Organic acid is one of the important components in plant roots and root exudates, which plays an important role in nutrient activation, changing soil microbial activity and alleviating metal element toxicity stress. To study the effect of endophytic fungi infection on the organic acid contents of plants, this study used the grass species Hordeum bogdanii, Endophytic Fungi (E+) and without endophytic fungi (E-) H. bogdanii grasses from Wensu County, Xinjiang Province, China, as test materials H. bogdanii plants were grown hydroponically and treated with 0, 50 and 150 mmol/L mixed alkali (Na2CO3:NaHCO3 = 1:1) for 1 and 21 h. Root exudates and fresh root samples were collected for determination the organic acid contents of malic acid, shikimic acid, citric acid, and oxalic acid. The results showed that under alkali stress, the oxalic acid content in roots and root exudates of E + and E- plants decreased. The contents of citric acid and shikimic acid in roots and root exudates of E + and E- plants increased first and then decreased. Malic acid content in root exudates did not change significantly, and malic acid content in roots increased with the increase of alkali stress time. Among them, the organic acid content of H. bogdanii E + plants was significantly higher than that of uninfected E- plants, indicating that endophytic fungi under alkali stress were helpful to increase the root exudates of H. bogdanii grass and the accumulation of malic acid, shikimic acid, citric acid and oxalic acid content in the root. The most obvious change was oxalic acid content, and there was interaction between endophytic fungi and alkali treatment. It indicated that endophytic fungi could increase the secretion of organic acids from plants under alkali stress, which was conducive to the survival of plants in alkaline soil.

Plants cope with abiotic stress in several ways, including by collaborating with microorganisms. Epichloë, an endophytic fungus, has been shown to improve plant tolerance to extreme external environments. Hordeum bogdanii is a known salt-tolerant plant with the potential to improve alkaline lands. NHX1 plays a key role in the transport of ions in the cell and is overexpressed in plants with increased salt tolerance. However, the expression levels of HbNHX1 in Epichloë endophytic fungal symbionts in H. bogdanii have not been elucidated. We used Hordeum bogdanii (E+) with the endophytic fungi Epichloë bromicola and H. bogdanii (E−) without the endophytic fungi and compared the differences in the ion content and HbNHX1 expression between the shoots and roots of E+ and E− plants under alkaline stress. The absorption capacity of both K+ and Na+ of H. bogdanii with endophytic fungi was higher than that without endophytic fungi. In the absence of alkaline stress, endophytic fungi significantly reduced the Cl− content in the host H. bogdanii. Alkaline stress reduced SO42− content in H. bogdanii; however, compared with E−, endophytic fungi increased the content of SO42− in E+ plants. With an increase in the alkaline concentration, the expression of HbNHX1 in the roots of H. bogdanii with endophytic fungus exhibited an upward trend, whereas the expression in the shoots exhibited a downward trend first and then an upward trend. Under 100 mmol·L−1 mixed alkaline stress, the expression of HbNHX1 in E+ was significantly higher than that in E−, indicating that endophytic fungi could increase the Na+ region in vacuoles. The external environment affects the regulation of endophytic fungi in H. bogdanii and that endophytic fungi can play a key role in soil salinization. Therefore, the findings of this study will provide technical support and a theoretical basis for better utilization of endophytic fungi from H. bogdanii in saline land improvement.