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

Comparison of the transcriptome and metabolome of wheat (Triticum aestivum L.) proteins content during grain formation provides insight

Introduction

Wheat is a food crop with a large global cultivation area, and the content and quality of wheat glutenin accumulation are important indicators of the quality of wheat flour.

Methods

To elucidate the gene expression regulation and metabolic characteristics related to the gluten content during wheat grain formation, transcriptomic and metabolomic analyses were performed for the high gluten content of the Xinchun 26 cultivar and the low proteins content of the Xinchun 34 cultivar at three periods (7 d, 14 d and 21 d) after flowering.

Results

Transcriptomic analysis revealed that 5573 unique differentially expressed genes (DEGs) were divided into two categories according to their expression patterns during the three periods. The metabolites detected were mainly divided into 12 classes. Lipid and lipid-like molecule levels and phenylpropanoid and polyketide levels were the highest, and the difference analysis revealed a total of 10 differentially regulated metabolites (DRMs) over the three periods. Joint analysis revealed that the DEGs and DRMs were significantly enriched in starch and sucrose metabolism; the citrate cycle; carbon fixation in photosynthetic organisms; and alanine, aspartate and glutamate metabolism pathways. The genes and contents of the sucrose and gluten synthesis pathways were analysed, and the correlation between gluten content and its related genes was calculated. Based on weighted correlation network analysis (WGCNA), by constructing a coexpression network, a total of 5 specific modules and 8 candidate genes that were strongly correlated with the three developmental stages of wheat grain were identified.

Discussion

This study provides new insights into the role of glutenin content in wheat grain formation and reveals potential regulatory pathways and candidate genes involved in this developmental process.

Development of sex-linked markers for gender identification of Actinidia arguta

The fruit of the dioecious plant Actinidia arguta has become a great attraction recently. It has long been difficult to distinguish the genders of hybrid seedlings before flowering, therefore increasing the expenditures of breeding. To produce reliable molecular marker for gender identification, this research utilized whole-genome re-sequencing of 15 males and 15 females from an 8-year-old cross population to develop gender specific markers. P51 and P11 were identified as sex-linked markers after verification. Both of these markers, according to the PCR results, only amplified a single band in male samples. These two markers were tested in 97 hybrids (52 females and 45 males) and 31 wild individuals (13 females and 18 males), with an accuracy of 96.88% and 96.09%, correspondingly. This research also verified the universalities of the two markers in Actinidia chinensis samples, and it could be inferred from the PCR results that neither marker was applicable to A. chinensis samples. The BLAST results of the two markers demonstrated that the two markers were closely aligned with different parts of the Y male-specific region of A. chinensis genome, thus they were likely to be useful for the research on the mechanism of sex determination of A. arguta. The two male-linked makers, P51 and P11, have already been used in sex-identification of A. arguta seedlings.

Highly efficient Agrobacterium-mediated transformation and plant regeneration system for genome engineering in tomato

Tomato (Solanum lycopersicum L.) is an important vegetable and nutritious crop plant worldwide. They are rich sources of several indispensable compounds such as lycopene, minerals, vitamins, carotenoids, essential amino acids, and bioactive polyphenols. Plant regeneration and Agrobacterium-mediated genetic transformation system from different explants in various genotypes of tomato are necessary for genetic improvement. Among diverse plant growth regulator (PGR) combinations and concentrations tested, Zeatin (ZEA) at 2.0 mg l⁻¹ in combination with 0.1 mg l⁻¹ indole-3-acetic acid (IAA) generated the most shoots/explant from the cotyledon of Arka Vikas (36.48 shoots/explant) and PED (24.68 shoots/explant), respectively. The hypocotyl explant produced 28.76 shoots/explant in Arka Vikas and 19.44 shoots/explant in PED. In contrast, leaf explant induced 23.54 shoots/explant in Arka Vikas and 17.64 shoots/explant in PED. The obtained multiple shoot buds from three explant types were elongated on a medium fortified with Gibberellic acid (GA₃) (1.0 mg l⁻¹), IAA (0.5 mg l⁻¹), and ZEA (0.5 mg l⁻¹) in both the cultivars. The rooting was observed on a medium amended with 0.5 mg l⁻¹ indole 3-butyric acid (IBA). The transformation efficiency was significantly improved by optimizing the pre-culture of explants, co-cultivation duration, bacterial density and infection time, and acetosyringone concentration. The presence of transgenes in the plant genome was validated using different methods like histochemical GUS assay, Polymerase Chain Reaction (PCR), and Southern blotting. The transformation efficiency was 42.8% in PED and 64.6% in Arka Vikas. A highly repeatable plant regeneration protocol was established by manipulating various plant growth regulators (PGRs) in two tomato cultivars (Arka Vikas and PED). The Agrobacterium-mediated transformation method was optimized using different explants like cotyledon, hypocotyl, and leaf of two tomato genotypes. The present study could be favourable to transferring desirable traits and precise genome editing techniques to develop superior tomato genotypes.

