Histidine kinase homologs that act as cytokinin receptors possess overlapping functions in the regulation of shoot and root growth in Arabidopsis

Cytokinins are plant hormones that may play essential and crucial roles in various aspects of plant growth and development. Although the functional significance of exogenous cytokinins as to the proliferation and differentiation of cells has been well documented, the biological roles of endogenous cytokinins have remained largely unknown. The recent discovery of the Arabidopsis Histidine Kinase 4 (AHK4)/CRE1/WOL cytokinin receptor in Arabidopsis thaliana strongly suggested that the cellular response to cytokinins involves a two-component signal transduction system. However, the lack of an apparent phenotype in the mutant, presumably because of genetic redundancy, prevented us from determining the in planta roles of the cytokinin receptor. To gain insight into the molecular functions of the three AHK genes AHK2, AHK3, and AHK4 in this study, we identified mutational alleles of the AHK2 and AHK3 genes, both of which encode sensor histidine kinases closely related to AHK4, and constructed a set of multiple ahk mutants. Application of exogenous cytokinins to the resultant strains revealed that both AHK2 and AHK3 function as positive regulators for cytokinin signaling similar to AHK4. The ahk2 ahk4 and ahk3 ahk4 double mutants and the ahk single mutants grew normally, whereas the ahk2 ahk3 double mutants exhibited a semidwarf phenotype as to shoots, such as a reduced leaf size and a reduced influorescence stem length. The growth and development of the ahk2 ahk3 ahk4 triple mutant were markedly inhibited in various tissues and organs, including the roots and leaves in the vegetative growth phase and the influorescence meristem in the reproductive phase. We showed that the inhibition of growth is associated with reduced meristematic activity of cells. Expression analysis involving AHK:beta-glucuronidase fusion genes suggested that the AHK genes are expressed ubiquitously in various tissues during postembryonic growth and development. Our results thus strongly suggest that the primary functions of AHK genes, and those of endogenous cytokinins, are triggering of the cell division and maintenance of the meristematic competence of cells to prevent subsequent differentiation until a sufficient number of cells has accumulated during organogenesis.     

Biochemical characteristics and ligand-binding properties of Arabidopsis cytokinin receptor AHK3 compared to CRE1/AHK4 as revealed by a direct binding assay

The cytokinin receptor AHK3 of Arabidopsis thaliana plays a predominant role in shoot development. A study of the hormone-binding characteristics of AHK3 compared with the mainly root-confined receptor CRE1/AHK4 has been accomplished using a live-cell binding assay on transgenic bacteria expressing individual receptor proteins. Both receptors bound trans-zeatin (tZ) with high affinity. Scatchard analysis showed a linear function corresponding to an apparent K(D) of 1-2 nM for the AHK3 receptor-hormone complex, which is close to the K(D) (2-4 nM) for the CRE1/AHK4 receptor-hormone complex. The specific binding of tZ to both receptors was pH dependent, AHK3 being more sensitive to pH changes than CRE1/AHK4. Hormone binding was reversible, at least for the bulk of (3)H-zeatin, and influenced by monovalent cations, while divalent cations (Ca(2+), Mg(2+), Mn(2+)) at physiological concentrations had no significant effect. AHK3 differed significantly from CRE1/AHK4 in relative affinity to some cytokinins. AHK3 had an approximately 10-fold lower affinity to isopentenyladenine (iP) and its riboside, but a higher affinity to dihydrozeatin than CRE1/AHK4. For AHK3, cytokinin ribosides (tZR, iPR) and cis-zeatin had true binding activity, although lower than that of tZ. The phenylurea-derived cytokinin thidiazuron was a strong competitor and bound to the same site as did adenine-derived cytokinins. The inhibitor of cytokinin action butan-1-ol had little effect on cytokinin-receptor complex formation. The revealed properties of AHK3 suggest its specific function in root-to-shoot communication.     

CREam of cytokinin signalling: receptor identified.

