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

Seed development in flowering plants is a critical part of plant life for successful reproduction. The formation of viable seeds requires the synchronous growth and development of the fruit and the three seed structures: the embryo, the endosperm, the seed coat. Molecular communication between these tissues is crucial to coordinate these developmental processes. The phytohormone auxin is a significant player in embryo, seed and fruit development. Its regulated local biosynthesis and its cell-to-cell transport capacity make of auxin the perfect candidate as a signaling molecule to coordinate the growth and development of the embryo, endosperm, seed and fruit. Moreover, newly formed seeds need nutrients and form new carbon sink, generating high sugar flow from vegetative tissues to the seeds. This review will discuss how auxin and sugars may be considered as signaling molecules to coordinate seed and fruit development.

• Keywords
• auxin, embryo, endosperm, fruit, molecular communication, seed, sucrose,
• MeSH
• Sugars metabolism   MeSH
• Indoleacetic Acids metabolism   MeSH
• Fruit growth & development   metabolism   MeSH
• Seeds growth & development   metabolism   MeSH
• Signal Transduction * MeSH
• Plant Development * MeSH
• Publication type
• Journal Article MeSH
• Review MeSH
• Names of Substances
• Sugars MeSH
• Indoleacetic Acids MeSH