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.

Contents

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]

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The meristem is a type of tissue found in plants. It consists of undifferentiated cells capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells continue to divide until a time when they get differentiated and then lose the ability to divide.

Cytokinin Class of plant hormones promoting cell division

Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. Folke Skoog discovered their effects using coconut milk in the 1940s at the University of Wisconsin–Madison.

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Primordium organ in the earliest recognizable stage of embryonic development

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Systemin Plant peptide hormone

Systemin is a plant peptide hormone involved in the wound response in the family Solanaceae. It was the first plant hormone that was proven to be a peptide having been isolated from tomato leaves in 1991 by a group led by Clarence A. Ryan. Since then, other peptides with similar functions have been identified in tomato and outside of the Solanaceae. Hydroxyproline-rich glycopeptides were found in tobacco in 2001 and AtPEPs were found in Arabidopsis thaliana in 2006. Their precursors are found both in the cytoplasm and cell walls of plant cells, upon insect damage, the precursors are processed to produce one or more mature peptides. The receptor for systemin was first thought to be the same as the brassinolide receptor but this is now uncertain. The signal transduction processes that occur after the peptides bind are similar to the cytokine-mediated inflammatory immune response in animals. Early experiments showed that systemin travelled around the plant after insects had damaged the plant, activating systemic acquired resistance, now it is thought that it increases the production of jasmonic acid causing the same result. The main function of systemins is to coordinate defensive responses against insect herbivores but they also affect plant development. Systemin induces the production of protease inhibitors which protect against insect herbivores, other peptides activate defensins and modify root growth. They have also been shown to affect plants' responses to salt stress and UV radiation. AtPEPs have been shown to affect resistance against oomycetes and may allow A. thaliana to distinguish between different pathogens. In Nicotiana attenuata, some of the peptides have stopped being involved in defensive roles and instead affect flower morphology.

Wall-associated kinase

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Two-component regulatory system

In the field of molecular biology, a two-component regulatory system serves as a basic stimulus-response coupling mechanism to allow organisms to sense and respond to changes in many different environmental conditions. Two-component systems typically consist of a membrane-bound histidine kinase that senses a specific environmental stimulus and a corresponding response regulator that mediates the cellular response, mostly through differential expression of target genes. Although two-component signaling systems are found in all domains of life, they are most common by far in bacteria, particularly in Gram-negative and cyanobacteria; both histidine kinases and response regulators are among the largest gene families in bacteria. They are much less common in archaea and eukaryotes; although they do appear in yeasts, filamentous fungi, and slime molds, and are common in plants, two-component systems have been described as "conspicuously absent" from animals.

Histidine kinase

Histidine kinases (HK) are multifunctional, and in non-animal kingdoms, typically transmembrane, proteins of the transferase class of enzymes that play a role in signal transduction across the cellular membrane. The vast majority of HKs are homodimers that exhibit autokinase, phosphotransfer, and phosphatase activity. HKs can act as cellular receptors for signaling molecules in a way analogous to tyrosine kinase receptors (RTK). Multifunctional receptor molecules such as HKs and RTKs typically have portions on the outside of the cell that bind to hormone- or growth factor-like molecules, portions that span the cell membrane, and portions within the cell that contain the enzymatic activity. In addition to kinase activity, the intracellular domains typically have regions that bind to a secondary effector molecule or complex of molecules that further propagate signal transduction within the cell. Distinct from other classes of protein kinases, HKs are usually parts of a two-component signal transduction mechanisms in which HK transfers a phosphate group from ATP to a histidine residue within the kinase, and then to an aspartate residue on the receiver domain of a response regulator protein. More recently, the widespread existence of protein histidine phosphorylation distinct from that of two-component histidine kinases has been recognised in human cells. In marked contrast to Ser, Thr and Tyr phosphorylation, the analysis of phosphorylated Histidine using standard biochemical and mass spectrometric approaches is much more challenging, and special procedures and separation techniques are required for their preservation alongside classical Ser, Thr and Tyr phosphorylation on proteins isolated from human cells.

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Response regulator

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Histidine phosphotransfer domain

Histidine phosphotransfer domains and histidine phosphotransferases are protein domains involved in the "phosphorelay" form of two-component regulatory systems. These proteins possess a phosphorylatable histidine residue and are responsible for transferring a phosphoryl group from an aspartate residue on an intermediate "receiver" domain, typically part of a hybrid histidine kinase, to an aspartate on a final response regulator.

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Ethylene signaling pathway

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References

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  11. ↑ Bacete, L; Mélida, H; López, G; Dabos, P; Tremousaygue, D; Denancé, N; Miedes, E; Bulone, V; Goffner, D; Molina, A (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.
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