Crown Gall Tumors

Crown gall is one of several plant tumor diseases typified by a non-self-limiting tissue overgrowth usually on the roots and bottom portions of stems of mainly woody plants. The appearance of tumors is rough on the surface with semi-soft, smooth, and spongy inner layers of tissue. With age, the tumors are easily dislodged, and their outer layers are friable. Unlike other tumor diseases, crown gall is the result of genetic transformation caused by Agrobacterium tumefaciens, a Gram-negative, rod-shaped bacterium that resides in soil preferably on the surface of roots. Very rare among bacteria is the ability of A. tumefaciens to transfer its tumor-inducing genes (oncogenes) into the host plant cell, culminating in the integration of the oncogenes into the plant chromosomes at one or more sites. The products of the integrated oncogenes modify synthesis of and sensitivity to the plant growth regulators cytokinin and auxin, causing the abnormal proliferation of the transformed cells and resulting in the neoplastic growth.