Molecular characterization of His-Asp phosphorelay signaling factors in maize leaves: implications of the signal divergence by cytokinin-inducible response regulators in the cytosol and the nuclei

Genome-wide identification and expression profiling of two-component system (TCS) genes in Brassica oleracea in response to shade stress

The Two-component system (TCS) consists of Histidine kinases (HKs), Phosphotransfers (HPs), and response regulator (RR) proteins. It has an important role in signal transduction to respond to a wide variety of abiotic stresses and hence in plant development. Brassica oleracea (cabbage) is a leafy vegetable, which is used for food and medicinal purposes. Although this system was identified in several plants, it had not been identified in Brassica oleracea yet. This genome-wide study identified 80 BoTCS genes consisting of 21 HKs, 8 HPs, 39 RRs, and 12 PRRs. This classification was done based on conserved domains and motif structure. Phylogenetic relationships of BoTCS genes with Arabidopsis thaliana, Oryza sativa, Glycine max, and Cicer arietinum showed conservation in TCS genes. Gene structure analysis revealed that each subfamily had conserved introns and exons. Both tandem and segmental duplication led to the expansion of this gene family. Almost all of the HPs and RRs were expanded through segmental duplication. Chromosomal analysis showed that BoTCS genes were dispersed across all nine chromosomes. The promoter regions of these genes were found to contain a variety of cis-regulatory elements. The 3D structure prediction of proteins also confirmed the conservation of structure within subfamilies. MicroRNAs (miRNAs) involved in the regulation of BoTCSs were also predicted and their regulatory roles were also evaluated. Moreover, BoTCSs were docked with abscisic acid to evaluate their binding. RNA-seq-based expression analysis and validation by qRT-PCR showed significant variation of expression for BoPHYs, BoERS1.1, BoERS2.1, BoERS2.2, BoRR10.2, and BoRR7.1 suggesting their importance in stress response. These genes showing unique expression can be further used in manipulating the plant's genome to make the plant more resistant the environmental stresses which will ultimately help in the increase of plant's yield. More specifically, these genes have altered expression in shade stress which clearly indicates their importance in biological functions. These findings are important for future functional characterization of TCS genes in generating stress-responsive cultivars.

Cytokinin signaling promotes salt tolerance by modulating shoot chloride exclusion in maize

Excessive accumulation of chloride (Cl^-) in the aboveground tissues under saline conditions is harmful to crops. Increasing the exclusion of Cl^- from shoots promotes salt tolerance in various crops. However, the underlying molecular mechanisms remain largely unknown. In this study, we demonstrated that a type A response regulator (ZmRR1) modulates Cl^- exclusion from shoots and underlies natural variation of salt tolerance in maize. ZmRR1 negatively regulates cytokinin signaling and salt tolerance, likely by interacting with and inhibiting His phosphotransfer (HP) proteins that are key mediators of cytokinin signaling. A naturally occurring non-synonymous SNP variant enhances the interaction between ZmRR1 and ZmHP2, conferring maize plants with a salt-hypersensitive phenotype. We found that ZmRR1 undergoes degradation under saline conditions, leading to the release of ZmHP2 from ZmRR1 inhibition, and subsequently ZmHP2-mediated signaling improves salt tolerance primarily by promoting Cl^- exclusion from shoots. Furthermore, we showed that ZmMATE29 is transcriptionally upregulated by ZmHP2-mediated signaling under highly saline conditions and encodes a tonoplast-located Cl^- transporter that promotes Cl^- exclusion from shoots by compartmentalizing Cl^- into the vacuoles of root cortex cells. Collectively, our study provides an important mechanistic understanding of the cytokinin signaling-mediated promotion of Cl^- exclusion from shoots and salt tolerance and suggests that genetic modification to promote Cl^- exclusion from shoots is a promising route for developing salt-tolerant maize.

Genome-wide identification and expression analysis of the TaRRA gene family in wheat (Triticum aestivum L.)

Cytokinin is an important endogenous hormone in plants performing a wide spectrum of biological roles. The type-A response regulators (RRAs) are primary cytokinin response genes, which are important components of the cytokinin signaling pathway and are involved in the regulation of plant growth and development. By analysis of the whole genome sequence of wheat, we identified 20 genes encoding RRAs which were clustered into eight homologous groups. The gene structure, conserved motifs, chromosomal location, and cis-acting regulatory elements of the TaRRAs were analyzed. Quantitative real-time polymerase chain reaction (qRT-PCR) results showed that the expression levels of most of the TaRRAs increased rapidly on exogenous cytokinin application. Moreover, the TaRRA family members displayed different expression profiles under the stress treatments of drought, salt, cold, and heat. This study provides valuable insights into the RRA gene family in wheat and promotes the potential application of these genes in wheat genetic improvement.

Cytokinin increases vegetative growth period by suppressing florigen expression in rice and maize

Increasing the vegetative growth period of crops can increase biomass and grain yield. In rice (Oryza sativa), the concentration of trans -zeatin, an active cytokinin, was high in the leaves during vegetative growth and decreased rapidly upon induction of florigen expression, suggesting that this hormone is involved in the regulation of the vegetative phase. To elucidate whether exogenous cytokinin application influences the length of the vegetative phase, we applied 6-benzylaminopurine (BAP) to rice plants at various developmental stages. Our treatment delayed flowering time by 8-9 days when compared with mock-treated rice plants, but only at the transition stage when the flowering signals were produced. Our observations also showed that flowering in the paddy field is delayed by thidiazuron, a stable chemical that mimics the effects of cytokinin. The transcript levels of florigen genes Heading date 3a (Hd3a) and Rice Flowering locus T1 (RFT1) were significantly reduced by the treatment, but the expression of Early heading date 1 (Ehd1), a gene found directly upstream of the florigen genes, was not altered. In maize (Zea mays), similarly, BAP treatment increased the vegetative phage by inhibiting the expression of ZCN8, an ortholog of Hd3a. We showed that cytokinin treatment induced the expression of two type-A response regulators (OsRR1 and OsRR2) which interacted with Ehd1, a type-B response regulator. We also observed that cytokinin did not affect flowering time in ehd1 knockout mutants. Our study indicates that cytokinin application increases the duration of the vegetative phase by delaying the expression of florigen genes in rice and maize by inhibiting Ehd1.

