Strain-dependent temperature-sensitive phase in crown gall tumorigenesis.

The effect of high temperature treatments on the early stages of crown gall tumorigenesis in sunflowers was investigated. Treatments of 32 C initiated at various times during the first ten days after infection had a similar effect on tumors induced by Agrobacterium tumefaciens strains B(6) and C58. Tumor growth was sensitive to 32 C until 60 to 72 hours after infection and was stimulated by 32 C after that time. Therefore, the "inception phase" for both C58 and B(6)-induced tumors ends between 60 to 72 hours after infection. In contrast, B(6) and C58 tumors varied in their response to 37 C treatments during the first 168 hours after infection. Both C58 and B(6) tumors were sensitive to 37 C during the first 72 hours; however, B(6) tumors became resistant to 37 C after 96 hours, whereas C58-induced tumors remained sensitive until 144 to 168 hours after infection.The growth rate of C58 and B(6) tumors in plants moved to 32 C at 90 hours after infection was the same. B(6) tumors in plants moved to 37 C at 90 hours after infection also continued to grow rapidly, whereas C58 tumors in plants moved to 37 C at 90 hours exhibited a complete cessation of tumor growth. The 37 C temperature-sensitive period specific to C58 tumors ceases by 168 hours after infection. This indicates that the temperature-sensitive functions are no longer required after 168 hours. The possible significance of this work with regard to the presence of a temperature-sensitive plasmid, pTiC58, in strain C58 and the mechanism of tumor-inducing plasmid DNA maintenance in incipient tumor cells is discussed.