Biology:Cytokinin signaling and response regulator protein

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A cytokinin signaling and response regulator protein is a plant protein that is involved in a two step cytokinin signaling and response regulation pathway. The current model of cytokinin signaling and response regulation shows that it works as a multi-step phosphorelay two-component signaling system.[1] This type of system is similar to two-component signaling systems in bacteria.[2] The cytokinin signaling pathway consists of sensor kinases, histidine phosphotransfer proteins, and response regulators.[2] In this system, cytokinin sensor kinases are activated by the presence of cytokinins.[2] The sensor kinase then autophosphorylates, transferring a phosphate from its kinase domain to its receiver domain.[2] The phosphate is then transferred to a histidine phosphotransfer protein which then phosphorylates a response regulator.[2] The response regulators can then serve as positive or negative regulators of the signaling mechanism and affect gene expression within the plant cells.[2] This system is a called a two-step system because it involves two steps to transfer the phosphate to the final target, the response regulators.[2] Cytokinin cause a rapid increase in the expression of response regulator genes Cytokinins are a class of phytohormones that promote cell division in plants.[3] Cytokinins participate in short and long-distance signaling and are transported for this signaling through the xylem of plants.[3] Cytokinins control the differentiation of meristem cells in plant development, particularly in shoots and roots where plants undergo growth.[4] Cytokinins act in a restricted region of the root meristem, and their signaling and regulation of genes occurs through a multi-step phosphorelay mediate by cytokinin histidine sensor kinases, histidine phosphotransfer proteins, and cytokinin response regulator proteins.[5]

Cytokinin sensor kinases

Cytokinin sensor kinases are the initial sensors that detect and are bound by cytokinins.[2] Research with maize and Arabidopsis thaliana suggest that some cytokinin sensor kinases bind multiple types of cytokinins while other cytokinin sensor kinases are specific for distinct cytokinins.[2]

AHK4, a cytokinin histidine kinase in Arabidopsis thaliana, is a cytokinin sensor that allows binding of multiple types of cytokinins.[2] AHK4 has been shown, through three-dimensional modeling, to completely surround bound cytokinin in the binding pocket.[2]

AHK2 and AHK3 have been shown to be critically involved in drought tolerance.[6] These receptors activate dehydration tolerance response within one hour of dehydration and continue activation through eight hours.[6]

Histidine phosphotransfer proteins

Histidine phosphotransfer proteins transfer the phosphate in the multistep phosphorelay signaling pathway from cytokinin sensor kinases to their final target, cytokinin response regulators.[7]

In Arabidopsis thaliana, most histidine phosphotransfer proteins are redundant, positive regulators in cytokinin signaling.[7] Most of the Arabidopsis thaliana histidine phosphotransfer proteins have functional overlap and affect many aspects of plant development.[7] AHP4, however, might play a negative role in cytokinin responses.[7]

Cytokinin response regulators

Cytokinin response regulators proteins are the final target of the two-step phosphorelay.[5] These response regulators fall into three known classes: type A response regulators, type B response regulators, and type C response regulators.[8]

Type A

Type A cytokinin response regulators serve as negative regulators for cytokinin signaling.[5] Cytokinin causes the rapid induction of type A response regulators.[5] The type A cytokinin response regulator family in Arabidopsis thaliana consists of 10 genes.[9] Expression of type A cytokinin response regulators decreases sensitivity to cytokinins, and a lack of type-A cytokinin response regulators leads to increased sensitivity to cytokinins.[10]

Type A cytokinin response regulators can act as negative regulators of cytokinin signaling by either competing with type-B positive regulators or by regulating the pathway through direct and indirect interactions with other pathway mechanisms.[5]

Type A cytokinin response regulators are also likely involved in other processes. One example is light signal transduction: ARR3 and ARR4 are involved in the synchronization of the circadian clock of Arabidopsis thaliana with external time and photoperiod.[10] Moreover, ARR6 is implied in the control of Arabidopsis thaliana disease-resistance and cell wall composition.[11]

Type B

Type B cytokinin response regulators are the positive regulators that oppose the negative regulation of type A cytokinin response regulators in the two-component cytokinin signaling pathway.[12] These regulators play a critical role in early response to cytokinin.[12] Differing expression of type-B cytokinin response regulators likely play a role in controlling cellular response to cytokinins.[13] The type-B cytokinin response regulator family consists of two subfamilies and one major subfamily.[13] The major family of type-B cytokinin response regulators are expressed in locations on the plant that are heavily influenced by cytokinins.[13] These regions where type-B cytokinin response regulators are heavily expressed include apical meristem regions and budding leaves.[13]

