... is a function mainly of their plasmids, not of the bacterial chromosome. The central role of plasmids in these bacteria can be shown easily by "curing" of strains. If the bacterium is grown near its maximum temperature (about 30oC in the case of Agrobacterium or Rhizobium) then the plasmid is lost and pathogenicity (of Agrobacterium) or nodule-forming ability (of Rhizobium) also is lost. However, loss of the plasmid does not affect bacterial growth in culture - the plasmid-free strains are entirely functional bacteria. Agrobacterium tumefaciens is found commonly on and around root surfaces - the region termed the rhizosphere - where it seems to survive by using nutrients that leak from the root tissues. But it infects only through wound sites, either naturally occurring or caused by transplanting of seedlings and nursery stock. In natural conditions, the motile cells of Agrobacterium tumefaciens are attracted to wound sites by chemotaxis. This is partly a ...

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