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1. Cytokinin: From autoclaved DNA to two-component signaling

   https://doi.org/10.1093/plcell/koad327  

   Argueso, Cristiana T; Kieber, Joseph J (January 2024, The Plant Cell)

   Abstract Since its first identification in the 1950s as a regulator of cell division, cytokinin has been linked to many physiological processes in plants, spanning growth and development and various responses to the environment. Studies from the last two and one-half decades have revealed the pathways underlying the biosynthesis and metabolism of cytokinin and have elucidated the mechanisms of its perception and signaling, which reflects an ancient signaling system evolved from two-component elements in bacteria. Mutants in the genes encoding elements involved in these processes have helped refine our understanding of cytokinin functions in plants. Further, recent advances have provided insight into the mechanisms of intracellular and long-distance cytokinin transport and the identification of several proteins that operate downstream of cytokinin signaling. Here, we review these processes through a historical lens, providing an overview of cytokinin metabolism, transport, signaling, and functions in higher plants. 

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2. TRANSPORTER OF IBA1 Links Auxin and Cytokinin to Influence Root Architecture

   https://doi.org/10.1016/j.devcel.2019.06.010  

   Michniewicz, Marta; Ho, Cheng-Hsun; Enders, Tara A.; Floro, Eric; Damodaran, Suresh; Gunther, Lauren K.; Powers, Samantha K.; Frick, Elizabeth M.; Topp, Christopher N.; Frommer, Wolf B.; et al (September 2019, Developmental Cell)
3. Isoprene enhances leaf cytokinin metabolism and induces early senescence

   https://doi.org/10.1111/nph.17833  

   Dani, Kaidala Ganesha; Pollastri, Susanna; Pinosio, Sara; Reichelt, Michael; Sharkey, Thomas D.; Schnitzler, Jörg‐Peter; Loreto, Francesco (May 2022, New Phytologist)
4. Role of the Cytokinin-Activated Type-B Response Regulators in Hormone Crosstalk

   https://doi.org/10.3390/plants9020166  

   Zubo, Yan O.; Schaller, G. Eric (February 2020, Plants)

   Cytokinin is an important phytohormone that employs a multistep phosphorelay to transduce the signal from receptors to the nucleus, culminating in activation of type-B response regulators which function as transcription factors. Recent chromatin immunoprecipitation-sequencing (ChIP-seq) studies have identified targets of type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs) and integrated these into the cytokinin-activated transcriptional network. Primary targets of the type-B ARRs are enriched for genes involved in hormonal regulation, emphasizing the extensive crosstalk that can occur between cytokinin, auxin, abscisic acid, brassinosteroids, gibberellic acid, ethylene, jasmonic acid, and salicylic acid. Examination of hormone-related targets reveals multiple regulatory points including biosynthesis, degradation/inactivation, transport, and signal transduction. Here, we consider this early response to cytokinin in terms of the hormones involved, points of regulatory crosstalk, and physiological significance. 

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5. Cytokinin and Ethylene Cell Signaling Pathways from Prokaryotes to Eukaryotes

   https://doi.org/10.3390/cells9112526  

   Bidon, Baptiste; Kabbara, Samar; Courdavault, Vincent; Glévarec, Gaëlle; Oudin, Audrey; Héricourt, François; Carpin, Sabine; Spíchal, Lukáš; Binder, Brad M.; Cock, J. Mark; et al (November 2020, Cells)

   null (Ed.)

   Cytokinins (CKs) and ethylene (ET) are among the most ancient organic chemicals on Earth. A wide range of organisms including plants, algae, fungi, amoebae, and bacteria use these substances as signaling molecules to regulate cellular processes. Because of their ancestral origin and ubiquitous occurrence, CKs and ET are also considered to be ideal molecules for inter-kingdom communication. Their signal transduction pathways were first historically deciphered in plants and are related to the two-component systems, using histidine kinases as primary sensors. Paradoxically, although CKs and ET serve as signaling molecules in different kingdoms, it has been supposed for a long time that the canonical CK and ET signaling pathways are restricted to terrestrial plants. These considerations have now been called into question following the identification over recent years of genes encoding CK and ET receptor homologs in many other lineages within the tree of life. These advances shed new light on the dissemination and evolution of these hormones as both intra- and inter-specific communication molecules in prokaryotic and eukaryotic organisms. 

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