Salt and alkalinity affect the physiological and metabolic pathways of plants, similar to other adverse factors. Adversity stress can significantly promote the synthesis of plant secondary metabolites. Secondary metabolism and its products are the material basis of plant response to environmental stress, and its content affects plant resistance to stress (Tu 2019). Polyphenols and flavonoids are important secondary metabolites of plants. To explore whether the mechanism via which endophytic fungi improve alkali tolerance of host H. bogdanii influences secondary metabolites, we investigate the contents of secondary metabolites and associated enzymes. In the present study, the infection of endophytic fungi and mixed alkali stress treatment on H. bogdanii had significant effects on phosphorus content in roots, flavonoids and alkaloid content in stems, polyphenol content in roots and stems, acid phosphatase activity in leaves, and polyphenol oxidase activity in stems and leaves.
At a concentration of 25 mmol/L, the phosphorus content in the roots of E+ and E- plants both reached the highest value and was significantly higher than that of the control. The reason for this may be due to the growth-promoting effect of low alkali concentration, which caused the phosphorus content at this treatment to be significantly higher than that of the control. Li and Tian (2014) also showed that appropriate salt stress can promote the absorption of phosphorus by plants. Malinowski et al (2000) found that the accumulation of phosphorus in the roots and stems of tall Festuca (genotype DN2) E+ plants was significantly higher than that of E- plants. Acid phosphatase is an important enzyme that regulates phosphorus metabolism in organisms. Acid phosphatase not only participates in the growth and metabolism of organisms and signal transduction pathways, but also enhances plant resistance to phosphorus deficiency, drought, low temperature, water, salt, and other adversities. Wang et al (2015), Liu et al (2017), and Javot et al (2007) also proved that arbuscular mycorrhizal (AM) fungi can significantly promote the absorption of phosphorus by roots after infecting plants, stimulate the secretion and activity of plant phosphatase, and induce an increase in phosphorus content in roots. Xie et al (2013) also showed that the acid phosphatase activity of plants inoculated with endophytic fungi increased significantly and that the total phosphorus content also increased to a greater extent. The findings of this study also showed that endophytic fungi promoted acid phosphatase activity in H. bogdanii leaves, which promoted an increase in phosphorus content. This difference was more obvious under alkaline stress; as the alkali concentration increased both the phosphorus content and acid phosphatase activity showed a downward trend.
Plant polyphenols are an inherent component of many plants. Under normal conditions, their content in plants is very low, but when plants are stimulated by foreign factors, the content of these substances will increase significantly to enhance the resistance to abiotic stress (Zhao 2004). Studies have shown that the content of polyphenols in plants can increase under adverse conditions (Wang et al. 2007). Phenolic compounds play a vital role in relieving oxidative stress because they are involved in the detoxification of reactive oxygen species (Wang et al. 2011). It has been reported that in several other grasses, endophytic fungal infection increases the content of phenolic compounds in plant roots (Ponce et al. 2009; Vázquez- de-Aldana et al. 2011). The present study also showed that with an increase in alkali stress concentration, the content of polyphenols in the roots and stems of H. bogdanii showed an increasing trend, especially in the stems. The content of polyphenols in E+ plants was higher than in E- plants. The study also showed that endophytic fungi increase the polyphenol content in the host H. bogdanii stem under alkali stress, which help the host to resist alkali stress.
Under alkaline stress, the polyphenol oxidase activity of H. bogdanii gradually decreased with the increase in alkali concentration. However, the E+ plant was significantly higher than E-, and the control E+ was also significantly higher than E-. Previous studies have shown that low concentrations of saline alkali stress can promote polyphenol oxidase activity. Yan et al (2021) and Zhao et al (2005) believe that the more alkaline the environment, the faster the enzyme activity decreases. This is consistent with the results of the trend of polyphenol oxidase activity in the stems and leaves of H. bogdanii obtained in the present study. Moreover, studies have also shown that when the activity of polyphenol oxidase decreases, the enzymatic reaction is inhibited, leading to an increase in polyphenol content (Zhang et al. 2013). The results of the present study also showed that under high alkali concentrations, the activity of polyphenol oxidase decreased and polyphenol content increased.
