Molecular mechanisms of crown gall tumorigenesis

Abstract Crown gall is a neoplastic tumor disease of many types of plants. The etiological agent of this economically important disease is Agrobacterium tumefaciens, a soil‐bome, Gram‐negative, rod‐shaped bacterium commonly associated with the roots of plants. The precise mechanism of crown gall tumorigenesis is not thoroughly understood. Studies over the past 10 years have shown that there are oncogenic genes carried on a large extrachromosomal element known as the Ti (for tumor‐inducing) plasmid. These genes are transferred to the plant by a novel mechanism carried out during infection (i.e., during the interaction between bacterial and host plant cells). The precise mechanism of this gene transfer remains unclear. Furthermore, how these genes are regulated and function in the transformed plant cell is not well understood. It is known, however, that these genes are directly involved in plant‐growth hormore synthesis and represent the oncogenic genes that operate in the host plant. A deregulatory effect also occurs when part of the Ti plasmid, known as the T‐DNA, is. incorporated into the plant genome as a static “transposable”; element. The stepwise stages leading to crown gall transformation are categorically described in this review.