Exogenous spermidine enhances Epichloe endophyte-induced tolerance to NaCl stress in wild barley (Hordeum brevisubulatum)

Background Soil salinity adversely affects plant growth. Microbial infection and exogenous polyamine application are effective approaches to ameliorate salt injury in plants. Our previous study showed that Epichloe endophytes play an important role in improving wild barley (Hordeum brevisubulatum) salinity tolerance through polyamine metabolism modification. However, the effects of polyamine (Spd) on Epichloe endophytes-induced host tolerance to salt stress are largely unknown. The aim of this work is to determine whether exogenous polyamine influences the function of Epichloe endophytes to improve salt tolerance of H. brevisubulatum plants. Methods Epichloe endophyte infected (E+) and endophyte free (E-) H. brevisubulatum plants were sprayed with distilled water or 1 mM spermidine (Spd) and treated with water or 200 mM NaCl. The experimental plots included four treatments: (a) CK, (b) NaCl, (c) Spd, and (d) Spd + NaCl. Dry weight, amino acid, sodium, potassium, calcium, nitrogen, phosphorus and carbon contents were quantified, Na+:K+ ratio and C:N:P stoichiometry were analyzed after 21 days exposure to NaCl treatment. Results There was greater accumulation of aspartic acid, threonine, glutamic acid, tyrosine, phenylalanine, histidine, arginine and proline but lower cysteine in endophyte-infected plants compared to endophyte-free counterparts in response to NaCl treatment. The presence of Epichloe endophyte also significantly increased dry weight, total amino acid, potassium, root calcium, nitrogen and phosphorus content but decreased sodium, shoot calcium content and ratios of sodium/potassium, carbon/nitrogen and carbon/phosphorus, nitrogen/phosphorus under NaCl stress. The effects of endophyte to decrease sodium content, sodium/potassium ratio and increase dry weight, total amino acid, potassium, nitrogen and shoot phosphorus were much more pronounced under dual application of NaCl and exogenous spermidine treatment compared to treatments only under NaCl treatment. Conclusion Epichloe endophytes may ameliorate the harmful effects of NaCl stress to H. brevisubulatum plants, whilst exogenous spermidine further enhanced this effect. Spermidine application to endophyte-infected seedlings decreased more of these harmful effects and mediated the function of the endophyte to suppress the negative effects of salinity stress.