Efficient colonization and harpins mediated enhancement in growth and biocontrol of wilt disease in tomato by Bacillus subtilis

UNLABELLED: Both Bacillus subtilis and harpins stimulate plant growth and defence against various plant pathogens. In this study, B. subtilis 168 and two derivatives, surfactin producer OKB105 and combined surfactin and HpaGX ooc producer OKBHF, were applied to tomato plants to investigate the mechanisms underlying this effect. To evaluate colonization ability, strains were labelled with green fluorescent protein (GFP). Although biofilm distribution of the three strains was similar on root surfaces, Colonization populations of the two surfactin producers were approximately 2- to 3-fold higher than that of strain 168, and this was accompanied by significantly increased tomato growth. These results suggest that efficient colonization, possibly facilitated by surfactin production, enhanced the efficiency of plant growth promotion by B. subtilis. All three B. subtilis treatments caused plants to have less severe disease symptoms after inoculation with Ralstonia solanacearum, with plants treated with OKBHF being the most resistant, suggesting that hpaGX ooc improves biocontrol efficiency of B. subtilis. Analysis of defence-related genes showed a synergistic effect of HpaGX ooc on B. subtilis enhancement of the expression of the pathogenesis-related genes PR1b1 and PR-P2. In contrast, expression of the defence-related genes PINI and PINII was suppressed. SIGNIFICANCE AND IMPACT OF THE STUDY: Bacillus subtilis and harpins are biological control agents with respective advantages. In this study, combinations of the both were applied to tomato in the form of hpaGX ooc -expressing B. subtilis, showed much better effects on resistance to wilt disease, and equivalent effects on plant growth promotion compared with the progenitor strain have a great potential in agricultural use.