Objective  Strains of plant growth promoting rhizobacteria (PGPR) with varied ACC metabolic strengths were constructed by promoter replacement technology, and the resulting chemotaxis and effects on the growth of wheat seedlings analyzed.

  Methods  Four gene fragments containing different promoter sequences were cloned and connected to the expression vector pBBR1MCS-2. Using the 3-parent hybridization method, strains of a similar growth rate was constructed.

  Results   Four strains with basically a same growth rate were obtained by the hybridization and transformation method. The AcdS gene expression and ACC deaminase activity were found positively correlated to the strength of the promoter transferred to the strain. The higher the ACC metabolism rate, the greater the chemotactic ability of the strain. In the experiment on wheat seedling root growth and plant biomass with or without the PGPR strains, the colonization number in rhizosphere of UW4△AcdS+Bra20A was the highest, while those of UW4△AcdS and UW4△AcdS+Bra1A lower; the stem and root weight of wheat treated with UW4△AcdS+Bra20A the highest, the stem weight of wheat treated with UW4△AcdS+Bra1A and UW4△AcdS lower, and the root weight of wheat treated with UW4△AcdS lowest.

  Conclusion   The promoter-replaced PGPRs with high enzyme activity displayed high ACC metabolic rate, which enhanced the chemotaxis and the plant-growth promotion ability.