Stress-induced expression of IPT gene in transgenic wheat reduces grain yield penalty under drought

Background

The heterologous expression of isopentenyl transferase (IPT) under the transcriptional control of the senescence-associated receptor-like kinase (SARK) promoter delayed cellular senescence and, through it, increased drought tolerance in plants. To evaluate the effect of pSARK::IPT expression in bread wheat, six independent transgenic events were obtained through the biolistic method and evaluated transgene expression, phenology, grain yield and physiological biomass components in plants grown under both drought and well-irrigating conditions. Experiments were performed at different levels: (i) pots and (ii) microplots inside a biosafety greenhouse, as well as under (iii) field conditions.

Results

Two transgenic events, called TR1 and TR4, outperformed the wild-type control under drought conditions. Transgenic plants showed higher yield under both greenhouse and field conditions, which was positively correlated to grain number (given by more spikes and grains per spike) than wild type. Interestingly, this yield advantage of the transgenic events was observed under both drought and well-watered conditions.

Conclusions

The results obtained allow us to conclude that the SARK promoter-regulated expression of the IPT gene in bread wheat not only reduced the yield penalty produced by water stress but also led to improved productivity under well-watered conditions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-021-00171-w.

Phytohormone cytokinin guides microtubule dynamics during cell progression from proliferative to differentiated stage

Cell production and differentiation for the acquisition of specific functions are key features of living systems. The dynamic network of cellular microtubules provides the necessary platform to accommodate processes associated with the transition of cells through the individual phases of cytogenesis. Here, we show that the plant hormone cytokinin fine‐tunes the activity of the microtubular cytoskeleton during cell differentiation and counteracts microtubular rearrangements driven by the hormone auxin. The endogenous upward gradient of cytokinin activity along the longitudinal growth axis in Arabidopsis thaliana roots correlates with robust rearrangements of the microtubule cytoskeleton in epidermal cells progressing from the proliferative to the differentiation stage. Controlled increases in cytokinin activity result in premature re‐organization of the microtubule network from transversal to an oblique disposition in cells prior to their differentiation, whereas attenuated hormone perception delays cytoskeleton conversion into a configuration typical for differentiated cells. Intriguingly, cytokinin can interfere with microtubules also in animal cells, such as leukocytes, suggesting that a cytokinin‐sensitive control pathway for the microtubular cytoskeleton may be at least partially conserved between plant and animal cells.

Plant growth-regulating molecules as thermoprotectants: functional relevance and prospects for improving heat tolerance in food crops

Among various abiotic stresses, heat stress is one of the most damaging, threatening plant productivity and survival all over the world. Warmer temperatures due to climatic anomalies above optimum growing temperatures have detrimental impacts on crop yield potential as well as plant distribution patterns. Heat stress affects overall plant metabolism in terms of physiology, biochemistry, and gene expression. Membrane damage, protein degradation, enzyme inactivation, and the accumulation of reactive oxygen species are some of the harmful effects of heat stress that cause injury to various cellular compartments. Although plants are equipped with various defense strategies to counteract these adversities, their defensive means are not sufficient to defend against the ever-rising temperatures. Hence, substantial yield losses have been observed in all crop species under heat stress. Here, we describe the involvement of various plant growth-regulators (PGRs) (hormones, polyamines, osmoprotectants, antioxidants, and other signaling molecules) in thermotolerance, through diverse cellular mechanisms that protect cells under heat stress. Several studies involving the exogenous application of PGRs to heat-stressed plants have demonstrated their role in imparting tolerance, suggesting the strong potential of these molecules in improving the performance of food crops grown under high temperature.