Cytokinins play a central role in the regulation of plant cell division and numerous developmental processes. Pleiotropic effects have made studies of this hormone difficult, and cytokinin signalling pathways have long remained elusive. The recent identification of CRE1 (a histidine kinase identical to AHK4 and WOL) as the cytokinin receptor of Arabidopsis thaliana is a landmark in cytokinin research. Mutations have been identified in CRE1, and the phenotype of loss-of-function mutations sheds new light on the role of cytokinins in plant development. This article describes the experimental paths leading to receptor identification and the current interpretation of its function.     

Two cytokinin receptors of Arabidopsis thaliana, CRE1/AHK4 and AHK3, differ in their ligand specificity in a bacterial assay

Strains of Escherichia coli that express two different cytokinin receptors of Arabidopsis thaliana, CRE1/AHK4 and AHK3, were used to study the relative sensitivity of these receptors to various cytokinins. Both receptors were most sensitive to the bases of the isoprenoid-type cytokinins trans-zeatin and isopentenyladenine but differed significantly in the recognition of other cytokinin compounds. In particular, CRE1/AHK4 recognized at 1 microm concentration only trans-zeatin while AHK3 recognized cis-zeatin and dihydrozeatin as well, although with a lower sensitivity. Similarly, CRE1/AHK4 was not activated by cytokinin ribosides and ribotides, but AHK3 was. Comparisons using the ARR5::GUS fusion gene as a cytokinin reporter in Arabidopsis showed similar relative degrees of responses in planta, except that cytokinins with aromatic side chains showed much higher activities than in the bacterial assay. These results indicate that the diverse cytokinin compounds might have specific functions in the numerous cytokinin-regulated processes, which may depend in turn on different receptors and their associated signalling pathways. The importance of precise control of local concentrations of defined cytokinin metabolites to regulate the respective downstream event is corroborated.     

Design and synthesis of photolabile caged cytokinin

Cytokinins are phytohormones that regulate diverse developmental processes throughout the life of a plant. trans-Zeatin, kinetin, benzyladenine and dihydrozeatin are adenine-type cytokinins that are perceived by the AHK cytokinin receptors. Endogenous cytokinin levels are critical for regulating plant development. To manipulate intracellular cytokinin levels, caged cytokinins were designed on the basis of the crystal structure of the AHK4 cytokinin receptor. The caged cytokinin was photolyzed to release the cytokinin molecule inside the cells and induce cytokinin-responsive gene expression. The uncaging of intracellular caged cytokinins demonstrated that cytokinin-induced root growth inhibition can be manipulated with photo-irradiation. This caged cytokinin system could be a powerful tool for cytokinin biology.     

Cytokinins regulate a bidirectional phosphorelay network in Arabidopsis

The cytokinin class of plant hormones plays key roles in regulating diverse developmental and physiological processes. Arabidopsis perceives cytokinins with three related and partially redundant receptor histidine kinases (HKs): CRE1 (the same protein as WOL and AHK4), AHK2, and AHK3 (CRE-family receptors). It is suggested that binding of cytokinins induces autophosphorylation of these HKs and subsequent transfer of the phosphoryl group to a histidine phosphotransfer protein (HPt) and then to a response regulator (RR), ultimately regulating downstream signaling events. Here we demonstrate that, in vitro and in a yeast system, CRE1 is not only a kinase that phosphorylates HPts in the presence of cytokinin but is also a phosphatase that dephosphorylates HPts in the absence of cytokinin. To explore the roles of these activities in planta, we replaced CRE1 with mutant versions of the gene or with AHK2. Replacing CRE1 with CRE1(T278I), which lacks cytokinin binding activity and is locked in the phosphatase form, decreased cytokinin sensitivity. Conversely, replacing CRE1 with AHK2, which favors kinase activity, increased cytokinin sensitivity. These results indicate that in the presence of cytokinins, cytokinin receptors feed phosphate to phosphorelay-integrating HPt proteins. In the absence of cytokinins, CRE1 removes phosphate from HPt proteins, decreasing the system phosphoload.     

