Studies on the classification and evolution of phytopathogenic bacteria of the genera Agrobacterium and Pseudomonas

Abstract

The genera Agrobacterium and Pseudomonas include numerous serious plant pathogens. Bacteria causing crown gall and hairy root belong to the genus Agrobacterium and can inflict severe damage to fruit trees and other agricultural plants. In both cases, it is suspected that the pathogens can be spread in contaminated scions and saplings, so there is growing demand to develop techniques for identifying and detecting the pathogens and to create systems that can trace their movements and identify carriers. However, in 1987, when I began work on these bacteria, relatively little was known about the characteristics of the Agrobacterium taxa; this lack was a major obstacle to the development of practical techniques. The Pseudomonas syringae complex is a representative set of plant pathogens in the genus Pseudomonas. Within the P. syringae complex are more than 60 pathovars that have been established so far based on differences in host plants, disease symptoms, etc. However, the genetic background that led to such diversification, as well as the actual state of diversification, were not satisfactorily understood. As with Agrobacterium, the knowledge needed to devise identification and detection techniques was lacking. To overcome obstacles that have hampered technical development, we need to clarify the characteristics of relevant taxa and thus enable their use as indices for identification and detection. In addition, when there are problems with the classification system itself, the system may need to be restructured in a more rational form. Recently in the field of bacterial taxonomy, genotypes and phenotypes related to core genomes (the fundamental framework of chromosomes) have been used as indices for classification. Taxa are defined based on such indices, arranged to follow an evolutionary course, and a classification system is constructed. Together with many co-workers, I have been conducting polyphasic analyses of the Agrobacterium taxa and P. syringae complex from an evolutionary perspective and working to use the results in a taxonomic study and in an applied study to develop practical techniques. In this paper, I will introduce the main findings of the studies.