Cultivar-strain specificity between Chrysanthemum morifolium and Agrobacterium tumefaciens

Abstract

We have examined cultivar specificity between Chrysanthemum morifolium and Agrobacterium tumefaciens strains Chry5 and B6. Although 42% of the 85 cultivars formed tumours after inoculation with both strains, some cultivars formed no tumours in response to Chry5, to B6, or to both strains. Four cultivars were utilized to examine these interactions in a leaf disc transformation system with a virE:: lacZ fusion. We specifically tested the hypothesis that cultivar-strain specificity is due to differential vir gene induction by conditioned media from the cultivars, but could find no evidence to support this possibility. We also tested the hypothesis that specificity is due to differential T-DNA transfer, integration or expression. This was accomplished by transferring into each strain a scorable marker (β-glucuronidase) contained between T-DNA borders. Leaf discs then were inoculated, and expression of enzyme activity was used to monitor transformation. Transfer of the T-DNA marker gene followed a pattern that reflected tumorigenicity in all cultivar-strain pairs, indicating that cultivar-strain specificity was due to differences in the transfer of T-DNA, or in subsequent integration processes. Analysis of the auxin and cytokinin levels of cultivars representative of different cultivar-strain interactions suggested that, although high hormone levels may be associated with high tumour mass, they do not appear to account for the innate susceptibility of a cultivar.