Dark septate endophytes improved the performance of Isatis indigotica by enhancing the degradation of lignocellulosic residues

To evaluate the lignocellulose degradation ability of dark septate endophytes (DSE) and the utilization efficiency of agricultural waste in medicinal plant cultivation, in this study, 44 DSE strains (isolated from various plots and plants) were inoculated on the medium with sodium carboxymethyl cellulose, xylan, and citrus pectin as the single carbon source or with additional guaiacol, and found that both DSE strains can selectively degrade different lignocellulose component (mainly cellulose, hemicellulose, and pectin), while the specificity of fungal genus was the main factor affecting the lignocellulosic degradation ability. Further, corn straw and licorice residue, were used as lignocellulosic residues (LR) in solid state fermentation to demonstrate the cellulase production ability of DSE. Among them, the activities of filter paper cellulase, carboxymethyl cellulase, and β-glucosidase of Paraphoma chlamydocopiosa were the highest at 0.721–2.055, 1.435–3.102, and 1.245–2.976 U/g, respectively, and followed by Paraboeremia selaginellae at 0.347–1.644, 1.386–2.864, and 1.068–2.878 U/g, respectively. In addition, P. chlamydocopiosa and P. selaginellae exhibited the Isatis indigotica growth promotion which could increase the shoot, root, and total biomass by 56.3 %, 93 %, and 58.4 % and 58.4 %, 139.8 %, 64.9 % respectively. Further the two-factor pot experiment investigated the synergetic effects of the two DSE strains plus LR on I. indigotica. Two strains improved the LR utilization efficiency by mobilizing soil C-hydrolytic enzyme activities. These results highlight potential DSE applications in medicinal plant cultivation under LR return.