Petal senescence: a hormone view

Flowers are highly complex organs that have evolved to enhance the reproductive success of angiosperms. As a key component of flowers, petals play a vital role in attracting pollinators and ensuring successful pollination. Having fulfilled this function, petals senesce through a process that involves many physiological and biochemical changes that also occur during leaf senescence. However, petal senescence is distinct, due to the abundance of secondary metabolites in petals and the fact that petal senescence is irreversible. Various phytohormones are involved in regulating petal senescence, and are thought to act both synergistically and antagonistically. In this regard, there appears to be developmental point during which such regulatory signals are sensed and senescence is initiated. Here, we review current understanding of petal senescence, and discuss associated regulatory mechanisms involving hormone interactions and epigenetic regulation.

Changes in the Metabolome of Picea balfouriana Embryogenic Tissues That Were Linked to Different Levels of 6-BAP by Gas Chromatography-Mass Spectrometry Approach

Embryogenic cultures of Picea balfouriana, which is an important commercial species for reforestation in Southern China, easily lose their embryogenic ability during long-term culture. Embryogenic tissue that proliferated at lower concentrations (3.6 μM and 2.5 μM) of 6-benzylaminopurine (6-BAP) were more productive, and generated 113 ± 6 and 89 ± 3 mature embryos per 100 mg embryogenic tissue, respectively. A metabolomic approach was used to study the changes in metabolites linked to embryogenic competence related to three different 6-BAP concentrations (2.5 μM, 3.6 μM, and 5 μM). A total of 309 compounds were obtained, among which 123 metabolites mapped to Kyoto Encyclopedia of Genes and genomes (KEGG) pathways. The levels of 35 metabolites were significantly differentially regulated among the three 6-BAP treatments, and 32 metabolites differed between the 2.5 μM and 5 μM treatments. A total of 17 metabolites appeared only once among the three comparisons. The combination of a score plot and a loading plot showed that in the samples with higher embryogenic ability (3.6 μM and 2.5 μM), up-regulated metabolites were mostly amino acids and down-regulated metabolites were mostly primary carbohydrates (especially sugars). These results suggested that 6-BAP may influence embryogenic competence by nitrogen metabolism, which could cause an increase in amino acid levels and higher amounts of aspartate, isoleucine, and leucine in tissues with higher embryogenic ability. Furthermore, we speculated that 6-BAP may affect the amount of tryptophan in tissues, which would change the indole-3-acetic acid levels and influence the embryogenic ability.

Dissecting the role of two cytokinin analogues (INCYDE and PI-55) on in vitro organogenesis, phytohormone accumulation, phytochemical content and antioxidant activity

There is a continuous search for new chemical entities to expand the collection of suitable compounds to increase the efficiency of micropropagation protocols. Two cytokinin (CK) analogues, 2-chloro-6-(3-methoxyphenyl)aminopurine (INCYDE) and CK antagonist 6-(2-hydroxy-3-methylbenzylamino)purine (PI-55) were used as a tool to elucidate the auxin-CK crosstalk under in vitro conditions in the medicinally important plant, Eucomis autumnalis subspecies autumnalis. These compounds were tested at 0.01, 0.1 and 10 μM alone as well as in combination with benzyladenine (BA) and naphthaleneacetic acid (NAA). The organogenesis, phytohormone content, phytochemical and antioxidant response in 10 week-old-in vitro regenerated E. autumnalis subspecies autumnalis was evaluated. INCYDE generally favoured shoot regeneration while the effect of PI-55 was more evident in root proliferation. Overall, INCYDE promoted the accumulation of higher concentrations and varieties of endogenous CK relative to the PI-55 treatments. In contrast, higher concentration of indole-3-acetic acid and 2-oxindole-3-acetic acid were generally observed in PI-55-supplemented cultures when compared to plantlets derived from INCYDE. Both CK analogues (individually and in-conjunction with exogenously applied PGRs) significantly influenced the phytochemicals and consequently the antioxidant potential of the in vitro regenerants. These results provided insight on how to alleviate root inhibition, a problem which causes considerable loss of several elite species during micropropagation.

Interactions between cytokinin signalling and abiotic stress responses

Plants have evolved elaborate mechanisms for sensing and responding to sub-optimal environmental conditions. Abiotic stresses caused by these conditions trigger a wide range of local and long-distance signals which must be co-ordinated and integrated into whole-plant processes, such as development, in order for the plant to respond properly and survive. Several hormones function as key regulators of stress tolerance, connecting local stimuli to systemic responses. Cytokinin is a hormone well known for its role in numerous aspects of growth and development, although abundant evidence also indicates that cytokinin functions in stress responses as well. At present, a full understanding of the effects of cytokinin on plant resistance to stress is lacking, possibly as a result of the complex interactions between cytokinin and stress signalling. Current knowledge of the physiological relationship between cytokinin and abiotic stress, based on measurements of cytokinin levels under stress conditions and the effects of cytokinin treatment on stress tolerance, has been examined here. A pattern of transcriptional regulation of stress-related genes by cytokinin in different plant species has also been identified. In addition, research regarding the role of specific cytokinin signalling components in a variety of stress responses is presented. We discuss what this body of research collectively implies with regard to cross-talk between cytokinin and abiotic stress tolerance.

