H2S responds to water deficiency by regulating lipid metabolism in leaves in a soybean-rhizobia symbiotic system
Non-targeted metabolomics data showed that soybean leaf metabolome had high plasticity as a strategy to regulate its metabolism under water deficiency condition. Previous studies indicated that metabolites appeared as biomarkers during the symbiosis of plants and microorganisms and have become an effective method of measuring plant performance (Fernandez et al., 2016). Moreover, the key metabolites (carbohydrates, amino acids, lipids, cofactors, nucleotides, peptides and secondary metabolites) were regulated and accumulated in plants in response to drought and high temperature stress (Das et al., 2017; Guy et al., 2007; Loskutov et al., 2017; Shulaev et al., 2010). In the present study, physiological data showed that H2S and rhizobia could regulate membrane lipid peroxidation and protect plants from severe oxidative damage. Interestingly, there have been significant changes in the regulation of lipids and lipid metabolites in metabonomics data. In this experiment, the up-regulated lipids and lipid metabolites of Q8-treated plant were markably increased compared with control under SW condition, and compared with Q8+NaHS-NW the metabolic compounds of Q8+NaHS-SW also changed significantly (P ≤0.05) (Fig. 11). Under the SW condition, we found that the up-regulated metabolites including PA, PG, galactaric acid, jaceidin 4’-glucuronide, and methyl furfuracrylate were notably increased by the inoculation of rhizobia in leaves (Fig. 12, Table 2). Furthermore, H2S and rhizobia synergistically regulated lipid metabolites including PE, PG, agavoside A, and dephospho-CoA in leaves under SW condition. These results may indicate that H2S and rhizobia synergistically showed more metabolites to participate in the regulation of water deficiency. Previous study reported that the metabolism involved in osmotic adjustment (proline, etc.) and active oxygen removal (L-glutamine and γ-L-glutamyl L-glutamate) was appreciably increased in tenuiflora seedlings inoculated with arbuscular mycorrhiza under alkali stress, suggesting that mycorrhizal colonization enhanced the alkali tolerance of plants (Yang et al., 2020). As expected, rhizobia catalyzed more metabolites in response to water stress under water deficiency. Phospholipid PG exists on the thylakoid membrane and participates in the photosynthesis of plants. Our study was in line with view taken by Sun et al. (2010) opinion, who pointed out that the increase of PG slowed the damage caused by salt stress in tomato plant. In addition, Jiao et al. (2018) found that soybean roots could resist neutral salt stress by regulating the metabolism of amino acids, carbohydrates and polyols. Of course, we also found that under water deficiency condition the H2S and rhizobia synergistically enhanced the metabolism of nutrients, including amino acids and organic acids (allantoic acid, D-pantethine, pentosidine) in soybean leaves. Nowadays, many lipids including phosphatidic acid, fatty acid, inositol phosphate, lysophospholipid, diacylglycerol, oxylipid, sphingolipid and N-acylethanolamine were found to play an important role in the signal transduction in plant response to abiotic stress (Chao et al., 2011; Kang et al., 2010; Kilaru et al., 2011; Wang, 2004; Zhang et al., 2019). Previous studies reported that lipid-mediated signal transduction responds to various environmental stresses (such as temperature, water shortage, salinity, etc), Phospholipase D and phosphatidic acid-mediated signal transduction results in less water loss by promoting the closure of stomata, thus becoming the key lipid in response to stress (Ji et al., 2018; Zhao, 2015). These results suggested that the H2S and rhizobia synergistically regulated metabolism of nutrients in leaves, including lipid and organic acids, which improved the ROS detoxification capacity, membrane stability and water tolerance.