Extremophilic bacterial strains as plant growth promoters and biocontrol agents against Pythium ultimum and Rhizocotnia solani

Plant growth-promoting rhizobacteria are helpful soil microorganisms that are used extensively in the agricultural industry. In the current investigation, a total of 120 extremophilic bacterial isolates were successfully obtained from a variety of drastic soils. These bacterial isolates were tested to determine their effectiveness as plant growth promoters and biocontrol agents against the two most frequent soil-borne plant pathogenic fungi, Pythium ultimum, and Rhizocotnia solani. Results indicated that numerous isolates demonstrated antifungal activity against P. ultimum and R. solani, through the production of diffusible antifungal metabolites, cell-wall degrading enzymes (chitinase, cellulase), and siderophores. In addition, many of these extremophilic bacteria fixed atmospheric nitrogen, solubilized insoluble phosphate and zinc, and produced indole-3-acetic acid (IAA) and salicylic acid. The 16 S rRNA analysis revealed that the most potent isolates with multiple modes of action were identified as Bacillus halotolerans, Massilia alkalitolerans, and Bacillus aryabhattai. Under greenhouse conditions, these three isolates, either individually or in combination were used as microbial inoculants, and their effects on the development of tomato and maize were evaluated both with and without the presence of the two pathogens. Application of the three isolates as individual or combined treatments significantly (P < 0.05) reduced the growth of P. ultimum and R. solani in vivo, and significantly (P < 0.05) increased the percentage of survived seedlings as evidenced by the significant (P < 0.05) increase in plant resistance as measured by the in planta contents of total phenols, polyphenol oxidase, and peroxidase. Additionally, the treatment of any one of the three isolates, either individually or in combination, significantly (P < 0.05) improved the development of tomato and maize, as evidenced by a considerable increase in the amount of total nitrogen and dry weight yield. The utilization of these bacterial isolates presents a potentially fruitful alternative to the excessive use of potentially harmful agrochemicals.