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Hydrogen sulphide (H 2 S) is emerging as an important signalling molecule involved in plant resistance to various stresses. However, the underlying mechanism of H 2 S in aluminium (Al) resistance and the crosstalk between H 2 S and nitric oxide (NO) in Al stress signalling remain elusive. Citrate secretion is a wide-spread strategy for plants against Al toxicity. Here, two citrate transporter genes, GmMATE13 and GmMATE47, were identified and characterized in soybean. Functional analysis in Xenopus oocytes and transgenic Arabidopsis showed that GmMATE13 and GmMATE47 mediated citrate exudation and enhanced Al resistance. Al treatment triggered H 2 S generation and citrate exudation in soybean roots. Pretreatment with an H 2 S donor significantly elevated Al-induced citrate exudation, reduced Al accumulation in root tips, and alleviated Al-induced inhibition of root elongation, whereas application of an H 2 S scavenger elicited the opposite effect. Furthermore, H 2 S and NO mediated Al-induced GmMATE expression and plasma membrane (PM) H + -ATPase activity and expression. Further investigation showed that NO induced H 2 S production by regulating the key enzymes involved in biosynthesis and degradation of H 2 S. These findings indicate that H 2 S acts downstream of NO in mediating Al-induced citrate secretion through the upregulation of PM H + -ATPase-coupled citrate transporter cotransport systems, thereby conferring plant resistance to Al toxicity.

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Spermidine (Spd) is a ubiquitous low-molecular-mass aliphatic amine that acts in abiotic stress tolerance in plants. We investigated how the treatment with exogenous Spd contributed to the protection against salt stress in rice chloroplasts. Analysis of the chlorophyll (Chl) fluorescence showed that there were many negative effects of salinity on different sites of the photosynthetic machinery; however, these were alleviated by adding Spd. Spd prevented the damage of structure and function of chloroplasts under salt stress; thylakoid membrane protein components and photosynthetic pigments were not severely affected by salinity in Spd-treated plants. Spd enhanced the activities of antioxidant enzymes and decreased contents of reactive oxygen species and malondialdehyde accumulation, suggesting that Spd may participate in the redox homeostasis in chloroplasts under salt stress. These results highlighted the positive effects of Spd on rice chloroplasts under salt stress through maintaining chloroplast structure stability.