Infection by the fungal endophyte Epichloë bromicola enhances the tolerance of wild barley (Hordeum brevisubulatum) to salt and alkali stresses

Abstract

Background and aims

Salinization is considered as a major environmental threat to agricultural systems. Infection with Epichloë fungal endophytes has been shown to increase tolerance to NaCl stress for several host grass species, but limited information is available regarding the effects of these endophytes under mixed salt (NaCl and Na₂SO₄) and mixed alkali (NaHCO₃ and Na₂CO₃) stresses. Since these four compounds are considered very harmful to many inland areas in China, we conducted a study to determine the impact of Epichloë fungal endophyte infection on wild barley (Hordeum brevisubulatum) under both salt stress (SS) and alkali stress (AS).

Methods

Wild barley with (E+) and without (E-) Epichloë endophyte was subjected to mixed salt (molar ratio of NaCl:Na₂SO₄ = 1:1) and mixed alkali (molar ratio of NaHCO₃:Na₂CO₃ = 1:1) treatments (0, 100, 200, 300 and 400 mM). Photosynthetic parameters and chlorophyll content were measured after 21 days exposure to stress, and growth parameters, physiological indexes, sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), nitrogen (N), phosphorus (P) and carbon (C) contents were determined after 22 days exposure to stress.

Results

The harmful effect of alkali stress on the growth of wild barley was stronger than those of salt stress, irrespective of endophyte infection. Alkali stress had a greater impact on photosynthesis and chlorophyll content compared to salt stress and also accumulated more of the osmoprotectant glycine betaine. However, salt stress appeared to increase total antioxidant capacity as well as increasing K⁺ content (resulting in a relative low Na⁺/K⁺ ratio). Under alkali stress, Ca²⁺ content sharply increased in roots as opposed to a decrease under salt stress. In roots, the C, N, P contents and the C:N ratio was higher under salt stress compared to alkali stress whereas the C:P and N:P ratios were lower. In shoots, the N and P contents were higher under salt stress compared to alkali stress whereas the C content and the C:P and C:N ratios were lower. Interestingly, the presence of Epichloë endophyte infection on wild barley under both salt stress and alkali stress led to significant amelioration of both stresses. Epichloë infection significantly increased photosynthesis, chlorophyll content, total antioxidant capacity and glycine betaine content, whilst lowering leaf malondialdehyde content. Furthermore, Epichloë infection reduced Na⁺ content, the Na⁺/K⁺ ratio and shoot Ca²⁺ content but increased K⁺ content and the root Ca²⁺ content. Epichloë infected plants also had higher C, N and P contents but lower ratios of C:N, C:P and N:P than uninfected plants.

Conclusions

The presence of the Epichloë endophyte suppresses the negative effect of salt stress and alkali stress on wild barley seedling growth. The possible mechanisms by which the presence of Epichloë endophyte enhances growth of plants exposed to those two stresses include improved photosynthetic ability, increased antioxidant potential, increased nutrient absorption, and osmotic and ionic adjustment. The study also found that alkali stress is more harmful to wild barley than salt stress.