Transcriptomic and proteomic analyses of embryogenic tissues in Picea balfouriana treated with 6-benzylaminopurine

The cytokinin 6-benzylaminopurine (6-BAP) influences the embryogenic capacity of the tissues of Picea balfouriana during long subculture (after 3 months). Tissues that proliferate in 3.6 and 5 µM 6-BAP exhibit the highest and lowest embryogenic capacity, respectively, generating 113 ± 6 and 23 ± 3 mature embryos per 100 mg of tissue. In this study, a comparative transcriptomic and proteomic approach was applied to characterize the genes and proteins that are differentially expressed among tissues under the influence of different levels of 6-BAP. A total of 51 375 unigenes and 2617 proteins were obtained after quality filtering. There were 2770 transcripts for proteins found among these unigenes. Gene ontology (GO) analysis of the differentially expressed unigenes and proteins showed that they were involved in cell and binding activity and were enriched in ribosome and glutathione metabolism pathways. Ribosomal proteins, glutathione S-transferase proteins, germin-like proteins and calmodulin-independent protein kinases were up-regulated in the embryogenic tissues with the highest embryogenic ability (treated with 3.6 µM 6-BAP), which was validated via quantitative real-time polymerase chain reaction (qRT-PCR) analysis, and these proteins might serve as molecular markers of embryogenic ability. Data are available via Sequence Read Archive (SRA) and ProteomeXchange with identifier SRP042246 and PXD001022, respectively.

Cytokinin signaling stabilizes the response activator ARR1

The cytokinins play essential roles in the development and environmental responses of higher plants. Cytokinin signaling leads to the phosphorylation-dependent activation of two classes of Arabidopsis response regulators (RRs): the type-B RR (RRB) transcriptional activators that promote the expression of cytokinin response genes and the type-A RRs (RRAs) that are encoded by primary cytokinin response genes and function as response inhibitors. We show that cytokinin signaling increases the abundance of ARR1, a ubiquitously expressed RRB, by preventing its degradation by the 26S proteasome. We also show that the RRAs act to suppress ARR1 accumulation, thus providing an explanation for their inhibitory action in cytokinin signaling. Collectively, our results reveal an additional regulatory mechanism in the cytokinin response pathway that involves the cytokinin-dependent stability control of a major RRB response activator.

Genetic variation in yield under hot ambient temperatures spotlights a role for cytokinin in protection of developing floral primordia

Unusually hot ambient temperatures (HAT) can cause pre-anthesis abortion of flowers in many diverse species, limiting crop production. This limitation is becoming more substantial with climate change. Flower primordia of passion fruit (Passiflora edulis Sims) vines exposed to HAT summers, normally abort. Flower abortion can also be triggered by gibberellin application. We screened for, and identified a genotype capable of reaching anthesis during summer as well as controlled HAT conditions, and also more resistant to gibberellin. Leaves of this genotype contained higher levels of endogenous cytokinin. We investigated a possible connection between higher cytokinin levels and response to gibberellin. Indeed, the effects of gibberellin application were partially suppressed in plants pretreated with cytokinin. Can higher cytokinin levels protect flowers from aborting under HAT conditions? In passion fruit, flowers at a specific stage showed more resistance in response to HAT after cytokinin application. We further tested this hypothesis in Arabidopsis. Transgenic lines with high or low cytokinin levels and cytokinin applications to wild-type plants supported a protective role for cytokinin on developing flowers exposed to HAT. Such findings may have important implications in future breeding programmes as well as field application of growth regulators.

