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1. Quantitative 3D imaging of cell level auxin and cytokinin response ratios in soybean roots and nodules: quantitative imaging of auxin and cytokinin

   https://doi.org/10.1111/pce.13169  

   Fisher, Jon ; Gaillard, Paul ; Fellbaum, Carl R. ; Subramanian, Senthil ; Smith, Steve ( March 2018 , Plant, Cell & Environment)
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. 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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4. Cytokinin–CLAVATA cross-talk is an ancient mechanism regulating shoot meristem homeostasis in land plants

   https://doi.org/10.1073/pnas.2116860119  

   Cammarata, Joseph ; Morales Farfan, Christopher ; Scanlon, Michael J. ; Roeder, Adrienne H. ( April 2022 , Proceedings of the National Academy of Sciences)

   Plant shoots grow from stem cells within shoot apical meristems (SAMs), which produce lateral organs while maintaining the stem cell pool. In the model flowering plant Arabidopsis , the CLAVATA (CLV) pathway functions antagonistically with cytokinin signaling to control the size of the multicellular SAM via negative regulation of the stem cell organizer WUSCHEL (WUS). Although comprising just a single cell, the SAM of the model moss Physcomitrium patens (formerly Physcomitrella patens ) performs equivalent functions during stem cell maintenance and organogenesis, despite the absence of WUS-mediated stem cell organization. Our previous work showed that the stem cell–delimiting function of the receptors CLAVATA1 (CLV1) and RECEPTOR-LIKE PROTEIN KINASE2 (RPK2) is conserved in the moss P. patens . Here, we use P. patens to assess whether CLV–cytokinin cross-talk is also an evolutionarily conserved feature of stem cell regulation. Application of cytokinin produces ectopic stem cell phenotypes similar to Ppclv1a , Ppclv1b , and Pprpk2 mutants. Surprisingly, cytokinin receptor mutants also form ectopic stem cells in the absence of cytokinin signaling. Through modeling, we identified regulatory network architectures that recapitulated the stem cell phenotypes of Ppclv1a , Ppclv1b , and Pprpk2 mutants, cytokinin application, cytokinin receptor mutations, and higher-order combinations of these perturbations. These models predict that Pp CLV1 and Pp RPK2 act through separate pathways wherein Pp CLV1 represses cytokinin-mediated stem cell initiation, and Pp RPK2 inhibits this process via a separate, cytokinin-independent pathway. Our analysis suggests that cross-talk between CLV1 and cytokinin signaling is an evolutionarily conserved feature of SAM homeostasis that preceded the role of WUS in stem cell organization. 

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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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