It has been known for quite some time that cytokinins, hormones typical of plants, are also produced and metabolized in bacteria. Most bacteria can only form the tRNA-bound cytokinins, but there are examples of plant-associated bacteria, both pathogenic and beneficial, that actively synthesize cytokinins to interact with their host. Similar to plants, bacteria produce diverse cytokinin metabolites, employing corresponding metabolic pathways. The identification of genes encoding the enzymes involved in cytokinin biosynthesis and metabolism facilitated their detailed characterization based on both classical enzyme assays and structural approaches. This review summarizes the present knowledge on key enzymes involved in cytokinin biosynthesis, modifications, and degradation in bacteria, and discusses their catalytic properties in relation to the presence of specific amino acid residues and protein structure.

• Keywords
• CKX, LOG, cytochrome P450 monooxygenase, cytokinin, isopentenyl transferase, tRNA modification,
• Publication type
• Journal Article MeSH
• Review MeSH

Cytokinins are plant hormones, derivatives of adenine with a side chain at the N6-position. They are involved in many physiological processes. While the metabolism of trans-zeatin and isopentenyladenine, which are considered to be highly active cytokinins, has been extensively studied, there are others with less obvious functions, such as cis-zeatin, dihydrozeatin, and aromatic cytokinins, which have been comparatively neglected. To help explain this duality, we present a novel hypothesis metaphorically comparing various cytokinin forms, enzymes of CK metabolism, and their signalling and transporter functions to the comics superheroes Hulk and Deadpool. Hulk is a powerful but short-lived creation, whilst Deadpool presents a more subtle and enduring force. With this dual framework in mind, this review compares different cytokinin metabolites, and their biosynthesis, translocation, and sensing to illustrate the different mechanisms behind the two CK strategies. This is put together and applied to a plant developmental scale and, beyond plants, to interactions with organisms of other kingdoms, to highlight where future study can benefit the understanding of plant fitness and productivity.

• Keywords
• Hulk/Deadpool, aromatic cytokinins, cis-zeatin, cytokinin biosynthesis, cytokinin oxidase/dehydrogenase, cytokinin signalling, cytokinin transport, cytokinins, isopentenyl transferase,
• MeSH
• Arabidopsis metabolism   MeSH
• Models, Biological MeSH
• Biological Transport MeSH
• Biological Assay MeSH
• Cytokinins metabolism   MeSH
• Plant Physiological Phenomena * MeSH
• Glycosylation MeSH
• Hydrolysis MeSH
• Kinetics MeSH
• Kinetin metabolism   MeSH
• Oxidoreductases metabolism   MeSH
• Plant Growth Regulators metabolism   MeSH
• Plants metabolism   MeSH
• Signal Transduction * MeSH
• Protein Binding MeSH
• Zeatin analogs & derivatives   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH
• Names of Substances
• cytokinin oxidase MeSH Browser
• Cytokinins MeSH
• dihydrozeatin MeSH Browser
• Kinetin MeSH
• Oxidoreductases MeSH
• Plant Growth Regulators MeSH
• Zeatin MeSH