Crown-gall and Agrobacterium tumefaciens: survey of a plant-cell-transformation system of interest to medicine and agriculture.

Crown-gall tumors (Figure 1) on various plants were described in Europe by several naturalists as early as the last century, and were generally ascribed to the action of insects or mechanical injury (Smith et al., 1911). According to Smith et al. (1911), Cavara (1897) in Italy was the first to demonstrate the bacterial nature of the disease around 1897 by means of inoculations from pure cultures. However, his studies, as well as those of other writers of southern Europe on this subject, were generally overlooked. In 1907, Smith and Townsend (1907) submitted a paper to Science in which they reported their findings on the causal agent of crown-gall tumors. Their results also showed that a bacterium was the etiological agent of the neoplasm, and they called it Bacterium tumefaciens. Their finding attracted immediate interest, especially from animal pathologists (Jensen, 1910, Levin, J., and Levine, 1918), since in their eyes it was the first instance in which a neoplasm could be associated with an infectious agent and therefore induced under defined experimental conditions. While some animal pathologists started to search for bacteria in animal neoplasms, it was shown that the causal agents of some animal tumors were filtrable (“viral”) and thus not of bacterial origin (Rous, 1911; Rous and Murphy, 1914). As a result, crown-gall disease and its infectious agent, more recently called Agrobacterium tumefaciens (Conn, 1942), became more and more the subject of studies of plant pathologists. However, with the years, fundamental similarities in the cellular processes of different cells were recognized. Much fundamental biological and especially genetic knowledge accumulated from the study of bacteria and their viruses. Cancer researchers made the finding, among other findings, that different agents such as viruses, chemicals, and irradiation could induce neoplasms, and some started to recognize that the question whether there is a common subcellular and molecular basis of neoplasms might be the fundamental problem to be answered. On the other hand, agricultural scientists, in their search for creating new genetic variability among plants, became interested in the possibility of modifying plant cells at the molecular level. With these prospects, the crown-gall disease, and its infectious agent, Agrobacterium tumefaciens, attracted more and more interest among biologists from different fields in succeeding years.