Plant Phosphoproteomics: Known Knowns, Known Unknowns, and Unknown Unknowns of an Emerging Systems Science Frontier

Plant systems science research depends on the dynamic functional maps of the biological substrates of plant phenotypes and host/environment interactions in diverse ecologies. In this context, high-resolution mass spectrometry platforms offer comprehensive insights into the molecular pathways regulated by protein phosphorylation. Reversible protein phosphorylation is a ubiquitous reaction in signal transduction mechanisms in biological systems. In contrast to human and animal biology research, a plethora of experimental options for functional mapping and regulation of plant biology are, however, not currently available. Plant phosphoproteomics is an emerging field of research that aims at addressing this gap in systems science and plant omics, and thus has a large scope to empower fundamental discoveries. To date, large-scale data-intensive identification of phosphorylation events in plants remained technically challenging. In this expert review, we present a critical analysis and overview of phosphoproteomic studies performed in the model plant Arabidopsis thaliana. We discuss the technical strategies used for the enrichment of phosphopeptides and methods used for their quantitative assessment. Various types of mass spectrometry data acquisition and fragmentation methods are also discussed. The insights gathered here can allow plant biology and systems science researchers to design high-throughput function-oriented experimental workflows that elucidate the regulatory signaling mechanisms impacting plant physiology and plant diseases.

Natural variation in a type-A response regulator confers maize chilling tolerance

Maize (Zea mays L.) is a cold-sensitive species that often faces chilling stress, which adversely affects growth and reproduction. However, the genetic basis of low-temperature adaptation in maize remains unclear. Here, we demonstrate that natural variation in the type-A Response Regulator 1 (ZmRR1) gene leads to differences in chilling tolerance among maize inbred lines. Association analysis reveals that InDel-35 of ZmRR1, encoding a protein harboring a mitogen-activated protein kinase (MPK) phosphorylation residue, is strongly associated with chilling tolerance. ZmMPK8, a negative regulator of chilling tolerance, interacts with and phosphorylates ZmRR1 at Ser15. The deletion of a 45-bp region of ZmRR1 harboring Ser15 inhibits its degradation via the 26 S proteasome pathway by preventing its phosphorylation by ZmMPK8. Transcriptome analysis indicates that ZmRR1 positively regulates the expression of ZmDREB1 and Cellulose synthase (CesA) genes to enhance chilling tolerance. Our findings thus provide a potential genetic resource for improving chilling tolerance in maize.

The genetic basis of low-temperature tolerance in maize is unclear. Here, the authors show that the type-A Response Regulator 1 (ZmRR1) and mitogen-activated protein kinase (MPK8) are positive and negative regulators of maize chilling tolerance, and ZmRR1 is phosphorylated by ZmMPK8 during cold treatment.

Root engineering in maize by increasing cytokinin degradation causes enhanced root growth and leaf mineral enrichment

Root-specific expression of a cytokinin-degrading CKX gene in maize roots causes formation of a larger root system leading to higher element content in shoot organs. The size and architecture of the root system is functionally relevant for the access to water and soil nutrients. A great number of mostly unknown genes are involved in regulating root architecture complicating targeted breeding of plants with a larger root system. Here, we have explored whether root-specific degradation of the hormone cytokinin, which is a negative regulator of root growth, can be used to genetically engineer maize (Zea mays L.) plants with a larger root system. Root-specific expression of a CYTOKININ OXIDASE/DEHYDROGENASE (CKX) gene of Arabidopsis caused the formation of up to 46% more root dry weight while shoot growth of these transgenic lines was similar as in non-transgenic control plants. The concentration of several elements, in particular of those with low soil mobility (K, P, Mo, Zn), was increased in leaves of transgenic lines. In kernels, the changes in concentration of most elements were less pronounced, but the concentrations of Cu, Mn and Zn were significantly increased in at least one of the three independent lines. Our data illustrate the potential of an increased root system as part of efforts towards achieving biofortification. Taken together, this work has shown that root-specific expression of a CKX gene can be used to engineer the root system of maize and alter shoot element composition.

OsRR6, a type-A response regulator in rice, mediates cytokinin, light and stress responses when over-expressed in Arabidopsis

Plants have evolved a complex network of components that sense and respond to diverse signals. In the present study, we have characterized OsRR6, a type-A response regulator, which is part of the two-component sensor-regulator machinery in rice. The expression of OsRR6 is induced by exogenous cytokinin and various abiotic stress treatments, including drought, cold and salinity stress. Organ-specific expression analysis revealed that its expression is high in anther and low in shoot apical meristem. The Arabidopsis plants constitutively expressing OsRR6 (OsRR6^OX) exhibited reduced cytokinin sensitivity, adventitious root formation and enhanced anthocyanin accumulation in seeds. OsRR6^OX plants were more tolerant to drought and salinity conditions when compared to wild-type. The hypocotyl growth in OsRR6^OX seedlings was significantly inhibited under red, far-red and blue-light conditions and also a decline in transcript levels of OsRR6 was observed in rice under the above monochromatic as well as white light treatments. Transcriptome profiling revealed that the genes associated with defense responses and anthocyanin metabolism are up-regulated in OsRR6^OX seedlings. Comparative transcriptome analysis showed that the genes associated with phenylpropanoid and triterpenoid biosynthesis are enriched among differentially expressed genes in OsRR6^OX seedlings of Arabidopsis, which is in conformity with reanalysis of the transcriptome data performed in rice transgenics for OsRR6. Further, genes like DREB1A/CBF3, COR15A, KIN1, ERD10 and RD29A are significantly upregulated in OsRR6^OX seedlings when subjected to ABA and abiotic stress treatments. Thus, a negative regulator of cytokinin signaling, OsRR6, plays a positive role in imparting abiotic stress tolerance.

Identification of AHK2- and AHK3-like cytokinin receptors in Brassica napus reveals two subfamilies of AHK2 orthologues

Cytokinin (CK) signalling is known to play key roles in the regulation of plant growth and development, crop yields, and tolerance to both abiotic stress and pathogen defences, but the mechanisms involved are poorly characterized in dicotyledonous crops. Here the identification and functional characterization of sensor histidine kinases homologous to Arabidopsis CK receptors AHK2 and AHK3 in winter oilseed rape are presented. Five CHASE-containing His kinases were identified in Brassica napus var. Tapidor (BnCHK1-BnCHK5) by heterologous hybridization of its genomic library with gene-specific probes from Arabidopsis. The identified bacterial artificial chromosome (BAC) clones were fingerprinted and representative clones in five distinct groups were sequenced. Using a bioinformatic approach and cDNA cloning, the precise gene and putative protein domain structures were determined. Based on phylogenetic analysis, four AHK2 (BnCHK1-BnCHK4) homologues and one AHK3 (BnCHK5) homologue were defined. It is further suggested that BnCHK1 and BnCHK3, and BnCHK5 are orthologues of AHK2 and AHK3, originally from the B. rapa A genome, respectively. BnCHK1, BnCHK3, and BnCHK5 displayed high affinity for trans-zeatin (1-3nM) in a live-cell competitive receptor assay, but not with other plant hormones (indole acetic acid, GA3, and abscisic acid), confirming the prediction that they are genuine CK receptors. It is shown that BnCHK1 and BnCHK3, and BnCHK5 display distinct preferences for various CK bases and metabolites, characteristic of their AHK counterparts, AHK2 and AHK3, respectively. Interestingly, the AHK2 homologues could be divided into two subfamilies (BnCHK1/BnCK2 and BnCHK3/BnCHK4) that differ in putative transmembrane domain topology and CK binding specificity, thus implying potential functional divergence.

