Bacterial and plant signal integration via D3-type cyclins enhances symptom development in the Arabidopsis thaliana—Rhodococcus fascians interaction

Polyamine and transcript profiling of wild-type and cytokinin receptor mutant plants revealed that the bacterial cytokinins directly stimulated the biosynthesis of plant putrescine by activating ADC expression. Pharmacological experiments showed that the increased levels of putrescine contributed to the severity of the symptoms. Thus, putrescine functions as a secondary signal that impinges on the cytokinin-activated pathway, amplifying the hormone-induced changes that lead to the formation of a leafy gall. Exogenous putrescine supply and treatment with polyamine biosynthesis inhibitors combined with transcript and polyamine analyses of wild-type and mutant plants indicated that the direct target of both the bacterial cytokinins and plant putrescine was the expression of D3-type cyclins. Hence, the activated D-type cyclin/retinoblastoma/E2F transcription factor pathway integrates both external and internal hormonal signals stimulating mitotic cell divisions and inducing pathological plant organogenesis.