Expression patterns and regulation of SlCRF3 and SlCRF5 in response to cytokinin and abiotic stresses in tomato (Solanum lycopersicum)

Cytokinin is an influential hormone involved in numerous aspects of plant growth and development. A group of transcription factors-cytokinin response factors (CRFs) has been included as a side branch to cytokinin signaling pathway which also constitute a subset of the AP2/ERF family of transcription factor proteins. This study examined the expression patterns of two transcription factor genes SlCRF3 and SlCRF5 in tomato (Solanum lycopersicum) to determine their regulation in response to cytokinin and a variety of abiotic stress conditions. Analyses conducted during different developmental stages by RT-PCR or GUS reporter gene expression revealed that these genes are differentially expressed in vegetative and reproductive organs. qRT-PCR experiments were also performed to study regulation by the hormone cytokinin and abiotic stress conditions such as flooding, drought, osmotic, oxidative, and temperature. These showed that SlCRF3 and SlCRF5 have different patterns of regulation in leaf, stem, and roots with SlCRF5 showing greater induction in leaf or root tissue compared to SlCRF3 in most cases. Additionally, knockdown analysis for SlCRF5 revealed defects across development including leaf morphology, primary root growth, and lateral root formation. Together, these findings indicate that SlCRF3 and SlCRF5 are potential regulators of tomato developmental processes associated with cytokinin or abiotic stresses.

The interaction between glucose and cytokinin signal transduction pathway in Arabidopsis thaliana

Cytokinins (CKs) and glucose (GLC) control a number of common responses in plants. We hypothesize that there may be an extensive overlap between CK- and GLC-signalling pathways. Microarray along with physiological analysis has been performed to find out the interdependence/overlap between CK and GLC signal transduction pathways in Arabidopsis seedlings. GLC could transcriptionally affect 76% of CK-regulated genes at whole genome level, 89% of which are agonistically regulated. GLC may also affect CK-regulated gene expression via non-transcriptional pathways. GLC can regulate several genes involved in CK metabolism and signalling. A number of gene families involved in development and stress are commonly regulated by CK and GLC. Physiologically, both GLC and CK could regulate hypocotyl length in dark. GLC and CK signalling may integrate at the level of type A Arabidopsis response regulators (ARRs) in controlling hypocotyl length. Both GLC and CK signalling cannot alter hypocotyl length in dark in auxin-signalling mutants auxin response2/indole-3-acetic acid7 (AXR2/IAA7) and AXR3/IAA17 suggesting that they may involve auxin-signalling component as a nodal point. Here, we demonstrate that there is an extensive overlap between CK- and GLC-regulated gene expression and physiological responses.

Cytokinin inhibition of leaf senescence

The senescence delaying effect of cytokinin is well known, however, the details behind how this process occurs remain unclear. Efforts to improve understanding of this phenomenon have led to the identification in Arabidopsis of specific cytokinin signaling components through which senescence signal responses are regulated. These include the cytokinin receptor (AHK3), the type-B response regulator (ARR2) and the recently identified cytokinin response factor (CRF6). At the mechanistic end of this process, it was found that increased cell-wall invertase activity which occurs in response to cytokinin is both necessary and sufficient for the inhibition of senescence. Yet, a direct link between the signaling and mechanistic steps of a cytokinin regulated senescence process has yet to be demonstrated. This may be in part because the relationship between senescence and primary metabolism implied by the key role of cell-wall invertase is the subject of two apparently opposing bodies of evidence. Here we briefly summarize and propose a model in which cytokinin mediated changes in sink/source relationships leads to delayed senescence which is consistent with existing evidence both for and against sugars as a trigger for developmental senescence.

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.

Auxin: a master regulator in plant root development

The demand for increased crop productivity and the predicted challenges related to plant survival under adverse environmental conditions have renewed the interest in research in root biology. Various physiological and genetic studies have provided ample evidence in support of the role of plant growth regulators in root development. The biosynthesis and transport of auxin and its signaling play a crucial role in controlling root growth and development. The univocal role of auxin in root development has established it as a master regulator. Other plant hormones, such as cytokinins, brassinosteroids, ethylene, abscisic acid, gibberellins, jasmonic acid, polyamines and strigolactones interact either synergistically or antagonistically with auxin to trigger cascades of events leading to root morphogenesis and development. In recent years, the availability of biological resources, development of modern tools and experimental approaches have led to the advancement of knowledge in root development. Research in the areas of hormone signal perception, understanding network of events involved in hormone action and the transport of plant hormones has added a new dimension to root biology. The present review highlights some of the important conceptual developments in the interplay of auxin and other plant hormones and associated downstream events affecting root development.

