The Hulks and the Deadpools of the Cytokinin Universe: A Dual Strategy for Cytokinin Production, Translocation, and Signal Transduction

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.

The LONELY GUY gene family: from mosses to wheat, the key to the formation of active cytokinins in plants

“…Indeed, they cite a yeast complementation assay that showed binding to receptors not only of free bases and ribosides but also of nucleotides and Oglucosides (Daudu et al, 2017) and infer that Brassica napus receptors (Kuderov a et al, 2015) also bind a wide range of cytokinin forms (see Nguyen et al, 2021b). This inference by Nguyen et al (2021b) is in contrast to that by Hluska et al (2021) who state that O-glucosides or ribosides exert cytokinin activity in various bioassays, but O-glucosides do not bind to receptors at all and ribosides very weakly. They are active because they can be metabolized to the active forms.…”

Biochemical and Structural Aspects of Cytokinin Biosynthesis and Degradation in Bacteria

“…It belongs to flavoprotein oxidoreductases from the vanillyl alcohol oxidase family containing covalently bound FAD cofactor [ 79 , 80 ]. While plant CKXs were extensively studied at the biochemical as well as a physiological level [ 5 , 81 ], R. fascians CKX (RfCKX or FasE) remains the only characterized functional CKX of bacterial origin [ 22 ]. The CKX gene of R. fascians was found on fas operon in 1994 [ 46 ] but its putative function in CK degradation was only revealed after the first CKX gene from maize ( ZmCKX1 ) was cloned and the encoded protein characterized as CKX [ 82 , 83 ].…”

In Planta Transcriptome and Proteome Profiles of Spongospora subterranea in Resistant and Susceptible Host Environments Illuminates Regulatory Principles Underlying Host–Pathogen Interaction

“…The tRNA pathway is an alternative route for cytokinin biosynthesis in many organisms [ 39 ]. Plant pathogens have developed several strategies to prevail over their hosts including the production of phytohormones such as cytokinin which interfere with host immune responses [ 40 ]. The tRNA-mediated cytokinin production by the rice blast fungus Magnaporthe oryzae reduces the defence responses from the host and alters the amino acid and sugar distribution to the favour of a pathogen [ 41 ].…”