The Role of Synergistic Action and Molecular Mechanism in the Effect of Genetically Engineered Strain Bacillus Subtilis Okbhf in Enhancing Tomato Growth and Cucumber Mosaic Virus Resistance

Bacillus subtilis is well known for its biocontrol activity against several plant pathogens and for its role in promoting plant growth. HpaG(Xooc), from rice pathogenic bacterium Xanthomonas oryzae pv. oryzicola, is a member of the harpin group of proteins. It is known to elicit hypersensitive cell death in non-host plants, thereby inducing disease and insect resistance in the plants and enhancing plant growth. In our previous experiment, we constructed a genetically engineered strain-B. subtilis OKBHF-through the introduction of the gene encoding HpaG(Xooc) into B. subtilis OKB105 in order to combine the effects of HpaG(Xooc) and wild-type PGPR in improving the plant growth rate and for biological control. In this study, we evaluated the use of treating the tomato plant with B. subtilis OKBHF. The results of greenhouse experiments demonstrated that OKBHF treatment had a significant effect on increasing the height, fresh weight, and flower and fruit number and obviously lowered the disease severity of Cucumber mosaic virus (CMV) infection at 28 days postinoculation (dpi). Subsequent reverse transcription-polymerase chain reaction analysis revealed the molecular mechanisms of HpaG(Xooc) and B. subtilis in the tomato plant, suggesting their synergistic roles in inducing enhanced expression of three expansin genes LeEXP2, LeEXP5, and LeEXP18, which regulate plant cell growth, and two defense-related genes Pti4 and Pti6, which activate the expression of a wide array of PR genes and one defense gene, PR-1a.