Cytokinin receptor-dependent and receptor-independent pathways in the dehydration response of Arabidopsis thaliana

Cytokinin signaling in Arabidopsis thaliana utilizes a multi-step two-component signaling (TCS) system comprised of sensor histidine kinases (AHKs), histidine phosphotransfer proteins (AHPs), and response regulators (ARRs). Recent studies have suggested that the cytokinin TCS system is involved in a variety of other signaling and metabolic pathways. To further explore a potential function of the cytokinin TCS in the Arabidopsis dehydration stress response, we investigated the expression of all type-A ARR genes and a type-C ARR, ARR22, in both wild type and ahk single, double, and triple mutants in response to dehydration compared to cytokinin as well as dehydration tolerance of ahk mutants. We found that drought significantly induced the expression of a subset of ARR genes, ARR5, ARR7, ARR15, and ARR22. The results of expression analyses in ahk single, double, and triple mutants demonstrated that the cytokinin receptors AHK2 and AHK3 are redundantly involved in dehydration-inducible expression of ARR7, but not that of ARR5, ARR15, or ARR22. Dehydration tolerance assays showed that ahk2 and ahk3 single mutants exhibited enhanced dehydration tolerance compared with that of wild-type plants and ahk4 mutants, and that ahk2 ahk3 double mutants exhibited stronger drought tolerance than that of ahk3 ahk4, which exhibited more enhanced drought tolerance than that of wild-type plants and ahk single mutants. Taken together, these results demonstrate that while the cytokinin receptors AHK2 and AHK3 are critically involved in the dehydration tolerance response, both cytokinin receptor-dependent pathway and receptor-independent pathway occur in the dehydration response regulating ARR gene expression. In addition, preincubating ahk2, ahk3, ahk4, and the wild-type plants with cytokinin induced enhanced dehydration stress tolerance in these plants, demonstrating that cytokinins are involved in regulating plant response to dehydration stress.     

The cytokinin receptors of Arabidopsis are located mainly to the endoplasmic reticulum

The plant hormone cytokinin is perceived by membrane-located sensor histidine kinases. Arabidopsis (Arabidopsis thaliana) possesses three cytokinin receptors: ARABIDOPSIS HISTIDINE KINASE2 (AHK2), AHK3, and CYTOKININ RESPONSE1/AHK4. The current model predicts perception of the cytokinin signal at the plasma membrane. However, cytokinin-binding studies with membrane fractions separated by two-phase partitioning showed that in the wild type, as well as in mutants retaining only single cytokinin receptors, the major part of specific cytokinin binding was associated with endomembranes. Leaf epidermal cells of tobacco (Nicotiana benthamiana) expressing receptor-green fluorescent protein fusion proteins and bimolecular fluorescence complementation analysis showed strong fluorescence of the endoplasmic reticulum (ER) network for all three receptors. Furthermore, separation of the microsomal fraction of Arabidopsis plants expressing Myc-tagged AHK2 and AHK3 receptors by sucrose gradient centrifugation followed by immunoblotting displayed the Mg2?-dependent density shift typical of ER membrane proteins. Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. A modified model for cytokinin signaling is proposed.     

The AHK4 gene involved in the cytokinin-signaling pathway as a direct receptor molecule in Arabidopsis thaliana

We previously identified a set of structurally related genes, AHK2, 3 and 4, each encoding a sensor histidine kinase in Arabidopsis thaliana. To determine the relevant biological functions, we identified a loss-of-function mutation of the AHK4 gene. The mutant exhibited the cytokinin-resistant phenotype not only in inhibition of root growth by cytokinin but also in greening and shoot induction of calli. Moreover, AHK4 expressed in budding yeast showed histidine kinase activity in a manner dependent on the presence of cytokinin. These results strongly suggested that AHK4 is involved in the cytokinin-signaling pathway, as a direct receptor molecule, in Arabidopsis.     

