The synergy of dark septate endophytes and organic residue on Isatis indigotica growth and active ingredients accumulation under drought stress

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 present study, a pot experiment was conducted to investigate the beneficial effects of dark septate endophytes (DSE) (Acrocalymma aquatica and Alternaria alstroemeriae) combine with organic residues from sterilized or non-sterilized fermented corn straw by Trichoderma afroharzianum on the growth and drought resistance of Isatis indigotica. Drought stress (30% field water capacity) reduced the shoot biomass, root biomass, and epigoitrin content of I. indigotica by 34.30%, 80.85%, and 16.51%, respectively. The DSE inoculation alone increased shoot biomass and epigoitrin content under well-watered condition (70% field water capacity) by 13.75%− 65.98% and 52.33%− 67.59%, respectively, and shoot biomass under drought stress by 37.27%− 84.91%. The organic residues application alone increased root biomass under well-watered condition and drought stress by 71.88%− 107.06% and 463.61%− 678.69%, respectively. The mixed application of DSE and organic residues increased root biomass and epigoitrin content under well-watered condition by 66.79%− 144.66% and 89.02%− 136.63%, respectively, and under drought stress by 410.66%− 580.49% and 28.26%− 88.37%, respectively. Structural equation modeling demonstrated that DSE inoculation, organic residue application, and the mixed application improved the growth of I. indigotica under drought stress through different direct and indirect pathways. Variation partitioning showed that DSE was the primary factor improving the epigoitrin content, and organic residue was the primary factor influencing plant biomass and morphological parameters. The mixed application of DSE and organic residue had further synergistic effects on soil enzyme activities, soil organic matter, root biomass, and epigoitrin content, especially the mixed application of A. aquatica and sterilized organic residue. These results contribute to a further understanding of the ecological function of DSE in dryland soils and reveal the synergistic application value of DSE and organic residue for medicinal plant cultivation.