Salicylic acid improves the salinity tolerance of Medicago sativa in symbiosis with Sinorhizobium meliloti by preventing nitrogen fixation inhibition

• Exogenous SA alleviates salt stress in M. sativa–S. meliloti symbiosis.
• SA improves growth and photosynthesis of alfalfa plants under salt stress.
• Negative effect of NaCl on nitrogen fixation and nodule biomass was reduced by SA.
• Antioxidants enzymes were induced by SA pretreatments in nodules of M. sativa.
• The accumulation of polyamines in response to salinity was prevented by SA.

In this work we have investigated the contribution of pretreatment with 0.1 and 0.5 mM salicylic acid (SA) to the protection against salt stress in root nodules of Medicago sativa in symbiosis with Sinorhizobium meliloti. SA alleviated the inhibition induced by salinity in the plant growth and photosynthetic capacity of M. sativa–S. meliloti symbiosis. In addition, SA prevented the inhibition of the nitrogen fixation capacity under salt stress since nodule biomass was not affected by salinity in SA pretreated plants. Antioxidant enzymes peroxidase (POX), superoxide dismutase (SOD), ascorbate peroxidase (APX), dehidroascorbate reductase (DHAR) and glutathione reductase (GR), key in the main pathway that scavenges H2O2 in plants, were induced by SA pretreatments which suggest that SA may participate in the redox balance in root nodules under salt stress. Catalase activity (CAT) was inhibited around 40% by SA which could be behind the increase of H2O2 detected in nodules of plants pretreated with SA. The accumulation of polyamines (PAs) synthesized in response to salinity was prevented by SA which together with the induction of 1-aminocyclopropane-l-carboxylic acid (ACC) content suggest the prevalence of the ethylene signaling pathway induced by SA in detriment of the synthesis of PAs. In conclusion, SA alleviated the negative effect of salt stress in the M. sativa–S. meliloti symbiosis through the increased level of nodule biomass and the induction of the nodular antioxidant metabolism under salt stress. The H2O2 accumulation and the PAs inhibition induced by SA in nodules of M. sativa suggest that SA activates a hypersensitive response dependent on ethylene.