Topolins and hydroxylated thidiazuron derivatives are substrates of cytokinin O-glucosyltransferase with position specificity related to receptor recognition

Glucosides of trans-zeatin occur widely in plant tissues, formed either by O-glucosylation of the hydroxylated side chain or N-glucosylation of the purine ring structure. O-Glucosylation is stereo-specific: the O-glucosyltransferase encoded by the Phaseolus lunatus ZOG1 gene has high affinity for trans-zeatin as the substrate, whereas the enzyme encoded by the maize (Zea mays) cisZOG1 gene prefers cis-zeatin. Here we show that hydroxylated derivatives of benzyladenine (topolins) are also substrates of ZOG1 and cisZOG1. The m-OH and o-OH derivatives are the preferred substrate of ZOG1 and cisZOG1, respectively. Among the hydroxylated derivatives of thidiazuron tested, the only enzyme/substrate combination resulting in conversion was cisZOG1/(o-OH) thidiazuron. The abilities of these cytokinins to serve as substrates to the glucosyltransferases were in a large part correlated with their biological activities in the P. lunatus callus bioassay, indicating that there may be similarities between cytokinin-binding sites on the enzymes and cytokinin receptors. Further support for this interpretation is provided by cytokinin recognition studies involving the Arabidopsis (Arabidopsis thaliana) CRE1/WOL/AHK4 and maize ZmHK1 receptors. The AHK4 receptor responded to trans-zeatin and m-topolin, while the ZmHK1 receptor responded also to cis-zeatin and o-topolin. Three-dimensional molecular models of the substrates were applied to explain the results.     

E. coli-Based Cell-Free Expression, Purification and Characterization of the Membrane-Bound Ligand-Binding CHASE-TM Domain of the Cytokinin Receptor CRE1/AHK4 of Arabidopsis thaliana

The plant hormone cytokinin is implicated in a large number of developmental and physiological processes. In the model plant Arabidopsis thaliana cytokinin is perceived by a class of membrane-bound receptor histidine kinases with three members, namely AHK2, AHK3, and CRE1/AHK4. These receptors possess an N-terminally located putative extracellular cyclases/histidine kinases associated sensor extracellular (CHASE) domain, which is responsible for hormone recognition. This hydrophilic domain and the two flanking transmembrane regions (CHASE-TM) were expressed using a cell-free protein expression system based on a bacterial ribosomal extract. To obtain soluble CHASE-TM protein, different detergents were directly added to the cell-free reaction and their effect on the yield of soluble protein was studied. After optimising the experimental set-up and employing Brij 58 as a detergent more than 3 mg/ml soluble protein of the CHASE-TM domain were obtained. Affinity purification via a C-terminally fused His-tag resulted in greater than 90% purity. The identity of the purified domain was confirmed by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) analysis. Circular dichroism spectroscopy was used and a predominantly α-helical folding pattern was shown, which is in good accordance with secondary structure prediction. A newly developed cytokinin binding assay confirmed the functionality of the thus expressed and purified CHASE-TM domain. The work presented clearly demonstrates the feasibility of producing high amounts of a plant membrane protein using a cell-free protein expression system. This opens the possibility of further biochemical and pharmacological analysis, as well as structural studies on this type of receptor protein.     

Preparation and biological activity of 6-benzylaminopurine derivatives in plants and human cancer cells

To study the structure-activity relationships of aromatic cytokinins, the cytokinin activity at both the receptor and cellular levels, as well as CDK inhibitory and anticancer properties of 38 6-benzylaminopurine (BAP) derivatives were compared in various in vitro assays. The compounds were prepared by the condensation of 6-chloropurine with corresponding substituted benzylamines. The majority of synthesised derivatives exhibited high activity in all three of the cytokinin bioassays employed (tobacco callus, wheat senescence and Amaranthus bioassay). The highest activities were obtained in the senescence bioassay. For some compounds tested, significant differences of activity were found in the bioassays used, indicating that diverse recognition systems may operate and suggesting that it may be possible to modulate particular cytokinin-dependent processes with specific compounds. Position-specific steric and hydrophobic effects of different phenyl ring substituents on the variation of biological activity were confirmed. In contrast to their high activity in bioassays, the BAP derivatives were recognised with much lower sensitivity than trans-zeatin in both Arabidopsis thaliana AHK3 and AHK4 receptor assays. The compounds were also investigated for their effects on cyclin-dependent kinase 2 (CDK2) and for antiproliferative properties on cancer and normal cell lines. Several of the tested compounds showed stronger inhibitory activity and cytotoxicity than BAP. There was also a significant positive correlation of the inhibitory effects on human and plant CDKs with cell proliferation of cancer and cytokinin-dependent tobacco cells, respectively. This suggests that at least a part of the antiproliferative effect of the new cytokinins was due to the inhibition of CDK activity.     

