Exogenous spermine-induced expression of SlSPMS gene improves salinity–alkalinity stress tolerance by regulating the antioxidant enzyme system and ion homeostasis in tomato

The study tested the function of exogenous spermine (Spm) in resisting salinity–alkalinity stress in tomato seedlings and found that tomato Spm synthase gene (SlSPMS) was involved in this regulation. The tomato seedlings cultivated in normal conditions or salinity–alkalinity conditions were irrigated with 100 ml one strength Hoagland nutrient solution 100 ml mixed solution (5 ml 300 mmol/L NaCl, 45 ml 300 mmol/L Na2SO4, 45 ml 300 mmol/L NaHCO3, and 5 ml 300 mmol/L Na2CO3 (pH = 8.90)) every 2 days, respectively. The 0.5 mM Spm pretreatment improved superoxide dismutase (SOD; EC 1.15.1.1) activity, catalase (CAT; EC 1.11.1.6) activity, ascorbate peroxidase (APX; EC 1.11.1.11) activity, and glutathione reductase (GR; EC 1.6.4.2) activity and decreased endogenous hydrogen peroxide (H2O2) content, malondialdehyde (MDA) content, and relative electrical conductivity (REC) in tomato leaves. Na+ content declined and K+ concentration rose in tomato seedlings when pre-treated with Spm. However the results showed that under salinity–alkalinity stress, silencing of SlSPMS with virus-induced gene silencing had lower antioxidant enzyme activities and higher Na+ content and lower K+ content than normal tomato seedlings, meaning that they had low salinity–alkalinity tolerance. Exogenous Spm could not reconstruct the tolerance to salinity–alkalinity stress in SlSPMS gene-silencing tomato seedlings. Taken together, exogenous Spm could induce the expression level of SlSPMS, which regulated the antioxidant enzyme system and ion homeostasis in tomato seedlings living in salinity–alkalinity environment, thereby improving the ability of tomato seedlings to resist salinity–alkalinity stress.