Structural basis for cytokinin receptor signaling: an evolutionary approach

Cytokinins are ubiquitous plant hormones; their signal is perceived by sensor histidine kinases-cytokinin receptors. This review focuses on recent advances on cytokinin receptor structure, in particular sensing module and adjacent domains which play an important role in hormone recognition, signal transduction and receptor subcellular localization. Principles of cytokinin binding site organization and point mutations affecting signaling are discussed. To date, more than 100 putative cytokinin receptor genes from different plant species were revealed due to the total genome sequencing. This allowed us to employ an evolutionary and bioinformatics approaches to clarify some new aspects of receptor structure and function. Non-transmembrane areas adjacent to the ligand-binding CHASE domain were characterized in detail and new conserved protein motifs were recovered. Putative mechanisms for cytokinin-triggered receptor activation were suggested.

Identification of five B-type response regulators as members of a multistep phosphorelay system interacting with histidine-containing phosphotransfer partners of Populus osmosensor

BACKGROUND

In plants, the multistep phosphorelay signaling pathway mediates responses to environmental factors and plant hormones. This system is composed of three successive partners: hybrid Histidine-aspartate Kinases (HKs), Histidine-containing Phosphotransfer proteins (HPts), and Response Regulators (RRs). Among the third partners, B-type RR family members are the final output elements of the pathway; they act as transcription factors and clearly play a pivotal role in the early response to cytokinin in Arabidopsis. While interactions studies between partners belonging to the multistep phosphorelay system are mainly focused on protagonists involved in cytokinin or ethylene pathways, very few reports are available concerning partners of osmotic stress signaling pathway.

RESULTS

In Populus, we identified eight B-type RR proteins, RR12-16, 19, 21 and 22 in the Dorskamp genotype. To assess HPt/B-type RR interactions and consequently determine potential third partners in the osmosensing multistep phosphorelay system, we performed global yeast two-hybrid (Y2H) assays in combination with Bimolecular Fluorescence Complementation (BiFC) assays in plant cells. We found that all B-type RRs are able to interact with HPt predominant partners (HPt2, 7 and 9) of HK1, which is putatively involved in the osmosensing pathway. However, different profiles of interaction are observed depending on the studied HPt. HPt/RR interactions displayed a nuclear localization, while the nuclear and cytosolic localization of HPt and nuclear localization of RR proteins were validated. Although the nuclear localization of HPt/RR interaction was expected, this work constitutes the first evidence of such an interaction in plants. Furthermore, the pertinence of this partnership is reinforced by highlighting a co-expression of B-type RR transcripts and the other partners (HK1 and HPts) belonging to a potential osmosensing pathway.

CONCLUSION

Based on the interaction studies between identified B-type RR and HPt proteins, and the co-expression analysis of transcripts of these potential partners in poplar organs, our results favor the model that RR12, 13, 14, 16 and 19 are able to interact with the main partners of HK1, HPt2, 7 and 9, and this HPt/RR interaction occurs within the nucleus. On the whole, the five B-type RRs of interest could be third protagonists putatively involved in the osmosensing signaling pathway in Populus.

A type-B response regulator drives the expression of the hydroxymethylbutenyl diphosphate synthase gene in periwinkle

In plant cytokinin (CK) signaling, type-B response regulators (RRs) act as major players in orchestrating the transcriptome changes in response to CK. However, their direct targets are poorly known. The identification of putative type-ARR1 motifs located within the promoter of the CK-responsive hydroxyl methyl butenyl diphosphate synthase (HDS) gene from the methyl erythritol phosphate (MEP) pathway prompted us to investigate the ability of a previously isolated periwinkle type-B RR (CrRR5) that presents high homologies with ARR1 to interact with the promoter. Electrophoretic mobility shift assays (EMSAs) demonstrated that the CrRR5 DNA-binding domain binds specifically type-ARR1 motifs within the HDS promoter. We also established through yellow fluorescent protein (YFP) imaging the targeting of CrRR5 into cell nucleus in accordance with its putative function of transcription factor. In transient assays performed on periwinkle cells cultivated with CK, overexpression of the full-length CrRR5 or a truncated CrRR5 engineering a constitutive active form (35S:ΔDDK) did not affect the HDS promoter activity that reached a threshold. By contrast, in absence of CK, overexpression of CrRR5ΔDDK enhanced promoter activity up to the threshold level observed in cells grown with CK. Our results strongly suggest that CrRR5 directly transactivates the HDS promoter. CrRR5 is the first identified transcription factor mediating the CK signaling that targets a gene from the MEP pathway involved in isoprenoid metabolism. Moreover, CrRR5 could play a role in a regulatory mechanism controlling CK homeostasis in periwinkle cells.