Antibodies against CKI1RD, a receiver domain of the sensor histidine kinase in Arabidopsis thaliana: from antigen preparation to in planta immunolocalization

Immunodetection is a powerful tool in functional studies of all organisms. In plants, the gene redundancy and presence of gene families composed of highly homologous members often impedes the unambiguous identification of individual gene products. A family of eight sensor histidine kinases (HKs) mediates the transduction of diverse signals into Arabidopsis thaliana cells, thereby ensuring the initiation of appropriate adaptive responses. Antibodies recognizing specific members of the HK family would be valuable for studying their functions in Arabidopsis and other plant species including important crops. We have focused on developing and applying antibodies against CYTOKININ-INDEPENDENT 1 (CKI1), which encodes a constitutively active membrane-bound sensor HK that regulates the development of female gametophytes and vascular tissue in Arabidopsis. A coding sequence delimiting the C-terminal receiver domain of CKI1 (CKI1(RD)) was expressed in Escherichia coli using the IPTG-inducible expression system and purified to give a highly pure target protein. The purified CKI1(RD) protein was then used as an antigen for anti-CKI1(RD) antibody production. The resulting polyclonal antibodies had a detection limit of 10 ng of target protein at 1:20,000 dilution and were able to specifically distinguish CKI1, both in vitro and in situ, even in a direct comparison with highly homologous members of the same HK family AHK4, CKI2 and ETR1. Finally, anti-CKI1(RD) antibodies were able to selectively bind CKI1-GFP fusion protein in a pull-down assay using crude lysate from an Arabidopsis cell suspension culture. Our results suggest that the receiver domain is a useful target for the functional characterization of sensor HKs in immunological and biochemical studies.

Updates on the model and the evolution of cytokinin signaling

Cytokinins represent a class of phytohormones, which are key players not only in many processes important for plant growth and development, but also in the response to changes in their environment. The model for the cytokinin signaling pathway was established at the turn of the last century and many experiments confirmed its validity. In recent years several changes and extensions to the model were necessary to accommodate new findings concerning its components, such as subcellular localization, selective protein degradation and new modes of cross talk. In addition phylogenetic analyses of components of the cytokinin circuitry started to reveal the origin and evolution of the cytokinin regulatory system.

Characterization of histidine-aspartate kinase HK1 and identification of histidine phosphotransfer proteins as potential partners in a Populus multistep phosphorelay

In poplar, we identified proteins homologous to yeast proteins involved in osmosensing multistep phosphorelay Sln1p-Ypd1p-Ssk1p. This finding led us to speculate that Populus cells could sense osmotic stress by a similar mechanism. This study focuses on first and second protagonists of this possible pathway: a histidine-aspartate kinase (HK1), putative osmosensor and histidine phosphotransfer proteins (HPt1 to 10), potential partners of this HK. Characterization of HK1 showed its ability to homodimerize in two-hybrid tests and to act as an osmosensor with a kinase activity in yeast, by functional complementation of sln1Δ sho1Δ strain. Moreover, in plant cells, plasma membrane localization of HK1 is shown. Further analysis on HPts allowed us to isolate seven new cDNAs, leading to a total of 10 different HPts identified in poplar. Interaction tests showed that almost all HPts can interact with HK1, but two of them exhibit stronger interactions, suggesting a preferential partnership in poplar. The importance of the phosphorylation status in these interactions has been investigated with two-hybrid tests carried out with mutated HK1 forms. Finally, in planta co-expression analysis of genes encoding these potential partners revealed that only three HPts are co-expressed with HK1 in different poplar organs. This result reinforces the hypothesis of a partnership between HK1 and these three preferential HPts in planta. Taken together, these results shed some light on proteins partnerships that could be involved in the osmosensing pathway in Populus.

Cytokinins résumé: their signaling and role in programmed cell death in plants

Cytokinins (CKs) are a large group of plant hormones which play a crucial role in many physiological processes in plants. One of the interesting functions of CKs is the control of programmed cell death (PCD). It seems that all CKs-dependent phenomena including PCD are accompanied by special multi-step phosphorelay signaling pathway. This pathway consists of three elements: histidine kinase receptors (HKs), histidine phosphotransfer proteins (HPs) and response regulators (RRs). This review shows the résumé of the latest knowledge about CKs signaling pathways in many physiological processes in plants with special attention paid to PCD process.

Down-stream components of cytokinin signaling and the role of cytokinin throughout the plant

Cytokinins constitute a class of plant hormones influencing numerous aspects of growth and development. These processes occur through the downstream components of the cytokinin signaling pathway after its perception and signal transduction. The importance of these downstream signaling components has been revealed through the use of both traditional genetic and advanced molecular approaches studying mutants and transgenic lines involving cytokinin and diverse plant growth and developmental processes. Interestingly, these effects are not always directly via cytokinin, but by interactions with other plants hormones or transcription factor cascades, which can involve regulatory loops that affect transcription as well as hormone concentrations. This review covers recent advancements in understanding the role of cytokinin via its signaling components, specifically the downstream responses regulators in controlling vital plant growth and developmental processes.

Ligand-binding properties and subcellular localization of maize cytokinin receptors

The ligand-binding properties of the maize (Zea mays L.) cytokinin receptors ZmHK1, ZmHK2, and ZmHK3a have been characterized using cytokinin binding assays with living cells or membrane fractions. According to affinity measurements, ZmHK1 preferred N(6)-(Δ(2)-isopentenyl)adenine (iP) and had nearly equal affinities to trans-zeatin (tZ) and cis-zeatin (cZ). ZmHK2 preferred tZ and iP to cZ, while ZmHK3a preferred iP. Only ZmHK2 had a high affinity to dihydrozeatin (DZ). Analysis of subcellular fractions from leaves and roots of maize seedlings revealed specific binding of tZ in the microsome fraction but not in chloroplasts or mitochondria. In competitive binding assays with microsomes, tZ and iP were potent competitors of [(3)H]tZ while cZ demonstrated significantly lower affinity; adenine was almost ineffective. The binding specificities of microsomes from leaf and root cells for cytokinins were consistent with the expression pattern of the ZmHKs and our results on individual receptor properties. Aqueous two-phase partitioning and sucrose density-gradient centrifugation followed by immunological detection with monoclonal antibody showed that ZmHK1 was associated with the endoplasmic reticulum (ER). This was corroborated by observations of the subcellular localization of ZmHK1 fusions with green fluorescent protein in maize protoplasts. All these data strongly suggest that at least a part of cytokinin perception occurs in the ER.

