Cytokinin-mediated resistance of tobacco

Cytokinins form one essential group of classic growth-regulating and -promoting phytohormones. Their structure is based on adenine with modification of the side chain at the N6 position. In recent years, CKs have been studied for their roles in plant interactions with their environment and particularly functions in regulating immunity and defence mechanisms against pathogens were identified. While some pathogens seem to hijack CK accumulation and signalling when establishing their infection niches, the identified resistance effects appear to be a wide-spread phenomenon in diverse pathosystems. Although several studies have been conducted to identify CK-mediated resistance effects, these only explain a part of the phenomenon. Hence, a more detailed analysis of the activities of individual CKs and components of the underlying mechanisms in relation to disease control is needed. To test our hypothesis that structurally different CKs will have differential impacts on plant immunity/resistance, we deployed the well-established system of exogenous cytokinin application by petiole feeding of N. tabacum leaves and the infection with Pseudomonas syringae pv. tabaci (Pst), which has been used before to identify CK-mediated resistance effects and components of the underlying mechanisms. To determine possible differences in the effects of six different CKs on this pathosystem, we comparatively scored symptom development (as an estimate of combined defence and tolerance effects) and pathogen proliferation (as an estimate of defence effects) of Pst infection after treatments with 2-iP, 6-BAP, cZ, tZ, Kin, and TDZ, as well as respective controls. Furthermore, we conducted targeted metabolome and whole transcriptome profiling to identify potential factors contributing to common and the expected differential effects of the individual CKs, which may be parts of CK-mediated resistance and tolerance.