Tumorigenesis and root nodulation by Agrobacterium tumefaciens carrying Rhizobium sym plasmids

Abstract

Agrobacterium and Rhizobium are closely related bacterial genera in the family Rhizobiceae. Agrobacterium and Rhizobium genes contributing to the induction of, respectively, plant tumors and root-nodules, are encoded on large plasmids. Agrobacterium tumefaciens is well known for its ability to cause tumorous growth of plant cells by transferring a segment of its own DNA (T-DNA) into the nuclear genome of plants. The mechanism of T-DNA transfer and integration is still unknown. Ti plasmid genes (other than T-DNA) required for these events have been identified within the virulence (vir) region, which encodes at least 6 complementation groups (1). In addition, A. tumefaciens chromosomal virulence genes are required. The Ti plasmid vir genes act in trans with respect to T-DNA (2) and many are conserved among various types of Ti plasmids: both those conferring a wide and a narrow host range for tumor formation. In addition, DNA homology with the vir region (but not necessarily functional homology) has been demonstrated for some sym plasmids of Rhizobium (3). The implication of this DNA homology remains unclear. Rhizobium genes determining root-nodule development (ndv) have both structural and functional homology with chromosomal vir genes (chv A and chv B) of Agrobacterium.