Cytokinins Mediate Resistance againstPseudomonas syringaein Tobacco through Increased Antimicrobial Phytoalexin Synthesis Independent of Salicylic Acid Signaling    

Abstract

Cytokinins are phytohormones that are involved in various regulatory processes throughout plant development, but they are also produced by pathogens and known to modulate plant immunity. A novel transgenic approach enabling autoregulated cytokinin synthesis in response to pathogen infection showed that cytokinins mediate enhanced resistance against the virulent hemibiotrophic pathogen Pseudomonas syringae pv tabaci. This was confirmed by two additional independent transgenic approaches to increase endogenous cytokinin production and by exogenous supply of adenine-and phenylurea-derived cytokinins. The cytokinin-mediated resistance strongly correlated with an increased level of bactericidal activities and up-regulated synthesis of the two major antimicrobial phytoalexins in tobacco (Nicotiana tabacum), scopoletin and capsidiol. The key role of these phytoalexins in the underlying mechanism was functionally proven by the finding that scopoletin and capsidiol substitute in planta for the cytokinin signal: phytoalexin pretreatment increased resistance against P. syringae. In contrast to a cytokinin defense mechanism in Arabidopsis (Arabidopsis thaliana) based on salicylic acid-dependent transcriptional control, the cytokininmediated resistance in tobacco is essentially independent from salicylic acid and differs in pathogen specificity. It is also independent of jasmonate levels, reactive oxygen species, and high sugar resistance. The novel function of cytokinins in the primary defense response of solanaceous plant species is rather mediated through a high phytoalexin-pathogen ratio in the early phase of infection, which efficiently restricts pathogen growth. The implications of this mechanism for the coevolution of host plants and cytokinin-producing pathogens and the practical application in agriculture are discussed.

Cis- and trans-zeatin differentially modulate plant immunity

“…The restriction of Pst proliferation through tZ appeared to be less distinct than reported for kinetin, while the suppression of symptoms seemed to be even stronger. 4 Considering the differences in the dynamics of SA accumulation and the physiological effects (symptoms and pathogen proliferation) between the very potent CKs kinetin 4 and tZ (this study), the involvement of distinct mechanisms in CK mediated resistance have to be assumed. Apparently, the combination of these different mechanisms by which the individual CKs function to restrict pathogen infection contribute to host plant and pathogen specific resistance.…”

“…The bacteria were derived from a exponentially growing liquid culture in LB medium supplemented with 20 mg/l tetracycline (Sigma-Aldrich). 4 At the time-point of infection, levels of cZ, tZ, SA and the phytoalexin scopoletin were determined in the leaves as reported…”

“…4 Detached leaves of 7-10-wk-old N. tabacum SR1 plants, grown under greenhouse conditions, 4 were fed via petioles with 10 μM cZ (Olchemim Ltd.,), tZ (Duchefa), diluted from 100 mM stock solutions in 0.5 M NaOH (Roth) or accordingly diluted NaOH solvent control for 24 h prior to Pst infection. 4,17 After this pre-treatment, leaves were infiltrated with 10 6 cfu/ml Pst in 10 mM MgCl 2 and leaves were put back on water. The bacteria were derived from a exponentially growing liquid culture in LB medium supplemented with 20 mg/l tetracycline (Sigma-Aldrich).…”

“…4 However, differences in the potential of the physiological effect on plant immunity are obvious. degradation and conjugation processes must be assumed.…”

“…Here, we comparatively study the differential impact of cZ and tZ treatment on tobacco-Pseudomonas syringae pv tabaci (Pst) interaction, a well-established pathosystem to analyze CK effects on plant immunity. 4 The analysis of free base cZ and tZ levels at the time-point of infection revealed a high and distinct accumulation of the according isomer applied ( Table 1). In samples treated with cZ only this isomer accumulated without increasing levels of tZ compared with the untreated control ( Table 1).…”