Correlation between susceptibility to crown gall and sensitivity to cytokinin in aspen cultivars

Closely related aspen cultivars (Populus tremula x P. alba) were ranked as resistant, intermediate, or susceptible to crown gall according to their tumor response to the selected strains C58, B6, and 354 of Agrobacterium tumefaciens, respectively. As shown with mutant agrobacteria that harbored derivatives of pTiB6S3, a strain retained its ability to define differences in susceptibility among aspen cultivars as long as it had a functional ipt gene and, therefore, could induce the biosynthesis of a cytokinin in transformed plants. This suggested that differences in susceptibility to crown gall were related to differences in sensitivity to cytokinin. Cultivar sensitivity to cytokinin was determined in vitro by a leaf disk assay. Aspen cultivars resistant to the highest cytokinin concentrations (32 mu M benzyladenine) were also found to be resistant to A. tumefaciens strain B6 and the most resistant to natural crown gall infections. This result showed that sensitivity to cytokinin was a plant factor controlling tumorigenesis.