Streptomyces strains alleviate water stress and increase peppermint (Mentha piperita) yield and essential oils

Drought is the most significant factor limiting plant production in a majority of agricultural fields worldwide. PGPRs (plant growth promoting rhizobacteria) are beneficial soil microorganisms, promote plant growth under normal and stress conditions. In a greenhouse study, the PGP potential of two salt-tolerant antagonistic Streptomyces (S. rimosus strain C-2012 and S. monomycini strain C 801) on peppermint growth and essential oils (EOs) content and composition under normal and water stress was evaluated. To realize the practical use of the PGPR strains, a field experiment was also carried out. In greenhouse and field experiments, soil inoculated with strain C-2012 and C 801 increased shoot fresh and dry weight compared to the non-inoculated controls. Strain C-2012 enhanced shoot dry weight 74% and 63% in greenhouse and field, respectively. Water stress decreased shoot fresh and dry weight but increased EOs and menthol content compared to normal irrigation. In stress conditions, inoculation with PGP strains increased plant growth, total EOs and menthol content. The frequency of soil inoculation with each strain resulted in different effects on plant growth and EOs content and composition. To our knowledge, this is the first report on the application of PGPR Streptomyces strains to increase peppermint and its EOs yield in greenhouse and field conditions. Our results certify the beneficial role of the salt-tolerant antagonistic Streptomyces strains to increase biomass and EOs content of peppermint and also to save water.