| Summary: | Drought is a major abiotic stress factor affecting plant growth and production, while utilizing beneficial endophytic fungi is one of the most promising strategies for enhancing plant growth and drought tolerance. In the current study, a pot experiment was conducted to investigate the beneficial effects of dark septate endophyte (DSE) (<i>Macrophomina pseudophaseolina</i>, <i>Paraphoma radicina</i>) and <i>Trichoderma</i> (<i>Trichoderma</i> <i>afroharzianum</i>, <i>Trichoderma</i> <i>longibrachiatum</i>) inoculum on <i>Astragalus mongholicus</i> grown in sterile soil under drought stress, alone, or in combination. The addition of <i>Trichoderma</i> enhanced the DSE colonization in roots regardless of the water condition. Under well-watered conditions, <i>M. pseudophaseolina</i> inoculation significantly enhanced the biomass and root length of <i>A. mongholicus</i>. The two DSE and <i>Trichoderma</i> inoculum significantly improved calycosin-7-O-β-D-glucoside content. However, <i>M</i>. <i>pseudophaseolina</i> + <i>T.</i> <i>afroharzianum</i> inoculation better promoted root growth, whereas co-inoculation had higher active ingredient contents compared with single inoculation, except for <i>P. radicina</i> + <i>T.</i> <i>afroharzianum</i>. Under drought stress, DSE and <i>Trichoderma</i> inoculum significantly improved root biomass, root length, calycosin-7-O-β-D-glucoside content, and activities of nitrate reductase and soil urease. <i>P. radicina</i> + <i>T. afroharzianum</i> and <i>P. radicina</i> + <i>T.</i> <i>longibrachiatum</i> better increased root length, and all combinations of DSE and <i>Trichoderma</i> had a greater impact on the increase in formononetin content compared with the single treatments. Additionally, <i>Trichoderma</i> relies on antioxidant enzymes, growth hormones, and the redox system (ascorbic acid–glutathione) to resist drought, while DSE strains have an additional osmotic regulation system in addition to the drought resistance function possessed by <i>Trichoderma</i>, and the effect of co-inoculation (especially <i>M</i>. <i>pseudophaseolina</i> + <i>T.</i> <i>longibrachiatum</i> and <i>P. radicina</i> + <i>T. afroharzianum</i>) on plant physiological parameters was greater than that of single inoculation. This study provides a new research direction for the effects of DSE and <i>Trichoderma</i> on medicinal plant cultivated in dryland.
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