The more, the merrier: cytokinin signaling beyond Arabidopsis

The phytohormone cytokinin is a key player in many developmental processes and in the response of plants to biotic and abiotic stress. The cytokinin signal is perceived and transduced via a multistep variant of the bacterial two-component signaling system. Most of the research on cytokinin signaling has been done in the model plant Arabidopsis thaliana. Research on cytokinin signaling has expanded to a much broader range of plants species in recent years. This is due to the natural limitation of Arabidopsis as a model species for the investigation of processes like nodulation or wood formation. The rapidly increasing number of sequenced plant genomes also facilitates the use of other species in this line of research. This review summarizes what is known about the cytokinin signaling in the different organisms and highlights differences to Arabidopsis.

Atypical response regulators expressed in the maize endosperm transfer cells link canonical two component systems and seed biology

BACKGROUND

Two component systems (TCS) are phosphotransfer-based signal transduction pathways first discovered in bacteria, where they perform most of the sensing tasks. They present a highly modular structure, comprising a receptor with histidine kinase activity and a response regulator which regulates gene expression or interacts with other cell components. A more complex framework is usually found in plants and fungi, in which a third component transfers the phosphate group from the receptor to the response regulator. They play a central role in cytokinin mediated functions in plants, affecting processes such as meristem growth, phyllotaxy, seed development, leaf senescence or tissue differentiation. We have previously reported the expression and cellular localization of a type A response regulator, ZmTCRR-1, in the transfer cells of the maize seed, a tissue critical for seed filling and development, and described its regulation by a tissue specific transcription factor. In this work we investigate the expression and localization of other components of the TCS signalling routes in the maize seed and initiate the characterization of their interactions.

RESULTS

The discovery of a new type A response regulator, ZmTCRR-2, specifically expressed in the transfer cells and controlled by a tissue specific transcription factor suggests a previously unknown role for TCS in the biology of transfer cells. We have characterized other canonical TCS molecules, including 6 histidine kinases and 3 phosphotransfer proteins, potentially involved in the atypical transduction pathway defined by ZmTCRR-1 and 2. We have identified potential upstream interactors for both proteins and shown that they both move into the developing endosperm. Furthermore, ZmTCRR-1 expression in an heterologous system (Arabidopsis thaliana) is directed to xylem parenchyma cells, probably involved in transport processes, one of the major roles attributed to the transfer cell layer.

CONCLUSIONS

Our data prove the expression of the effector elements of a TCS route operating in the transfer cells under developmental control. Its possible role in integrating external signals with seed developmental processes is discussed.

Overexpression of type-A rice response regulators, OsRR3 and OsRR5, results in lower sensitivity to cytokinins

Response regulators are part of a two-component regulatory system. The type-A Arabidopsis response regulators act as negative regulators. To understand the function of type-A response regulators in the model monocot plant, rice (Japonica cultivar-group: Zhonghua11), we overexpressed two type-A OsRR genes, OsRR3 and OsRR5 (pACT1:OsRR3 and pACT1:OsRR5). We hoped to gain insight into their molecular function in cytokinin-signaling pathways. Both OsRR3 and OsRR5 overexpressors had longer roots and more lateral roots compared with Zhonghua11, when treated with exogenous cytokinin. Using callus formation and chlorophyll content assays, we found that Zhonghua11 was more sensitive to cytokinin compared with other cultivars of rice, expressing high transcript levels of OsRR3 and OsRR5. The expression of most type-A OsRR genes was repressed by OsRR3 and OsRR5 overexpression. However, semi-quantitative RT-PCR showed that three type-A OsRR genes showed increased expression. Our results suggest that both OsRR3 and OsRR5 mainly act as negative regulators of cytokinin signaling, as indicated by the reduced sensitivity of OsRR3 and OsRR5 overexpressors to exogenous cytokinins.

Unraveling the evolution of cytokinin signaling

The conquest of the land by plants required dramatic morphological and metabolic adaptations. Complex developmental programs under tight regulation evolved during this process. Key regulators of plant development are phytohormones, such as cytokinins. Cytokinins are adenine derivatives that affect various processes in plants. The cytokinin signal transduction system, which is mediated via a multistep variant of the bacterial two-component signaling system, is well characterized in the model plant Arabidopsis (Arabidopsis thaliana). To understand the origin and evolutionary pattern of this signaling pathway, we surveyed the genomes of several sequenced key plant species ranging from unicellular algae, moss, and lycophytes, to higher land plants, including Arabidopsis and rice (Oryza sativa), for proteins involved in cytokinin signal transduction. Phylogenetic analysis revealed that the hormone-binding receptor and a class of negative regulators first appeared in land plants. Other components of the signaling pathway were present in all species investigated. Furthermore, we found that the receptors evolved under different evolutionary constraints from the other components of the pathway: The number of receptors remained fairly constant, while the other protein families expanded.

Characterization of new maize genes putatively involved in cytokinin metabolism and their expression during osmotic stress in relation to cytokinin levels

Plant hormones, cytokinins (CKs), have been for a long time considered to be involved in plant responses to stress. However, their exact roles in processes linked to stress signalization and acclimatization to adverse environmental conditions are unknown. In this study, expression profiles of the entire gene families of CK biosynthetic and degradation genes in maize (Zea mays) during development and stress responses are described. Transcript abundance of particular genes is discussed in relation to the levels of different CK metabolites. Salt and osmotic stresses induce expression of some CK biosynthetic genes in seedlings of maize, leading to a moderate increase of active forms of CKs lasting several days during acclimatization to stress. A direct effect of CKs to mediate activation of stress responses does not seem to be possible due to the slow changes in metabolite levels. However, expression of genes involved in cytokinin signal transduction is uniformly down-regulated within 0.5 h of stress induction by an unknown mechanism. cis-Zeatin and its derivatives were found to be the most abundant CKs in young maize seedlings. We demonstrate that levels of this zeatin isomer are significantly enhanced during early stress response and that it originates independently from de novo biosynthesis in stressed tissues, possibly by elevated specific RNA degradation. By enhancing their CK levels, plants could perhaps undergo a reduction of growth rates maintained by abscisic acid accumulation in stressed tissues. A second role for cytokinin receptors in sensing turgor response is hypothesized besides their documented function in CK signaling.

