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In this article it is shown that the T-DNA of Agrobacterium tumefaciens contains besides the well-known cyt and aux genes another gene with an oncogenic effect in plants. The gene in question is called 6(b) and causes the formation of small tumors in plant species such as Nicotiana glauca and Kalanchoe tubiflora.

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The induction of vir gene expression in different types of Agrobacterium strains shows different pH sensitivity profiles. The pH sensitivity pattern demonstrated by octopine Ti strains was similar to that of a supervirulent leucinopine Ti strain, whereas this was different from that shown by nopaline Ti strains and agropine Ri strains. Data are given which indicate that these differences are due to different properties of the virA genes of these wild types. An exceptional case was formed by strains with the limited-host-range plasmid pTiAG57 which showed AS-dependent vir induction only if reduced inoculum sizes were used and the temperature was 28 degrees C or below.