A cytokinin signaling and response regulator protein is a plant
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
that is involved in a two step
cytokinin
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 (biology), cell growth and cellular differentiation, differentiation, but also affect apical ...
signaling and
response regulation pathway.
The current model of
cytokinin
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 (biology), cell growth and cellular differentiation, differentiation, but also affect apical ...
signaling and response regulation shows that it works as a multi-step
phosphorelay two-component signaling system.
This type of system is similar to two-component signaling systems in bacteria.
The cytokinin signaling pathway consists of sensor
kinase
In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...
s,
histidine phosphotransfer proteins, and response regulators.
In this system, cytokinin sensor kinases are activated by the presence of cytokinins.
The sensor kinase then
autophosphorylates, transferring a phosphate from its kinase domain to its receiver domain.
The phosphate is then transferred to a histidine phosphotransfer protein which then
phosphorylates
In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols:
:
This equation can be writt ...
a response regulator.
The response regulators can then serve as positive or negative regulators of the signaling mechanism and affect
gene expression
Gene expression is the process (including its Regulation of gene expression, regulation) by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, ...
within the plant cells.
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.
Cytokinin cause a rapid increase in the expression of response regulator genes
Cytokinins are a class of
phytohormone
Plant hormones (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, including embryogenesis, the regulation of organ si ...
s that promote cell division in plants.
Cytokinins participate in short and long-distance signaling and are transported for this signaling through the
xylem
Xylem is one of the two types of transport tissue (biology), tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts o ...
of plants.
Cytokinins control the differentiation of
meristem
In cell biology, the meristem is a structure composed of specialized tissue found in plants, consisting of stem cells, known as meristematic cells, which are undifferentiated cells capable of continuous cellular division. These meristematic c ...
cells in plant development, particularly in shoots and roots where plants undergo growth. 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.
Cytokinin sensor kinases
Cytokinin sensor kinases are the initial sensors that detect and are bound by cytokinins.
Research with
maize
Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout grass that produces cereal grain. It was domesticated by indigenous peoples in southern Mexico about 9,000 years ago from wild teosinte. Native American ...
and ''
Arabidopsis thaliana
''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small plant from the mustard family (Brassicaceae), native to Eurasia and Africa. Commonly found along the shoulders of roads and in disturbed land, it is generally ...
'' suggest that some cytokinin sensor kinases bind multiple types of cytokinins while other cytokinin sensor kinases are specific for distinct cytokinins.
AHK4, a cytokinin
histidine kinase in ''Arabidopsis thaliana'', is a cytokinin sensor that allows binding of multiple types of cytokinins.
AHK4 has been shown, through three-dimensional modeling, to completely surround bound cytokinin in the binding pocket.
AHK2 and AHK3 have been shown to be critically involved in drought tolerance.
These receptors activate dehydration tolerance response within one hour of dehydration and continue activation through eight hours.
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.
In ''Arabidopsis thaliana'', most histidine phosphotransfer proteins are redundant, positive regulators in cytokinin signaling.
Most of the ''Arabidopsis thaliana'' histidine phosphotransfer proteins have functional overlap and affect many aspects of plant development.
AHP4, however, might play a negative role in cytokinin responses.
Cytokinin response regulators
Cytokinin response regulators proteins are the final target of the two-step phosphorelay.
These response regulators fall into three known classes: type A response regulators, type B response regulators, and type C response regulators.
Type A
Type A cytokinin response regulators serve as negative regulators for cytokinin signaling.
Cytokinin causes the rapid induction of type A response regulators.
The type A cytokinin response regulator family in ''Arabidopsis thaliana'' consists of 10 genes.
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.
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.
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.
Moreover, ARR6 is implied in the control of ''Arabidopsis thaliana'' disease-resistance and cell wall composition.
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.
These regulators play a critical role in early response to cytokinin.
Differing expression of type-B cytokinin response regulators likely play a role in controlling cellular response to cytokinins.
The type-B cytokinin response regulator family consists of two subfamilies and one major subfamily.
The major family of type-B cytokinin response regulators are expressed in locations on the plant that are heavily influenced by cytokinins.
These regions where type-B cytokinin response regulators are heavily expressed include apical meristem regions and budding leaves.
ARR1, ARR10, and ARR12 have been indicated to mediate root growth response.
Each of ARR1, ARR10, and ARR12 vary in their effect on root growth response, likely related to differences in root expression patterns.
ARR1, ARR10, and ARR12 have been determined to have a functional overlap with type B response regulators.
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.
ARR22 and ARR22 and ARR24 are the two known type-C cytokinin response regulators in ''Arabidopsis thaliana''.
Research suggests that ARR22 plays a positive role in stress tolerance by improving cell membrane integrity.
Increases in expression of ARR22 modulates abiotic stress-responsive genes, possibly aiding in drought and
freezing tolerance.
However, the role of ARR24 in ''Arabidopsis'' plant signaling remains undetermined.
References
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Plant proteins
Cytokinins