H2S and rhizobia regulates nodule formation and
nitrogenase activity in soybean plants under water deficiency
Symbiotic nitrogen fixation increases the available nitrogen content of
plants and promotes legumes growth and development. Many small signaling
molecules, such as NO, H2O2, and
phytohormones have been reported to be involved in the regulation of
nodulation between legumes and rhizobia (Hérouart et al., 2002; Puppo et
al., 2013). In this experiment, our results showed water deficiency
significantly inhibited the accumulation of nodules weight, endogenous
H2S content, and nitrogenase activities (Fig. 2C, D and
E), suggesting that water deficiency affected the establishment of
symbiotic system between rhizobia and soybean. Besides, under SW
condition, the endogenous H2S content was higher in the
Q8+NaHS-treated plants than in the control treatment (Fig. 2D, E).
Similarly, Chen et al. (2016) found that NaHS application enhanced the
endogenous H2S content in S. oleracea leaves
under drought stress. There was no significant difference in the nodule
nitrogen fixation area between NaHS+Q8 and the Q8 treatments under NW
and MW conditions (Fig. S1). The nodule nitrogen fixation area was
notably reduced under SW condition compared with NW condition.
Interestingly, the nitrogen fixation area was partially rescued in
NaHS-treated nodules under SW condition (Fig. S1). These results
indicated that under water deficiency condition H2S
could alleviate water deficiency-induced the decrease of the nitrogenase
activity. Additionally, an interesting phenomenon was found that under
MW condition the nodules number was greatly increased, but nodules
biomass was not obviously affected under the same condition (Fig. 2B,
C). The possible reason was that water deficiency stimulated root
nodulation to resist water stress. Additionally, previous studies have
shown that plant can continue to nodule in the water deficiency by
stimulating the early nodulation factor (Paula et al., 2016). In the
present study, we investigated the expression of symbiosis-related genes
in root nodules. GmENOD40 is a downstream component of NFs
perception (Ferguson et al., 2010), which is expressed in peripheral
cells, cortical cells, nodule primordia and developing nodules of root
vascular bundles (Ferguson & Mathesius, 2014). Charon et al. (1999)
reported that the change of ENOD40 expression abundance could
affect nodulation, suggesting that it plays an important role in the
organogenesis of nodules. In the present study, the expression abundance
of GmENOD40 was up-regulated by H2S and rhizobia
in soybean roots under whatever water condition (Fig. 10A). Besides,
another nodulation marker genes involved in the pathway of NFs
nodulation, such as GmERN , was also activated by
H2S and rhizobia (Fig. 10B). NIN is essential for
nodule organogenesis and nitrogen-fixing symbiosis in Medicago
truncatula roots (Tatiana et al., 2015). Under any water condition, we
found that the expression levels of three NIN genes were
up-regulated by Q8+NaHS treatment in soybean root (Fig. 10C, D, E).
Above results showed that H2S and rhizobia stimulated
the expression of GmENOD40 , GmERN , and GmNIN genes
under water deficiency condition in soybean roots. Moreover, the
moderate water deficiency may stimulate the nodule formation in the
soybean symbiosis system and help plants to adaption the water
deficiency environment. Therefore, our results suggested under water
deficiency condition H2S promoted nodule development and
enhanced water-deficiency tolerance in the soybean-rhizobia symbiotic
system.