Abstract:

Bacillus subtilis B931, a biological control bacterium isolated from wheat field, protects plants against many soil-borne diseases and promotes plant growth. A mutant library of B931 containing over 3 000 Tn917 mutants was constructed by transposon mutagenesis with plasmid pTV1::Tn917. Six mutants defective in antibiotic production (B931-A－) were selected by dual culture test with Gaeumannomyces graminis var. tritici on PDA plates. Another six mutants which over-produced or less-produced phytohormones 3-Indoleacetic acid (IAA), gibberellin (GA₃) or cytokinin (CTK), respectively, were selected by ELISA and HPLC methods. Greenhouse experiments showed that, compared with the wild type, the antibiotic-defective mutant was less effective in biological control of wheat take-all caused by G. graminis var. tritici and cotton damping-off caused by Rhizoctonia solani, indicating that the antibiotic production played a major role in the biocontrol activity of B931. The mutants that over-produced or less-produced phytohormones had no significant influence on seedling growth of wheat and cotton. However, the mutants that overproduced IAA or CTK could significantly promote root growth of sweet potato seedlings.

Key words: Bacillus subtilis, Transposon mutagenesis, Antibiotic, Phytohormone