Biosynthesis Pathway
Signaling Pathway
R. fascians
Rhodococcus fascians, R. fascians, is a cytokinin secreting pathogen. Pertry et al., 2009 showed that R. fascians uses derivatives of cytokinin to manipulate host defenses and requires the two cytokinin ARABIDOPSIS HISTIDINE KINASE receptors (AHK3 and AHK4). Since these receptors are required, this indicates that cytokinin-mediated alternation of host physiology is likely resulting in better conditions for the pathogen in planta.
M. oryzae
Magnaporthe oryzae, M. oryzae, is a fungus that was shown by Chanclud et al., 2016 to have the ability to synthesis and secrete cytokinin derivatives to promote pathogen growth. It's suspected that this is not due to decreased defense activation, but rather an increase of nutrient allocation at site of infection that is contributing to increased virulence of M. oryzae.
HopQ1
Hann et al., 2014 showed that HopQ1 is a type III secretion effector from Pseudomonas syringae pv. tomato DC3000 that acts as a negative regulator of PTI (PAMP-Triggered Immunity) by increasing the levels of cytokinin in plants. By increasing the internal levels of cytokinin, this results in lower levels of the PTI receptor FLAGELLIN SENSING 2 (FLS2) causing a decrease in host immune responses and increased bacterial growth.
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CK signaling pathway: The CK signaling pathway involves a two-component system. The first step of the two-component system involves binding of CK to AHK receptors resulting in an auto-phosphorylation. This leads to a phosphorylation of the AHP proteins, to further phosphorylate and activate ARR proteins in the nucleus. Type-A ARRs are negative regulators of the CK pathway resulting in inhibition of CK-mediated responses, while type-B ARRs are positive regulators resulting in activation of CK-dependent genes. Arrows indicate positive interaction; blunt-end indicates negative interaction (inhibition).
CK biosynthetic pathway: Majority of the bioactive CKs synthesized in plants are isoprenoid-derived. Isoprenoid CK are synthesized by the addition of an isopentenyl moiety from DMAPP, dimethylallyl diphosphate, to the N6 position of various adenine derivatives such as ATP, ADP, or AMP. The addition of the isopentenyl moiety is catalyzed by the enzyme IPT, which is the rate-limiting step of this reaction. Following this trans-hydroxylation by the cytochrome P450 monooxygenases CYP735A1 and CYP735A2 to result in zeatin ribotides. Lastly, LOG phosphoribohydrolase enzymes, such as LOG, results in hydroxylation to form bioactive isoprenoid CKs. Arrows indicate positive interaction; blunt-end indicates negative interaction (inhibition).