ARR1, ARR10, and ARR12 have been indicated to mediate root growth response.[12] Each of ARR1, ARR10, and ARR12 vary in their effect on root growth response, likely related to differences in root expression patterns.[12] ARR1, ARR10, and ARR12 have been determined to have a functional overlap with type B response regulators.[12]

Type C

Type-C cytokinin response regulators are unique in that their expression is not induced by cytokinins like type-A cytokinin response regulators and type-B cytokinin response regulators.[1] ARR22 and ARR22 and ARR24 are the two known type-C cytokinin response regulators in Arabidopsis thaliana.[1] Research suggests that ARR22 plays a positive role in stress tolerance by improving cell membrane integrity.[1] Increases in expression of ARR22 modulates abiotic stress-responsive genes, possibly aiding in drought and freezing tolerance.[1] However, the role of ARR24 in Arabidopsis plant signaling remains undetermined.[1]

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 "Inducible expression of Arabidopsis response regulator 22 (ARR22), a type-C ARR, in transgenic Arabidopsis enhances drought and freezing tolerance". PLOS ONE 8 (11): e79248. November 2013. doi:10.1371/journal.pone.0079248. PMID 24244460. 
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 "Cytokinin signaling". Current Opinion in Plant Biology 8 (5): 518–25. October 2005. doi:10.1016/j.pbi.2005.07.013. PMID 16054432. 
  3. ↑ 3.0 3.1 "Cytokinins: activity, biosynthesis, and translocation". Annual Review of Plant Biology 57: 431–49. 2006. doi:10.1146/annurev.arplant.57.032905.105231. PMID 16669769. 
  4. ↑ "Cytokinins determine Arabidopsis root-meristem size by controlling cell differentiation". Current Biology 17 (8): 678–82. April 2007. doi:10.1016/j.cub.2007.02.047. PMID 17363254. 
  5. ↑ 5.0 5.1 5.2 5.3 5.4 "Cytokinin regulates type-A Arabidopsis Response Regulator activity and protein stability via two-component phosphorelay". The Plant Cell 19 (12): 3901–14. December 2007. doi:10.1105/tpc.107.052662. PMID 18065689. 
  6. ↑ 6.0 6.1 "Cytokinin receptor-dependent and receptor-independent pathways in the dehydration response of Arabidopsis thaliana". Journal of Plant Physiology 169 (14): 1382–91. September 2012. doi:10.1016/j.jplph.2012.05.007. PMID 22704545. 
  7. ↑ 7.0 7.1 7.2 7.3 "The Arabidopsis histidine phosphotransfer proteins are redundant positive regulators of cytokinin signaling". The Plant Cell 18 (11): 3073–87. November 2006. doi:10.1105/tpc.106.045674. PMID 17122069. 
  8. ↑ "Nomenclature for two-component signaling elements of rice". Plant Physiology 143 (2): 555–7. February 2007. doi:10.1104/pp.106.093666. PMID 17284581. 
  9. ↑ "Type-A Arabidopsis response regulators are partially redundant negative regulators of cytokinin signaling" (in en). The Plant Cell 16 (3): 658–71. March 2004. doi:10.1105/tpc.018978. PMID 14973166. 
  10. ↑ 10.0 10.1 "Arabidopsis response regulators ARR3 and ARR4 play cytokinin-independent roles in the control of circadian period". The Plant Cell 18 (1): 55–69. January 2006. doi:10.1105/tpc.105.037994. PMID 16326927. 
  11. ↑ Bacete, L; Mélida, H; López, G; Dabos, P; Tremousaygue, D; Denancé, N; Miedes, E; Bulone, V et al. (12 Mar 2020). "Arabidopsis Response Regulator 6 (ARR6) Modulates Plant Cell-Wall Composition and Disease Resistance". Molecular Plant-Microbe Interactions 33 (5): 767–780. doi:10.1094/MPMI-12-19-0341-R. PMID 32023150. 
  12. ↑ 12.0 12.1 12.2 12.3 12.4 "Type B response regulators of Arabidopsis play key roles in cytokinin signaling and plant development". The Plant Cell 20 (8): 2102–16. August 2008. doi:10.1105/tpc.108.059584. PMID 18723577. 
  13. ↑ 13.0 13.1 13.2 13.3 "Type-B response regulators display overlapping expression patterns in Arabidopsis". Plant Physiology 135 (2): 927–37. June 2004. doi:10.1104/pp.103.038109. PMID 15173562. 



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