Arabidopsis cytokinin receptor mutants reveal functions in shoot growth, leaf senescence, seed size, germination, root development, and cytokinin metabolism

We used loss-of-function mutants to study three Arabidopsis thaliana sensor histidine kinases, AHK2, AHK3, and CRE1/AHK4, known to be cytokinin receptors. Mutant seeds had more rapid germination, reduced requirement for light, and decreased far-red light sensitivity, unraveling cytokinin functions in seed germination control. Triple mutant seeds were more than twice as large as wild-type seeds. Genetic analysis indicated a cytokinin-dependent endospermal and/or maternal control of embryo size. Unchanged red light sensitivity of mutant hypocotyl elongation suggests that previously reported modulation of red light signaling by A-type response regulators may not depend on cytokinin. Combined loss of AHK2 and AHK3 led to the most prominent changes during vegetative development. Leaves of ahk2 ahk3 mutants formed fewer cells, had reduced chlorophyll content, and lacked the cytokinin-dependent inhibition of dark-induced chlorophyll loss, indicating a prominent role of AHK2 and, particularly, AHK3 in the control of leaf development. ahk2 ahk3 double mutants developed a strongly enhanced root system through faster growth of the primary root and, more importantly, increased branching. This result supports a negative regulatory role for cytokinin in root growth regulation. Increased cytokinin content of receptor mutants indicates a homeostatic control of steady state cytokinin levels through signaling. Together, the analyses reveal partially redundant functions of the cytokinin receptors and prominent roles for the AHK2/AHK3 receptor combination in quantitative control of organ growth in plants, with opposite regulatory functions in roots and shoots.     

The specificity of cytokinin signalling in Arabidopsis thaliana is mediated by differing ligand affinities and expression profiles of the receptors

Arabidopsis thaliana has three membrane‐located cytokinin receptors (AHK2, AHK3 and CRE1/AHK4), which are sensor histidine kinases containing a ligand‐binding CHASE domain. Despite their structural similarity the role of these receptors differs in planta. Here we have explored which parameters contribute to signal specification. In a bacterial assay, the CHASE domain of AHK2 has a similar ligand binding spectrum as CRE1/AHK4. It shows the highest affinity for isopentenyladenine (iP) and trans‐zeatin (tZ) with an apparent K_D of 1.4 and 4.0 nm , respectively. Real‐time PCR analysis of cytokinin primary response genes in double mutants retaining only single receptors revealed that all receptors are activated in planta by cytokinin concentrations in the low nanomolar range. However, there are differences in sensitivity towards the principal cytokinins iP and tZ. The activation of the cytokinin‐sensitive P_ARR5:GUS reporter gene in three different double mutants shows specific, but also overlapping, spatial domains of activity, which were for all receptors predominantly in the shoot apical meristems and root cap columella. AHK2 and AHK3 signal specifically in leaf parenchyma cells, AHK3 in stomata cells, and CRE1/AHK4 in the root vasculature. Promoter‐swap experiments demonstrate that CRE1/AHK4 can functionally replace AHK2 but not AHK3. However, the cytoplasmic AHK3 histidine kinase (Hk) domain can be replaced by the CRE1/AHK4 Hk domain, which suggests that functionality is mediated in this case by the extracytosolic domain. Together, the data show that both differential gene expression and ligand preference contribute to specify the receptor activity.     

