Growth-Promoting Characteristics of Fungal and Bacterial Endophytes Isolated from a Drought-Tolerant Mint Species Endostemon obtusifolius (E. Mey. ex Benth.) N. E. Br

Endophytes are primarily endosymbiotic bacteria and fungi that colonize the interior tissues of their host plant. They enhance the host plant’s growth and attenuate adverse effects of biological stress. Endophytic species of many indigenous plants are an untapped resource of plant growth-promoting microorganisms that can mitigate abiotic stress effects. Thus, this study aimed to isolate endophytes from the roots and leaves of the medicinal plant Endostemon obtusifolius to evaluate their in vitro growth-promoting capacities and drought tolerance and to characterize the most promising species. Twenty-six endophytes (fourteen bacteria and twelve fungi) were isolated and cultured from the roots and leaves of E. obtusifolius. All 26 endophytes produced flavonoids, and 14 strains produced phenolic compounds. Of the 11 strains that displayed good free radical scavenging capability (low IC(50)) in the 1-1-diphenyl-1-picryhydrazyl radical scavenging assay, only three strains could not survive the highest drought stress treatment (40% polyethylene glycol). These 11 strains were all positive for ammonia and siderophore production and only one strain failed to produce hydrogen cyanide and solubilize phosphate. Seven isolates showed aminocyclopropane-1-carboxylate deaminase activity and differentially synthesized indole-3-acetic acid. Using molecular tools, two promising symbiotic, drought stress tolerant, and plant growth-enhancing endophytic species (EORB-2 and EOLF-5) were identified as Paenibacillus polymyxa and Fusarium oxysporum. The results of this study demonstrate that P. polymyxa and F. oxysporum should be further investigated for their drought stress mitigation and plant growth enhancement effects as they have the potential to be developed for use in sustainable agricultural practices.