Saline-alkali stress is one of the most important abiotic stresses that severely threatens plant growth and influences the productivity of agriculture. It limits different physiological and biochemical mechanism of plants and results in serious reduction in growth and development (Chen et al. 2019; Zhang et al. 2019; Liu et al. 2023b). The accumulation and allocation of plant biomass is an important function for evaluating resource acquisition and environmental adaptation strategies. In this study, we found that saline-alkali stress significantly decreased the shoot, root and total biomass of tall fescue, and they decreased with the increase of saline-alkali stress level. However, inoculation with AMF significantly offset the negative effects of saline-alkali stress and appreciably improved the growth and biomass production of tall fescue. Furthermore, M + plants had higher root:shoot compared to M- plants at relatively low saline-alkali stress levels (200 and 400 mmol/L), which could increase the root surface area for efficient water uptake and help the plant survive water shortage (Chen et al. 2023). Similar results were found by Gao et al. (2023) and Zhang et al. (2024a). We infer that adverse conditions (e.g., excessive Na+ injury, osmotic stress) may increase the benefit of AMF to the plants.
Related studies have recognized that changes in soil microbial diversity, structure, and function affect the ability of plants to withstand stress (Hao et al. 2022; Liu et al. 2022c). AMF has the potential to alter plant root exudates and produce new biochemicals to reshape the microbial community composition in the rhizosphere (Xing et al. 2024). In this study, we found that AMF inoculation alleviated the negative effect of saline-alkali stress on bacterial diversity in rhizosphere soil of tall fescue, with higher bacterial diversity in M + than in M- treatment whether under 200, 400 or 600 mmol/L saline-alkali stress conditions. In addition, AMF inoculation altered the abundance of several bacterial groups, including increasing the relative abundance of Proteobacteria, Actinobacteriota and Firmicutes and decreasing the relative abundance of Acidobacteriota and Chloroflexi. Proteobacteria are one of the most abundant bacterial groups in terrestrial soils (Wang et al. 2021) and survive in some extreme conditions (Liao et al. 2023), which can take part in soil nitrification and oxidation processes, thereby increasing soil nutrients. Actinobacteriota generally control the utilization of sugar in the soil, and Firmicutes are concentrated in contaminated soils and are effective in bioremediation and stress tolerance (Ci et al. 2021; Wang et al. 2024). These results suggest that the resistance of AMF to saline-alkali stress may be enhanced by regulating the rhizosphere bacterial community, and AMF may attract some bacteria in the tall fescue rhizosphere soil, which is useful for plant growth.
Similar to bacteria, the fungal community diversity in tall fescue rhizosphere soil of M- treatment decreased with increasing saline-alkali stress levels. However, AMF presence increased the fungal community diversity in the rhizosphere soil of tall fescue, and changed fungal community composition, with lower relative abundance of dominant Ascomycota and higher relative abundance of Mortierellomycota in M + than M- treatment. Most species of Mortierellomycota are saprophytic and mainly participate in decomposing soil organic carbon (Liu et al. 2022c; Bai et al. 2024). Therefore, through their symbiotic relationship with plants, AMF and its associated rhizosphere microbes may play significant roles in assisting tall fescue growth and adaptability under saline-alkali stress.
Our results showed that saline-alkali stress and AMF inoculation caused differences in tall fescue rhizosphere soil microbial community both bacterial and fungal diversity and composition. An important question is, “Which bacteria or fungi contributes most to the tall fescue growth differences between M + and M- treatments under saline-alkali stress conditions?” In our study, SEM was performed to examine the relationships between microbial community and tall fescue total biomass under saline-alkali stress conditions and found that AMF inoculation directly increased tall fescue total biomass on one hand and indirectly synergistically promoted tall fescue growth via increasing bacterial community diversity and regulating bacterial community composition rather than fungi on the other hand under saline-alkali stress conditions. Similar results were supported by Lu et al. (2023), who demonstrated that AMF symbiosis recruited rhizosphere bacterial communities to improve soil phosphate mobilization and regulate sulfur uptake and by Kavadia et al. (2021), who found that inoculation with AMF and nitrogen-fixing bacteria (Sinorhizobium meliloti) led to high shoot biomass and accumulation of N.
Above all, this study suggested that AMF coordinated soil bacterial communities to promote tall fescue growth under saline-alkali stress conditions. The interaction between the AMF, plant, and rhizosphere bacteria under saline-alkali stress conditions could pave the way for technologies to regulate the rhizosphere bacteria to increase plants growth.