Study on the Biological Characteristics of Dark Septate Endophytes under Drought and Cadmium Stress and Their Effects on Regulating the Stress Resistance of <i>Astragalus membranaceus</i>

<i>Astragalus membranaceus</i> is a famous traditional medicinal plant. However, drought and cadmium (Cd) pollution are the main abiotic stress factors that affect plant growth and yield and the ability to improve the host’s stress resistance through the use of beneficial endophytic fungi. To evaluate the tolerance of dark septate endophytes (DSE) to various abiotic stresses, 10 DSE strains [<i>Microsphaeropsis cytisi</i> (<i>Mc</i>), <i>Alternaria alstroemeriae</i> (<i>Aa</i>), <i>Stagonosporopsis lupini</i> (<i>Sl</i>), <i>Neocamarosporium phragmitis</i> (<i>Np</i>), <i>Paraphoma chlamydocopiosa</i> (<i>Pc</i>), <i>Macrophomina phaseolina</i> (<i>Mp’</i>), <i>Papulaspora equi</i> (<i>Pe</i>), <i>Alternaria tellustris</i> (<i>At</i>), <i>Macrophomina pseudophaseolina</i> (<i>Mp</i>), and <i>Paraphoma radicina</i> (<i>Pr</i>)] were investigated under different drought and Cd stressors in vitro by using solid-plate cultures and liquid-shaker cultures in the current study. The experiments involved using varying concentrations of PEG (0, 9, 18, and 27%) and Cd²⁺ (0, 25, 50, and 100 mg/L) to simulate different stress conditions on DSE. Additionally, the effect of DSE (<i>Np</i> and <i>At</i>) on the growth of <i>A. membranaceus</i> at different field water capacities (70% and 40%) and at different CdCl₂ concentrations (0, 5, 10, and 15 mg Cd/kg) in soil was studied. The results demonstrated that the colony growth rates of <i>Aa</i>, <i>Np</i>, <i>Pc</i>, <i>Mp’</i>, and <i>Mp</i> were the first to reach the maximum diameter at a PEG concentration of 18%. <i>Aa</i>, <i>Np</i>, and <i>At</i> remained growth-active at 100 mg Cd/L. In addition, <i>Aa</i>, <i>Np</i>, and <i>At</i> were selected for drought and Cd stress tests. The results of the drought-combined-with-Cd-stress solid culture indicated that the growth rate of <i>Np</i> was significantly superior to that of the other strains. In the liquid culture condition, the biomasses of <i>Np</i> and <i>Aa</i> were the highest, with biomasses of 1.39 g and 1.23 g under the concentration of 18% + 25 mg Cd/L, and <i>At</i> had the highest biomass of 1.71 g at 18% + 50 mg Cd/L concentration, respectively. The CAT and POD activities of <i>Np</i> reached their peak levels at concentrations of 27% + 50 mg Cd/L and 27% + 25 mg Cd/L, respectively. Compared to the control, these levels indicated increases of 416.97% and 573.12%, respectively. <i>Aa</i>, <i>Np</i>, and <i>At</i> positively influenced SOD activity. The glutathione (GSH) contents of <i>Aa</i>, <i>Np</i>, and <i>At</i> were increased under different combined stressors of drought and Cd. The structural-equation-modeling (SEM) analysis revealed that <i>Aa</i> positively influenced biomass and negatively affected Cd content, while <i>Np</i> and <i>At</i> positively influenced Cd content. Under the stress of 40% field-water capacity and the synergistic stress of 40% field-water capacity and 5 mg Cd/kg soil, <i>Np</i> and <i>At</i> significantly increased root weight of <i>A. membranaceus</i>. This study provides guidance for the establishment of agricultural planting systems and has good development and utilization value.