Abstract
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.
Highlights
As the plant tRNA contains mainly cis-zeatin, which is the main CK involved in stress responses, it is logical that a specific compound released from RNA can serve as a stress signal because the RNA hydrolysis is stress sensitive [60]
We presented a novel hypothesis showing that the Cytokinin Universe is full of both (i) strong Hulks with high CK activity, metabolite conversion, and sensing power that drives fast growth spurts and (ii) immortal Deadpools sustaining basal survival, metabolites, enzymes, and sensing mechanisms with lower activity but a long-lasting effect
This demonstrates that the production levels, and different forms of CK can determine which of the two different Hulk/Deadpool strategies take place
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The first compound with CK-like activity isolated from a natural source (that is, apart from degraded DNA) was N,N0 -diphenylurea [2] It was isolated from coconut milk, as that was previously recognised to induce cytokinesis. While the tRNAs recognising UNN codons contain mostly CKs or wybutosine, a hypermodified guanine, those recognising ANN codons contain almost exclusively N6 -threonylcarbamoyladenosine No such conservation is present in tRNAs binding to CNN or GNN codons [7]. The functions of CKs can be summarised as triggering cellular changes that are essential for numerous possible fates throughout the plant’s life This includes both developmental processes and adaptive responses to various abiotic and biotic inputs. The response is further influenced by crosstalk with other signals that vary with the cell’s history and context [12]
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