Environmental perception avenues: the interaction of cytokinin and environmental response pathways

Cytokinins were discovered in the 1950s by their ability to promote cell division in cultured plant cells. Recently, there have been significant breakthroughs in our understanding of the biosynthesis, metabolism, perception and signal transduction of this phytohormone. These advances, coupled with physiological and other approaches, have enabled remarkable progress to be made in our understanding of the interactions between cytokinin function and environmental inputs. In this review, we first highlight the most recent advances in our understanding of cytokinin biosynthesis, metabolism and signalling. We then discuss how various environmental signals interact with these pathways to modulate plant growth, development and physiology.

Cytokinin signaling during root development

The cytokinin class of phytohormones regulates division and differentiation of plant cells. They are perceived and signaled by a phosphorelay mechanism similar to those observed in prokaryotes. Research into the components of phosphorelay had previously been marred by genetic redundancy. However, recent studies have addressed this with the creation of high-order mutants. In addition, several new elements regulating cytokinin signaling have been identified. This has uncovered many roles in diverse developmental and physiological processes. In this review, we look at these processes specifically in the context of root development. We focus on the formation and maintenance of the root apical meristem, primary and secondary vascular development, lateral root emergence and development, and root nodulation. We believe that the root is an ideal organ with which to investigate cytokinin signaling in a wider context.

Ehd2, a rice ortholog of the maize INDETERMINATE1 gene, promotes flowering by up-regulating Ehd1

Recent research into the flowering of rice (Oryza sativa) has revealed both unique and conserved genetic pathways in the photoperiodic control of flowering compared with those in Arabidopsis (Arabidopsis thaliana). We discovered an early heading date2 (ehd2) mutant that shows extremely late flowering under both short- and long-day conditions in line with a background deficient in Heading date1 (Hd1), a rice CONSTANS ortholog that belongs to the conserved pathway. This phenotype in the ehd2 mutants suggests that Ehd2 is pivotal for the floral transition in rice. Map-based cloning revealed that Ehd2 encodes a putative transcription factor with zinc finger motifs orthologous to the INDETERMINATE1 (ID1) gene, which promotes flowering in maize (Zea mays). Ehd2 mRNA in rice tissues accumulated most abundantly in developing leaves, but was present at very low levels around the shoot apex and in roots, patterns that are similar to those of ID1. To assign the position of Ehd2 within the flowering pathway of rice, we compared transcript levels of previously isolated flowering-time genes, such as Ehd1, a member of the unique pathway, Hd3a, and Rice FT-like1 (RFT1; rice florigens), between the wild-type plants and the ehd2 mutants. Severely reduced expression of these genes in ehd2 under both short- and long-day conditions suggests that Ehd2 acts as a flowering promoter mainly by up-regulating Ehd1 and by up-regulating the downstream Hd3a and RFT1 genes in the unique genetic network of photoperiodic flowering in rice.

The Maize Viviparous8 locus, encoding a putative ALTERED MERISTEM PROGRAM1-like peptidase, regulates abscisic acid accumulation and coordinates embryo and endosperm development

We describe a mutant of Zea mays isolated from a W22 inbred transposon population, widow's peak mutant1 (wpk1), with an altered pattern of anthocyanin synthesis and aleurone cell differentiation in endosperm. In addition, a failure of the developing mutant embryo to form leaf initials is associated with decreased expression of a subset of meristem regulatory genes that includes Abphyl1 and Td1. We show that the viviparous8 (vp8) mutant has a similar pleiotropic phenotype in the W22 inbred background in contrast to the viviparous embryo phenotype exhibited in the standard genetic background, and we confirmed that wpk1 is allelic to vp8. Further genetic analysis revealed that the standard vp8 stock contains an unlinked, partially dominant suppressor of the vp8 mutation that is not present in W22. Consistent with the early-onset viviparous phenotype of vp8, expression of several embryonic regulators, including LEC1/B3 domain transcription factors, was reduced in the mutant embryo. Moreover, reduced abscisic acid (ABA) content of vp8/wpk1 embryos was correlated with altered regulation of ABA biosynthesis, as well as ABA catabolic pathways. The ABA biosynthetic gene Vp14 was down-regulated in the nonsuppressed background, whereas the ZmABA8'oxA1a ABA 8'-hydroxylase gene was strongly up-regulated in both genetic backgrounds. Molecular analysis revealed that Vp8 encodes a putative peptidase closely related to Arabidopsis thaliana ALTERED MERISTEM PROGRAM1. Because the Vp8 regulates meristem development as well as seed maturation processes, including ABA accumulation, we propose that VP8 is required for synthesis of an unidentified signal that integrates meristem and embryo formation in seeds.

Cytokinin signaling: two-components and more

The hormone cytokinin is perceived via a phosphorelay that is similar to the two-component systems with which bacteria sense and respond to environmental stimuli. Characterization of two-component elements in Arabidopsis, maize and rice shows that cytokinin responses are mediated via partially redundant two-component protein families: histidine kinases, histidine phosphotransfer proteins and response regulators, as well as by novel players, such as the cytokinin response factors. Recent studies in the model plant Arabidopsis have revealed that cytokinin regulates these signaling components through a variety of mechanisms, including modulating transcription, controlling phosphorelay and regulating protein localization and stability. Genetic analyses of cytokinin signaling components have clarified the roles of cytokinin signaling in development and revealed novel functions for this 'classic' phytohormone.

Transcript profiles of the cytokinin response regulator gene family in Populus imply diverse roles in plant development

Cytokinins are plant hormones that influence diverse processes of growth and development. In this study the cytokinin response regulators (RRs) were identified, annotated and characterized at the transcript level in Populus balsamifera ssp. trichocarpa genotype Nisqually 1. The Populus genome was searched for genes that exhibit high sequence identity across their receiver domains. Gene structure was determined by prediction software and, where possible, corroborated by publicly available expressed sequence tags (ESTs). Thirty-three genes belonging to the cytokinin RR gene family were identified in Populus: 11 type As, 11 type Bs and 11 pseudo-RRs. Developmental and cytokinin-responsive expression of the Populus RRs was assessed by whole-genome microarrays and semiquantitative reverse transcription polymerase chain reaction (RT-PCR). Populus RR type As and type Bs appear to be preferentially expressed in nodes, while pseudo-RRs are preferentially expressed in mature leaves. Seven type As and three type Bs were rapidly induced by exogenous cytokinin. Organ-preferred expression patterns suggest possible roles for type As and Bs in development and for pseudo-RRs in integration of environmental signals with plant function.

