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 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 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 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 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 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 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]
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 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 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 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 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 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 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]