Classical anticytokinins do not interact with cytokinin receptors but inhibit cyclin-dependent kinases

Cytokinins are a class of plant hormones that regulate the cell cycle and diverse developmental and physiological processes. Several compounds have been identified that antagonize the effects of cytokinins. Based on structural similarities and competitive inhibition, it has been assumed that these anticytokinins act through a common cellular target, namely the cytokinin receptor. Here, we examined directly the possibility that various representative classical anticytokinins inhibit the Arabidopsis cytokinin receptors CRE1/AHK4 (cytokinin response 1/Arabidopsis histidine kinase 4) and AHK3 (Arabidopsis histidine kinase 3). We show that pyrrolo[2,3-d]pyrimidine and pyrazolo[4,3-d]pyrimidine anticytokinins do not act as competitors of cytokinins at the receptor level. Flow cytometry and microscopic analyses revealed that anticytokinins inhibit the cell cycle and cause disorganization of the microtubular cytoskeleton and apoptosis. This is consistent with the hypothesis that they inhibit regulatory cyclin-dependent kinase (CDK) enzymes. Biochemical studies demonstrated inhibition by selected anti-cytokinins of both Arabidopsis and human CDKs. X-ray determination of the crystal structure of a human CDK2-anticytokinin complex demonstrated that the antagonist occupies the ATP-binding site of CDK2. Finally, treatment of human cancer cell lines with anticytokinins demonstrated their ability to kill human cells with similar effectiveness as known CDK inhibitors.     

Pseudo-Response Regulators (PRRs) or True Oscillator Components (TOCs)

In Arabidopsis thaliana, AUTHENTIC RESPONSE REGULATORS (ARRs) act as downstream components of the His-to-Asp phosphorelay (two-component) signaling pathway that is propagated primarily by the cytokinin receptor kinases, AUTHENTIC HIS-KINASES (AHK2, AHK3 and AHK4/CRE1). Thus, this bacterial type of signaling system is essential for responses to a class of hormones in plants. Interestingly, this higher plant has also evolved its own atypical (or unique) variants of two-component signal transducers, PSEUDO-RESPONSE REGULATORS (PRRs). Several lines of recent results suggest that the functions of PRRs are closely relevant to the plant clock (oscillator) that is central to circadian rhythms, the underlying mechanisms of which have long been the subject of debate. Through an overview of recent results, the main issue addressed here is whether or not the pseudo-response regulators (PRRs) are true oscillator components (TOCs).     

Structural basis for cytokinin recognition by Arabidopsis thaliana histidine kinase 4

Cytokinins are classic hormones that orchestrate plant growth and development and the integrity of stem cell populations. Cytokinin receptors are eukaryotic sensor histidine kinases that are activated by both naturally occurring adenine-type cytokinins and urea-based synthetic compounds. Crystal structures of the Arabidopsis thaliana histidine kinase 4 sensor domain in complex with different cytokinin ligands now rationalize the hormone-binding specificity of the receptor and may spur the design of new cytokinin ligands.     

The Arabidopsis sensor His-kinase, AHk4, can respond to cytokinins

His-to-Asp (His-->Asp) phosphorelay mechanisms are presumably involved in propagation of certain environmental stimuli, including phytohormones, in Arabidopsis thaliana. In addition to the previously characterized His-kinases, namely, the ETR1 family of ethylene receptors, CKI1 cytokinin-sensor, and ATHK1 osomo-sensor, this higher plant has three more His-kinases (named AHK2, AHK3, and AHK4). By employing the well-known His-->Asp phosphorelay systems in both the fission yeast and Escherichia coli, evidence is presented showing that the AHK4 His-kinase has an ability to serve as a cytokinin-responsive environmental sensor. Taking advantage of this AHK4-dependent His-->Asp phosphorelay system in E. coli, a phosphorelay interaction between the Arabidopsis His-kinase and histidine-containing phosphotransmitters (AHPs) was also demonstrated for the first time.