Cytokinin regulates type-A Arabidopsis Response Regulator activity and protein stability via two-component phosphorelay

The plant hormone cytokinin regulates many aspects of growth and development. Cytokinin signaling involves His kinase receptors that perceive cytokinin and transmit the signal via a multistep phosphorelay similar to bacterial two-component signaling systems. The final targets of this phosphorelay are a set of Arabidopsis thaliana Response Regulator (ARR) proteins containing a receiver domain with a conserved Asp phosphorylation site. One class of these, the type-A ARRs, are negative regulators of cytokinin signaling that are rapidly transcriptionally upregulated in response to cytokinin. In this study, we tested the role of phosphorylation in type-A ARR function. Our results indicate that phosphorylation of the receiver domain is required for type-A ARR function and suggest that negative regulation of cytokinin signaling by the type-A ARRs most likely involves phosphorylation-dependent interactions. Furthermore, we show that a subset of the type-A ARR proteins are stabilized in response to cytokinin in part via phosphorylation. These studies shed light on the mechanism by which type-A ARRs act to negatively regulate cytokinin signaling and reveal a novel mechanism by which cytokinin controls type-A ARR function.

Comparison of gene expression between upland and lowland rice cultivars under water stress using cDNA microarray

To elucidate the differences in the regulation of water stress tolerance between two genotypes of rice, upland-rice (UR, resistant to water stress) and lowland-rice (LR, susceptible to water stress), we constructed subtracted cDNA libraries from polyethyleneglycol (PEG)-treated and non-treated rice seedlings (IRAT109, an upland-rice variety) by suppression subtractive hybridization (SSH), from which about 2,000 recombinant colonies were picked and amplified. Then, a cDNA microarray containing these expressed sequence tags (ESTs) was used to analyze the gene expression profiles in UR and LR in response to PEG treatment. Microarray data revealed that the majority of genes expressed in UR and LR are almost identical and Student's t test showed that 13% of all the ESTs detected in leaves and 7% of that in roots expressed differentially in transcripts abundance between the two genotypes. After sequencing, it was found that 64 and 79 unique ESTs expressed at higher levels in UR and LR, respectively. Many of the ESTs that showed higher expression in UR upon PEG treatment represented genes for transcription factors, genes playing roles in detoxification or protection against oxidative stress, and genes that help in maintaining cell turgor. In contrast, some ESTs that showed higher expression in LR were genes functioning in the degradation of cellular components. Based on data from this study and previous reports, we suggest that overexpression of some genes that expressed at higher level in UR may improve water stress tolerance in LR and other plant species.

The two-component signal system in rice (Oryza sativa L.): A genome-wide study of cytokinin signal perception and transduction

In this report we define the genes of two-component regulatory systems in rice through a comprehensive computational analysis of rice (Oryza sativa L.) genome sequence databases. Thirty-seven genes were identified, including 5 HKs (cytokinin-response histidine protein kinase) (OsHK1-4, OsHKL1), 5 HPs (histidine phosphotransfer proteins) (OsHP1-5), 15 type-A RRs (response regulators) (OsRR1-15), 7 type B RR genes (OsRR16-22), and 5 predicted pseudo-response regulators (OsPRR1-5). Protein motif organization, gene structure, phylogenetic analysis, chromosomal location, and comparative analysis between rice, maize, and Arabidopsis are described. Full-length cDNA clones of each gene were isolated from rice. Heterologous expression of each of the OsHKs in yeast mutants conferred histidine kinase function in a cytokinin-dependent manner. Nonconserved regions of individual cDNAs were used as probes in expression profiling experiments. This work provides a foundation for future functional dissection of the rice cytokinin two-component signaling pathway.

Laser microdissection of narrow sheath mutant maize uncovers novel gene expression in the shoot apical meristem

Microarrays enable comparative analyses of gene expression on a genomic scale, however these experiments frequently identify an abundance of differentially expressed genes such that it may be difficult to identify discrete functional networks that are hidden within large microarray datasets. Microarray analyses in which mutant organisms are compared to nonmutant siblings can be especially problematic when the gene of interest is expressed in relatively few cells. Here, we describe the use of laser microdissection microarray to perform transcriptional profiling of the maize shoot apical meristem (SAM), a ~100-μm pillar of organogenic cells that is required for leaf initiation. Microarray analyses compared differential gene expression within the SAM and incipient leaf primordium of nonmutant and narrow sheath mutant plants, which harbored mutations in the duplicate genes narrow sheath1 (ns1) and narrow sheath2 (ns2). Expressed in eight to ten cells within the SAM, ns1 and ns2 encode paralogous WUSCHEL1-like homeobox (WOX) transcription factors required for recruitment of leaf initials that give rise to a large lateral domain within maize leaves. The data illustrate the utility of laser microdissection-microarray analyses to identify a relatively small number of genes that are differentially expressed within the SAM. Moreover, these analyses reveal potentially conserved WOX gene functions and implicate specific hormonal and signaling pathways during early events in maize leaf development.

Unlike animals, plants exhibit a prolonged period of organogenesis, generating new leaves throughout their life cycle. This ability to maintain an embryo-like state is dependent upon the activity of shoot meristems, whose dual functions are to supply an inner core of pluripotent cells that sustain the shoot meristem while simultaneously generating new leaves derived from cells at the meristem periphery. Deciphering the complex combinations of molecular signals that transform meristematic cells into leaf primordia is a central question in plant developmental biology. In this study, we used the power of focused laser light to microdissect shoot meristems from neighboring leaf and stem tissue in the maize plant. Once isolated, we compared patterns of gene expression in normal shoot meristems to those of genetically mutant shoot meristems that form abnormal, narrow leaves. Out of more than 21,000 maize genes analyzed, 66 genes were identified as misexpressed in the mutant shoot meristems. All but one of the differentially expressed genes are previously unstudied in maize, and the majority are predicted to function during cell division, growth, or developmental signaling. Many of these novel genes are expressed in specific domains of the shoot meristem, consistent with their predicted function during maize leaf initiation.

Overexpression of a type-A response regulator alters rice morphology and cytokinin metabolism

Genome-wide analyses of rice (Oryza sativa L.) cytokinin (CK)-responsive genes using the Affymetrix GeneChip(R) rice genome array were conducted to define the spectrum of genes subject to regulation by CK in monocotyledonous plants. Application of trans-zeatin modulated the expression of a wide variety of genes including those involved in hormone signaling and metabolism, transcriptional regulation, macronutrient transport and protein synthesis. To understand further the function of CK in rice plants, we examined the effects of in planta manipulation of a putative CK signaling factor on morphology, CK metabolism and expression of CK-responsive genes. Overexpression of the CK-inducible type-A response regulator OsRR6 abolished shoot regeneration, suggesting that OsRR6 acts as a negative regulator of CK signaling. Transgenic lines overexpressing OsRR6 (OsRR6-ox) had dwarf phenotypes with poorly developed root systems and panicles. Increased content of trans-zeatin-type CKs in OsRR6-ox lines indicates that homeostatic control of CK levels is regulated by OsRR6 signaling. Expression of genes encoding CK oxidase/dehydrogenase decreased in OsRR6-ox plants, possibly accounting for elevated CK levels in transgenic lines. Expression of a number of stress response genes was also altered in OsRR6-ox plants.

