similar results have been reported that the application of exogenous
H2S regulated the metabolism of AsA and GSH in wheat
leaves by increasing the activities of APX, GR and DHAR under water
stress (Shan et al., 2011). Previous studies have shown that the higher
accumulation of the AsA-GSH cycle and APX activity synergistically
reduced the excess production of H2O2 in
cashew plants (Lima et al., 2018). Khan et al. (2017) reported that
NO-induced H2S alleviated osmotic stress in wheat
seedlings by enhancing the activities of APX and GR. Interestingly, the
regeneration of GSH from oxidized glutathione (GSSG) is catalyzed by GR
and improved the antioxidant capacity of cells (Hasanuzzaman et al.,
2017). These results suggested that H2S and rhizobia
synergistically triggered the up-regulation of the AsA-GSH cycle related
enzymes under water deficiency, which further enhanced abiotic stress
tolerance in plants.