Abstract: The transconjugant of Agrobacterium tumefaciens with C58 genetic background containing plasmid pSym3622 of Rhizobium leguminosarum by. phaseoli was capable of infecting the roots of Phaseolus vulgaris of the wounding sites and inducing the formation of nodule-like structures near the sites of inoculation. These structures were different from the nodule and crown galls induced respectively by R. leguminosarumbv, phaseoli 3622--15 and a virulant strain A208 (pTiT37) of A. tumefaciens. The vascular bundles of the nodule-like structures were located at the central region of the structures and were surrounded by highly vacuolated thin-walled cells rich in starch grains. There was no bacterium neither in cell nor in the intercellular space of these structures evidenced by light and electron microscopic observations. In contrast, many metabolically active bacteroids were observed within the cells and in the intercellular spaces of the effective nodules, in the case of crown galls, there was also no bacterium in any part of the tumours. The internal structures of tumour on the root of Phaseolus vuigaris were visualized as distinguishable coherent areas of cells partitioned by disorganized cell zones. These areas of cells were traced back to be derived from different meristematic zones. At some instance, a number of root-like protrusions which were likely to develop into new roots were observed. In other parts, host cells were visible as a result of cell degeneration.

Key words: Rhizobium leguminosarum bv. phaseoli, Agrobacterium tumefaciens, Nodule, Nodule-like structure, Crown gall