Identification and characterization of cytokinin-signalling gene families in rice

We identified four histidine kinase (HK) genes of a cytokinin receptor family, two histidine-containing phosphotransmitter (HPt) genes, thirteen A-type response regulator (RR) genes and six B-type RR genes in the rice genome. The HK genes (OHK2, OHK3, OHK4 and OHK5 for Oryza sativa HK), the HPt genes (OHP1 and OHP2 for O. sativa HPt) and the B-type RR genes (ORR1, ORR2, ORR3, ORR4 and ORR6 for O. sativa RR) except one (ORR5) showed expression in various organs. ORR5 was expressed in callus and flower. Three A-type RR genes (OsRR4, OsRR9 and OsRR10 for O. sativa RR) showed cytokinin-induced expression, and three (OsRR8, OsRR12 and OsRR13) showed expression in flower. We also identified two other genes named OHK1 and CHARK (CHASE domain Receptor-like serine/threonine Kinase). OHK1 encodes an HK similar to Arabidopsis CKI1, which is involved in female gametophyte development. CHARK encodes a protein with an extracellular cytokinin-perceiving CHASE domain and a cytoplasmic serine/threonine kinase domain which are connected with a single transmembrane domain. The presence of all four gene families and CHARK in the rice genome suggests that a cytokinin signal is transduced by the phosphotransfer mechanism as is the case in Arabidopsis, and that rice may have an additional novel signalling pathway involving serine/threonine phosphorylation.

Cytokinin represses phosphate-starvation response through increasing of intracellular phosphate level

The involvement of cytokinins (CTKs) in the repression of phosphate (Pi)-starvation signalling has been widely documented. However, the full physiological and molecular relevance of this role remains unclear. To gain further insights into the regulation system of CTK repression of Pi-starvation signalling, a global analysis of gene expression events in rice seedlings under Pi starvation, and the exogenous CTK treatment under Pi-sufficient (+P) and Pi-deficient (-P) conditions, was conducted using oligonucleotide array analysis. Physiological and biochemical adaptation was observed after 10 d Pi starvation in rice seedlings. A global reduction of the Pi-starvation signalling was detected after 3 d treatment of exogenous CTK. Expression profiling data indicate that, together with a significant increase of intracellular Pi content, many expression changes responsive to Pi starvation were reversed by exogenous CTK treatment while CTK-responsive genes behaved normally under -P condition. These results suggest that the interplay of CTK signal and Pi-starvation response can be partially explained by the rise of Pi concentration after exogenous CTK treatment. Microarray data also revealed that a small number of genes have different CTK response patterns under different Pi levels, suggesting a subtle interaction between CTK and Pi-starvation signalling pathway.

The maize transfer cell-specific type-A response regulator ZmTCRR-1 appears to be involved in intercellular signalling

Response regulators are signal-transduction molecules present in bacteria, yeast and plants, acting as relays for environmental challenges. This paper reports the characterization of a Zea mays gene, ZmTCRR-1, that codes for a member of the type-A response regulator class of proteins. The gene was found to be expressed exclusively in the endosperm transfer-cell layer 8-14 days after pollination, when transfer-cell differentiation is most active. The promoter of ZmTCRR-1 was strongly transactivated in heterologous systems by the transfer cell-specific transcription factor ZmMRP-1. The ZmTCRR-1 protein was detected not only in the transfer-cell layer, but also in the conductive tissue deep inside the endosperm, where there is no transcription of the gene. This suggests that two-component systems might be involved in intercellular signal transmission, in contrast to the generally held belief that these systems are involved only in cell-autonomous pathways.

Interactions between nitrogen and cytokinin in the regulation of metabolism and development

Inorganic nitrogen is a substrate for nitrogen assimilation and also functions as a signal triggering widespread changes in gene expression that modulate metabolism and development. To integrate the actions of the nitrogen signal at the whole plant level, plants use multiple signaling routes that communicate internal and external nitrogen status. One route depends on nitrate itself and one uses cytokinin as a messenger. Recent genome-wide research has shown that the nitrate-specific signal regulates a wide variety of metabolic processes including nitrogen and carbon metabolism, and cytokinin biosynthesis. Cytokinin-mediated signaling is related to the control of development, protein synthesis and acquisition of macronutrients. The coordination and interaction of both regulatory pathways is important for normal plant growth under variable nitrogen supply conditions.

Molecular characterization and differential expression of cytokinin-responsive type-A response regulators in rice (Oryza sativa)

BACKGROUND

The response regulators represent the elements of bacterial two-component system and have been characterized from dicot plants like Arabidopsis but little information is available on the monocots, including the cereal crops. The aim of this study was to characterize type-A response regulator genes from rice, and to investigate their expression in various organs as well as in response to different hormones, including cytokinin, and environmental stimuli.

RESULTS

By analysis of the whole genome sequence of rice, we have identified ten genes encoding type-A response regulators based upon their high sequence identity within the receiver domain. The exon-intron organization, intron-phasing as well as chromosomal location of all the RT-PCR amplified rice (Oryza sativa) response regulator (OsRR) genes have been analyzed. The transcripts of OsRR genes could be detected by real-time PCR in all organs of the light- and dark-grown rice seedlings/plants, although there were quantitative differences. The steady-state transcript levels of most of the OsRR genes increased rapidly (within 15 min) on exogenous cytokinin application even in the presence of cycloheximide. Moreover, the expression of the OsRR6 gene was enhanced in rice seedlings exposed to salinity, dehydration and low temperature stress.

CONCLUSION

Ten type-A response regulator genes identified in rice, the model monocot plant, show overlapping/differential expression patterns in various organs and in response to light. The induction of OsRR genes by cytokinin even in the absence of de novo protein synthesis qualifies them to be primary cytokinin response genes. The induction of OsRR6 in response to different environmental stimuli indicates its role in cross-talk between abiotic stress and cytokinin signaling. These results provide a foundation for further investigations on specific as well as overlapping cellular functions of type-A response regulators in rice.

