The emergence of dark septate endophytic fungi in the microbial world
Dark septate endophytic (DSE) fungi are diverse facultative ascomycetes found inside the root tissues without any pathogenicity to the host. Like arbuscular mycorrhizal fungi, DSE has predominant positive ecological roles in plants. The DSE fungal associations exhibit positive effects on plant growth, water, and nutrient uptake especially, the least available nutrient, phosphorus (P). Phytohormones secretion by the DSE adds up in the crop improvement. Nevertheless, the presence of melanin in DSE has a wide range of photoprotection and antioxidant properties which could scavenge free radicals under stress condition. Four DSE fungi, Curvularia geniculata RSL06, Phoma multirostrata RSL03, P. multirostrata RSL04 and Eurotiomycetes sp. RSL05 were isolated from roots of different plants exposed to light stress such as UV and a different wavelength of lights. The pH of the broth, biomass, amylase enzyme and specific activity, and their melanin production were significantly varied among the fungal isolates and light conditions. The FTIR spectroscopic analysis of melanin in all DSE showed distinct variation in the functional groups. Further, the higher radical scavenging activity of melanin was recorded in C. geniculata exhibited 29.65%-51.94% in UV and 10.52%-39.38% in the wavelength of light. Previously, we proved C. geniculata improved plant growth through phosphate solubilization and phytohormone production. Hence, this study suggested that fungal melanin acts as a potential agent against various light stress and contains antioxidant activity, which extends our knowledge to analyze the activity of DSE fungi under molecular level. In the future, DSE fungi could replace the usage of chemical fertilizers in agriculture. Dark septate endophytic (DSE) fungi is a diverse group of Ascomycetes fungi that colonize the plants roots, and may facilitate plant growth and fitness, however, their ecological roles need further clarification. This study aimed to evaluate the growth promoting effects of DSE fungi in a medicinal plant, liquorice (Glycyrrhiza uralensis), under additional organic residues. First, we isolated, identified and characterized, two DSE fungal species (Acrocalymma vagum and Paraboeremia putaminum) harboring inside the roots of liquorice growing in arid areas of China. Second, we examined the performance and rhizosphere soil parameters of liquorice plants inoculated with these fungi under additional sterilized organic residues and unsterilized organic residue (containing Trichoderma viride population) in a growth chamber. The results showed that two DSE strains could effectively colonize plant roots and formed a strain-dependent symbiosis with liquorice. DSE inoculation alone increased the plant biomass, and glycyrrhizic acid and glycyrrhizin content. It also improved the root system and N and P absorption by plants, consequently depleting these macronutrients in the soil. Residues alone increased soil organic matter, available phosphorus (P), and available nitrogen (N) content, and plant biomass, N, P, glycyrrhizic acid, and glycyrrhizin content. Mantel test and structural equation model (SEM) analysis demonstrated that DSE associated with residues significantly positively influenced soil organic matter, available P and available N, and plant biomass, glycyrrhizin, N, P, and root surface area. Variation in plant growth and glycyrrhizic acid and glycyrrhizin accumulation can be attributed to the effects of DSE inoculation. DSE associated with residues exhibited a general synergistic effect on the growth and accumulation of glycyrrhizic acid and glycyrrhizin of liquorice.