Molecular Strategies in the Interaction Between Agrobacterium and its Hosts

Summary

The formation of hyperplasias termed “galls” produced by P.savastanoi and the formation of crown gall tumors by Agrobacterium tumefaciens have a number of features in common, In both cases: the unregulated production of IAA arid cytokintn at the site of-infection is-required for gaii formation. Thus, both compounds, the product of bacteria! genes, are virulence factors in both diseases, and therefore, it is not surprising that the disease symptoms are similar. Further, both violence factors are codecs by genes on large plasmids and have a remarkable degree of homology with each other. However, in Pseudcmonas the genes are constitutively expressed in the bacterium, whereas in Agrobacterium. expression occurs oniy after the genes are integrated into the plant chromosome. Apparently, the structura, component of the gene is prokaryotic in nature whereas the upstream regulatory sequences are eukaryotic and respond to plant transcription and translation machinery. Disease production:by P. savssianos appears to be a much simpler process than that by Agrohacterium. in the former ease, the two phytohormones are synthesized by the bacteria at the site of infection and flying bacteria are necessary for continued symptomatology. On the other hand, Agrobactenaum has evolved a very compfex mechanism.for introducing a piece of its plasmid into plant ceiis where it functions. Consequently, the continuecl growth of the bacteria at the wound site is not necessary for tumor formation. The vir genes of Agrobacte rium apparently do not have a counterpart in .Pseudomonas. although its virulence piasmid has not been subjected to extensive mutagenesis.