Crystallization and preliminary X-ray diffraction study of the histidine-containing phosphotransfer protein ZmHP1 from maize

In histidine-aspartate phosphorelays (two-component systems) involved in plant-hormone signalling, histidine-containing phosphotransfer (HPt) proteins mediate the transfer of a phosphoryl group from the sensory histidine kinase to the response regulator. The maize HPt protein ZmHP1 has been crystallized. Although ZmHP1 with an N-terminal His tag could be crystallized using sodium chloride as a precipitant, the crystals diffracted poorly to only 3.2 A resolution. When the His tag was removed, ZmHP1 crystals were obtained using polyethylene glycol 4000 as a precipitant and the diffraction data were greatly enhanced to 2.4 A resolution. The crystals belonged to the space group P4(1)2(1)2, with one ZmHP1 molecule in the asymmetric unit.

Crystal structure of the histidine-containing phosphotransfer protein ZmHP2 from maize

In higher plants, histidine-aspartate phosphorelays (two-component system) are involved in hormone signaling and stress responses. In these systems, histidine-containing phosphotransfer (HPt) proteins mediate the signal transmission from sensory histidine kinases to response regulators, including integration of several signaling pathways or branching into different pathways. We have determined the crystal structure of a maize HPt protein, ZmHP2, at 2.2 A resolution. ZmHP2 has six alpha-helices with a four-helix bundle at the C-terminus, a feature commonly found in HPt domains. In ZmHP2, almost all of the conserved residues among plant HPt proteins surround this histidine, probably forming the docking interface for the receiver domain of histidine kinase or the response regulator. Arg102 of ZmHP2 is conserved as a basic residue in plant HPt proteins. In bacteria, it is replaced by glutamine or glutamate that form a hydrogen bond to Ndelta atoms of the phospho-accepting histidine. It may play a key role in the complex formation of ZmHP2 with receiver domains.

Plant response regulators implicated in signal transduction and circadian rhythm

The so-called 'response regulators' were originally discovered as common components of the widespread histidine (His)-->aspartate (Asp) phosphorelay signal transduction system in prokaryotes. Through the course of evolution, higher plants have also come to employ such prokaryotic response regulators (RRs) for their own signal transduction, such as the elicitation of plant hormone (e.g. cytokinin) responses. Furthermore, plants have evolved their own atypical variants of response regulators, pseudo response regulators (PRRs), which are used to modulate sophisticated biological processes, including circadian rhythms and other light-signal responses. Recent studies using the model plant Arabidopsis thaliana have begun to shed light on the interesting functions of these plant response regulators.

Suppression of anoikis and induction of metastasis by the neurotrophic receptor TrkB

Metastasis is a major factor in the malignancy of cancers, and is often responsible for the failure of cancer treatment. Anoikis (apoptosis resulting from loss of cell-matrix interactions) has been suggested to act as a physiological barrier to metastasis; resistance to anoikis may allow survival of cancer cells during systemic circulation, thereby facilitating secondary tumour formation in distant organs. In an attempt to identify metastasis-associated oncogenes, we designed an unbiased, genome-wide functional screen solely on the basis of anoikis suppression. Here, we report the identification of TrkB, a neurotrophic tyrosine kinase receptor, as a potent and specific suppressor of caspase-associated anoikis of non-malignant epithelial cells. By activating the phosphatidylinositol-3-OH kinase/protein kinase B pathway, TrkB induced the formation of large cellular aggregates that survive and proliferate in suspension. In mice, these cells formed rapidly growing tumours that infiltrated lymphatics and blood vessels to colonize distant organs. Consistent with the ability of TrkB to suppress anoikis, metastases--whether small vessel infiltrates or large tumour nodules--contained very few apoptotic cells. These observations demonstrate the potent oncogenic effects of TrkB and uncover a specific pro-survival function that may contribute to its metastatic capacity, providing a possible explanation for the aggressive nature of human tumours that overexpress TrkB.

Molecular characterization of cytokinin-responsive histidine kinases in maize. Differential ligand preferences and response to cis-zeatin

Genes for cytokinin-responsive His-protein kinases (ZmHK1, ZmHK2, and ZmHK3a) were isolated from maize (Zea mays). Heterologous expression of each of the ZmHKs in Escherichia coli having the DeltarcsC and cpslacZ genetic background conferred cytokinin-inducibility of lacZ expression on the bacteria. In the recombinant E. coli system, ZmHK1 and ZmHK3a were more sensitive to free-base cytokinins than to the corresponding nucleosides; isopentenyladenine was most effective for ZmHK1, while ZmHK2 tended to be most sensitive to trans-zeatin and the riboside. In contrast to a known cytokinin receptor of Arabidopsis (AHK4/CRE1/WOL), all ZmHKs responded to cis-zeatin (cZ), which generally is believed to be inactive or only weakly active. In cultured maize cells, expression of ZmRR1, a cytokinin-inducible response regulator, was induced by cZ as well as by trans-zeatin. These results strongly suggest that maize cytokinin receptors differ in ligand preference, and that cZ is an active cytokinin at least in maize.

Histidine-containing phosphotransfer domain extinction by RNA interference turns off a cytokinin signalling circuitry in Catharanthus roseus suspension cells

We previously reported that cytokinins (CK) induce the fast and specific transcription of CrRR1, a gene encoding a type A response regulator in Catharanthus roseus cell cultures. Here, we characterized the CrHPt1 gene that encodes a histidine-containing phosphotransfer domain. CrHPt1 was silenced through RNA interference (RNAi) to test its possible implication in the CK signalling pathway. In transgenic lines stably transformed with an intron-spliced construct, the degradation of CrHPt1 transcripts abolishes the CK inductive effect on CrRR1 transcription. These result give a new in vivo functional argument for the crucial role of HPt proteins in the CK signalling pathway leading to the expression of the genes encoding type A response regulators. They also show that RNAi is a powerful strategy to turn off the CK signalling circuitry.

Type-A Arabidopsis response regulators are partially redundant negative regulators of cytokinin signaling

Type-A Arabidopsis (Arabidopsis thaliana) response regulators (ARRs) are a family of 10 genes that are rapidly induced by cytokinin and are highly similar to bacterial two-component response regulators. We have isolated T-DNA insertions in six of the type-A ARRs and constructed multiple insertional mutants, including the arr3,4,5,6,8,9 hextuple mutant. Single arr mutants were indistinguishable from the wild type in various cytokinin assays; double and higher order arr mutants showed progressively increasing sensitivity to cytokinin, indicating functional overlap among type-A ARRs and that these genes act as negative regulators of cytokinin responses. The induction of cytokinin primary response genes was amplified in arr mutants, indicating that the primary response to cytokinin is affected. Spatial patterns of ARR gene expression were consistent with partially redundant function of these genes in cytokinin signaling. The arr mutants show altered red light sensitivity, suggesting a general involvement of type-A ARRs in light signal transduction. Further, morphological phenotypes of some arr mutants suggest complex regulatory interactions and gene-specific functions among family members.