Under different concentrations of alkali stress, endophytic fungi had a significant effect on the alkaloid content in the stems of H. bogdanii. Alkali treatment reduced the alkaloid content of H. bogdanii. It has been reported that the total alkaloid content of plants decreases with an increase in pH (Tang and Chen 2011), which is consistent with the results of the present study. The alkaloid content of E+ plants was significantly higher than that of E- plants. Gao and Nan (2007) believed that endophytic fungi can significantly increase the alkaloid content of plants, which is consistent with this result.
Under different concentrations of alkali stress, endophytic fungi had a significant effect on the flavonoid content in the stems of H. bogdanii. The flavonoid content in E+ plants was significantly lower than that in E- plants, and the content of flavonoids in high alkali concentrations was significantly higher than that in low alkali concentrations. Chen (2019) suggested that endophytic fungal infection significantly reduced the content of flavonoids in H. bogdanii, and the results are similar to those of the present study; salt stress increases the total flavonoid content in roots, stems, leaves, and flowers (Yan 2011; Hou et al. 2016). Other studies (Zhou et al. 2004) showed that flavonoids accumulate under stress. In this experiment, as the alkali concentration increased, the flavonoid content increased, and the high alkali concentration flavonoid content was significantly higher than the low alkali concentration. This result is similar to that of salt stress, indicating that both salt and alkali stress can increase flavonoid content.
Latch et al (1985) suggested that the increase in plant biomass of ryegrass infected with endophytic fungi may be related to gibberellin (GA). Wang et al (2007) reported that gibberellin can inhibit the synthesis of flavonoids by reducing the activity of chalcone synthase (CHS). In the present study, we measured the hormones of E + and E- plants of Burton barley and found that the content of gibberellin GA3 in E + leaves of H. bogdanii, planted indoors and outdoors, was higher than that of E - (data to be published). The higher the content of GA3, the more it inhibited the content of flavonoids, which was consistent with the result that the content of flavonoids in E + plants was significantly lower than that of E-.
Alkaloids are N-based secondary metabolites, and flavonoids are C-based secondary metabolites. According to the hypothesis of "carbon nutrient balance" proposed by Bryant et al (1983), there is a balance between C-based secondary metabolites (such as terpenes and phenols) and N-based secondary metabolites (such as alkaloids). In the present study, endophytic fungi significantly promoted the content of N-based secondary metabolites alkaloids, but significantly reduced the content of C-based secondary metabolites flavonoids to maintain the nutrient balance in the plant. The experimental results also align with the "carbon nutrient balance" hypothesis.
In summary, endophytic fungi promote the increase of phosphorus content by promoting acid phosphatase activity in plant leaves, and the effect is more significant under alkali stress. Endophytic fungi increased plant polyphenol content and polyphenol oxidase activity under alkali stress, and plant polyphenol oxidase activity decreased gradually with an increase in alkali concentration. Endophytic fungi could significantly increase alkaloid content in plants, but alkali treatment decreased alkaloid content in plants. With an increase of alkali concentration, flavonoid content increased, and flavonoid contents under high alkali concentrations were significantly higher those under low alkali concentrations. However, endophytic fungi decreased the content of plant flavonoids. Endophytic fungi could have significantly promoted the alkaloid contents of secondary metabolites, based on N, and significantly reduced flavone contents, based on carbon, as a strategy of maintaining nutritional balance in plants.
Therefore, the mechanism via which endophytic fungi improve alkali tolerance of host plants influences secondary metabolites. Endophytic fungi can improve the alkali resistance of host plants by increasing the contents of secondary metabolites, polyphenols and alkaloids, and enhancing polyphenol oxidase and acid phosphatase activities, which in turn increase phosphorus contents in plants. The results of the present study could facilitate the enhancement of alkali tolerance in plants.