Inhibitory effect of ACC deaminase-producing bacteria on crown gall formation in tomato plants infected by Agrobacterium tumefaciens or A. vitis

This study showed that various rhizosphere bacteria producing the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase (ACCD), which can degrade ACC, the immediate precursor of ethylene in plants, and thereby lower plant ethylene levels, can act as promising biocontrol agents of pathogenic strains of Agrobacterium tumefaciens and A. vitis. Soaking the roots of tomato (Solanum lycopersicum) seedlings in a suspension of the ACCD-producing Pseudomonas putida UW4, Burkholderia phytofirmans PsJN or Azospirillum brasilense Cd1843 transformed by plasmid pRKTACC carrying the ACCD-encoding gene acdS from UW4, significantly reduced the development of tumours on tomato plants injected 4-5 days later with pathogenic Agrobacterium strains via wounds on the plant stem. The fresh mass of tumours formed by plants pretreated with ACCD-producing strains was typically four- to fivefold less than that of tumours formed on control plants inoculated only with a pathogenic Agrobacterium strain. Simultaneously, the level of ethylene evolution per amount of tumour mass on plants pretreated with ACCD-producing bacteria decreased four to eight times compared with that from tumours formed on control plants or plants pretreated with bacteria deficient in ACCD production. Moreover, transgenic tomato plants expressing a bacterial ACCD were found to be highly resistant to crown gall formation relative to the parental, non-transformed tomato plants. The results support the hypothesis that ethylene is a crucial factor in Agrobacterium tumour formation, and that ACCD-produced rhizosphere bacteria may protect plants infected by pathogenic Agrobacteria from crown gall disease.