Conclusions


The results from our studies tell us several things about the Agrobacterium-plant interaction. First, the opines are particularly important to this interaction; they provide a nutritional advantage to the agrobacteria and they function as signals by which the bacterium senses it is in the environment of a crown gall tumor. Our analyses of the gene systems involved in mannityl opine biosynthesis and catabolism strongly suggest that this opine system arose by evolution of pre-existing gene sets, and that this evolution was driven by competition from other microorganisms that the agrobacteria co-exist with in soils. Second, it is important to the Ti plasmid that opines regulate conjugation. Otherwise, it is unlikely that the strategy of placing the expression of traR under the control of the opine regulon would be so pervasive. Third, accomplishing this regulatory integration by placing traR in an opine-regulated operon indicates that serendipitous gene insertions can result in favorable genetic outcomes, and suggests a novel mechanism for tieing one regulatory system to another. Finally, that conjugation also is regulated by quorum sensing via TraR/AAI suggests two things. First, it apparently is important to the biology of the Ti plasmid that the donor population be at some critical level before conjugation is induced. Second, in that quorum sensing depends upon the extracellular accumulation of the acyl-homoserine lactone signal molecule, this suggests that the niche inhabited by A. tumefaciens is diffusion-limited.