The relationships between plant pathogenic bacteria and their hosts

Abstract

The criteria generally used in the classification of the genus Agrobacterium and the genus Rhizobium have been evaluated and found to be unsuitable for both differentiation of the genera and the species. Other tests based on the types of reaction expected to occur in the plant have been examined. No in vitro method was found to differentiate the saprophytic Agrobacterium radiobacter from the pathogenic form, A.tumefaciens, but it is suggested that tests based on organic nitrogen metabolizm can be used to differentiate both genera. A selective medium has been developed for agrobacteria which is also useful in distinguishing between agrobacteria and rhizobia. The reactions leading to the production of 3- ketolactose from lactose by some agrobacteria have been examined. It is shown that the production is temperature dependent and phosphate sensitive. The reasons why certain strains of agrobacteria do not form 3- ketolactose have been determined. All attempts to induce such strains to form excess 3- ketolactose, and so achieve uniformity within the genus, failed. It was suggested that this reducing sugar intermediate may aid in the survival of the pathogen in the host plant, and a detailed examination was made of this suggestion in relation to the effects of temperature on gall initiation. Phenols appeared to be essential in plant defence against bacterial invasion. The resistance of the plant might be increased or decreased depending on whether the phenol systems were augmented or inhibited. The phenols seemed to have a dual role, as indole acetic acid synergists or inhibitors, and an examination of their effect on destruction of IAA and on peroxidase reactions showed that plant "indole acetic acid oxidase" is probably associated with peroxidation.