Gamma-induced mutants of <i>Bacillus</i> and <i>Streptomyces</i> display enhanced antagonistic activities and suppression of the root rot and wilt diseases in pulses

Abstract

This study aims to increase <i>Bacillus</i> and <i>Streptomyces</i> antagonistic activity against the root rot and wilt diseases of pulses caused by <i>Macrophomina phaseolina</i> and <i>Fusarium oxysporum</i> f. sp. <i>udum</i>, respectively. To increase antagonistic action, <i>Bacillus subtilis</i> BRBac4, <i>Bacillus siamensis</i> BRBac21, and <i>Streptomyces cavourensis</i> BRAcB10 were subjected to random mutagenesis using varying doses of gamma irradiation (0.5-3.0 kGy). Following the irradiation, 250 bacterial colonies were chosen at random for each antagonistic strain and their effects against pathogens were evaluated in a plate assay. The ERIC, BOX, and random amplified polymorphic studies demonstrated a clear distinction between mutant and wild-type strains. When mutants were compared to wild-type strains, they showed improved plant growth-promoting characteristics and hydrolytic enzyme activity. The disease suppression potential of the selected mutants, <i>B</i>. <i>subtilis</i> BRBac4-M6, <i>B</i>. <i>siamensis</i>i BRBac21-M10, and <i>S</i>. <i>cavourensis</i> BRAcB10-M2, was tested in green gram, black gram, and red gram. The combined inoculation of <i>B</i>. <i>siamensis</i> BRBac21-M10 and <i>S</i>. <i>cavourensis</i> BRAcB10-M2 reduced the incidence of root rot and wilt disease. The same treatment also increased the activity of the defensive enzymes peroxidase, polyphenol oxidase, and phenylalanine ammonia-lyase. These findings suggested that gamma-induced mutation can be exploited effectively to improve the biocontrol characteristics of <i>Bacillus</i> and <i>Streptomyces</i>. Following the field testing, a combined bio-formulation of these two bacteria may be utilised to address wilt and root-rot pathogens in pulses.