Multiple mutations in the transferred regions of the Agrobacterium rhizogenes root-inducing plasmids

Agrobacterium rhizogenes induces root formation at the site of inoculation in plants and inserts fragments of its Ri plasmid into the plant nuclear DNA. The transferred region (T-DNA) of the Ri plasmid of the A. rhizogenes strain A4 is made of two fragments, namely TL and TR; the latter harbors a sequence homology with the tms loci (responsible for auxin synthesis) of A. tumefaciens. On Daucus carota slices, single insertion mutations on the TL region of A. rhizogenes do not confer a mutant phenotype while an insertion-deletion in the TR region do confer a Basatt phenotype. Six double mutants with a single insertion in the TL region and the same deletion-insertion of the TR region were constructed. Three of these double mutants were avirulent on D. carota which indicates that in A. rhizogenes A4 the TL and the TR regions cooperate to confer a full infectious phenotype.