The bases of crown gall tumorigenesis.

Abstract

The nine decades since Smith and Townsend demonstrated that Agrobacterium tumefaciens causes plant tumors (95) have been marked by a series of surprises. Among the most important of these was the report in 1958 that these tumors could be excised and propagated in vitro without exogenous plant hormones (7). Equally important were a series of reports beginning about the same time that tumors released compounds that agrobacteria could use as nutrients (24). Perhaps the most exciting discoveries, reported in the 1970s and 1980s, were that tumorigenesis required the transfer of fragments of oncogenic DNA to infected plant cells (10), that this process evolved from a conjugal transfer system (99), and that the genes that direct this process are expressed in response to host-released chemical signals (47). This DNA transfer process has become a cornerstone of plant molecular genetics. The genus Agrobacterium also has provided excellent models for several aspects of host-pathogen interactions, including intercellular transport of macromolecules (11), bacterial detection of host organisms (47), targeting of proteins to plant cell nuclei (3), and interbacterial chemical signaling via autoinducer-type pheromones (120).