Cytokinins

Cytokinin Transporters: GO and STOP in Signaling

Cytokinins are phytohormones essential for cytokinesis and many other physiological and developmental processes in planta. Long-distance transport and intercellular transport have been postulated. For these processes, the existence of cytokinin transporters has been suggested. Recently, a transporter loading the xylem (AtABCG14) and another for cellular import (AtPUP14) have been discovered. AtABCG14 participates in the xylem loading process of cytokinins and contributes to the positive regulation of shoot growth. The cellular importer AtPUP14 is required to suppress cytokinin signaling. A role of a transporter as stop signal is a new paradigm for a hormone transporter.

Type-B ARABIDOPSIS RESPONSE REGULATORs Specify the Shoot Stem Cell Niche by Dual Regulation of WUSCHEL

Plants are known for their capacity to regenerate the whole body through de novo formation of apical meristems from a mass of proliferating cells named callus. Exogenous cytokinin and auxin determine cell fate for the establishment of the stem cell niche, which is the vital step of shoot regeneration, but the underlying mechanisms remain unclear. Here, we show that type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs), critical components of cytokinin signaling, activate the transcription of WUSCHEL (WUS), which encodes a key regulator for maintaining stem cells. In parallel, type-B ARRs inhibit auxin accumulation by repressing the expression of YUCCAs, which encode a key enzyme for auxin biosynthesis, indirectly promoting WUS induction. Both pathways are essential for de novo regeneration of the shoot stem cell niche. In addition, the dual regulation of type-B ARRs on WUS transcription is required for the maintenance of the shoot apical meristem in planta. Thus, our results reveal a long-standing missing link between cytokinin signaling and WUS regulator, and the findings provide critical information for understanding cell fate specification.

Analysis of CFB, a cytokinin-responsive gene of Arabidopsis thaliana encoding a novel F-box protein regulating sterol biosynthesis

Protein degradation by the ubiquitin-26S proteasome pathway is important for the regulation of cellular processes, but the function of most F-box proteins relevant to substrate recognition is unknown. We describe the analysis of the gene Cytokinin-induced F-box encoding (CFB, AT3G44326), identified in a meta-analysis of cytokinin-related transcriptome studies as one of the most robust cytokinin response genes. F-box domain-dependent interaction with the E3 ubiquitin ligase complex component ASK1 classifies CFB as a functional F-box protein. Apart from F-box and transmembrane domains, CFB contains no known functional domains. CFB is expressed in all plant tissues, predominantly in root tissue. A ProCFB:GFP-GUS fusion gene showed strongest expression in the lateral root cap and during lateral root formation. CFB-GFP fusion proteins were mainly localized in the nucleus and the cytosol but also at the plasma membrane. cfb mutants had no discernible phenotype, but CFB overexpressing plants showed several defects, such as a white upper inflorescence stem, similar to the hypomorphic cycloartenol synthase mutant cas1-1. Both CFB overexpressing plants and cas1-1 mutants accumulated the CAS1 substrate 2,3-oxidosqualene in the white stem tissue, the latter even more after cytokinin treatment, indicating impairment of CAS1 function. This suggests that CFB may link cytokinin and the sterol biosynthesis pathway.

Interspecies hormonal control of host root morphology by parasitic plants

Parasitic plants share a common anatomical feature, the haustorium. Haustoria enable both infection and nutrient transfer, which often leads to growth penalties for host plants and yield reduction in crop species. Haustoria also reciprocally transfer substances, such as RNA and proteins, from parasite to host, but the biological relevance for such movement remains unknown. Here, we studied such interspecies transport by using the hemiparasitic plant Phtheirospermum japonicum during infection of Arabidopsis thaliana Tracer experiments revealed a rapid and efficient transfer of carboxyfluorescein diacetate (CFDA) from host to parasite upon formation of vascular connections. In addition, Phtheirospermum induced hypertrophy in host roots at the site of infection, a form of enhanced secondary growth that is commonly observed during various parasitic plant-host interactions. The plant hormone cytokinin is important for secondary growth, and we observed increases in cytokinin and its response during infection in both host and parasite. Phtheirospermum-induced host hypertrophy required cytokinin signaling genes (AHK3,4) but not cytokinin biosynthesis genes (IPT1,3,5,7) in the host. Furthermore, expression of a cytokinin-degrading enzyme in Phtheirospermum prevented host hypertrophy. Wild-type hosts with hypertrophy were smaller than ahk3,4 mutant hosts resistant to hypertrophy, suggesting hypertrophy improves the efficiency of parasitism. Taken together, these results demonstrate that the interspecies movement of a parasite-derived hormone modified both host root morphology and fitness. Several microbial and animal plant pathogens use cytokinins during infections, highlighting the central role of this growth hormone during the establishment of plant diseases and revealing a common strategy for parasite infections of plants.

The interaction between glucose and cytokinin signaling in controlling Arabidopsis thaliana seedling root growth and development

Cytokinin (CK) and glucose (GLC) control several common responses in plants. There is an extensive overlap between CK and GLC signal transduction pathways in Arabidopsis. Physiologically, both GLC and CK could regulate root length in light. CK interacts with GLC via HXK1 dependent pathway for root length control. Wild-type (WT) roots cannot elongate in the GLC free medium while CK-receptor mutant ARABIDOPSIS HISTIDINE KINASE4 (ahk4) and type B ARR triple mutant ARABIDOPSIS RESPONSE REGULATOR1, 10,11 (arr1, 10,11) roots could elongate even in the absence of GLC as compared with the WT. The root hair initiation was also found defective in CK signaling mutants ahk4, arr1,10,11 and arr3,4,5,6,8,9 on increasing GLC concentration (up to 3%); and lesser number of root hairs were visible even at 5% GLC as compared with the WT. Out of 941 BAP regulated genes, 103 (11%) genes were involved in root growth and development. Out of these 103 genes, 60 (58%) genes were also regulated by GLC. GLC could regulate 5736 genes, which include 327 (6%) genes involved in root growth and development. Out of these 327 genes, 60 (18%) genes were also regulated by BAP. Both GLC and CK signaling cannot alter root length in light in auxin signaling mutant AUXIN RESPONSE3/INDOLE-3-ACETIC ACID17 (axr3/iaa17) suggesting that they may involve auxin signaling component as a nodal point. Therefore CK- and GLC- signaling are involved in controlling different aspects of root growth and development such as root length, with auxin signaling components working as downstream target.

Novel in vivo screening design for the rapid and cost-effective identification of transcriptional regulators

Genetic screens are a common tool to identify new modulators in a defined context, e.g. hormonal response or environmental stress. However, most screens are either in vitro or laborious and time-and-space inefficient. Here we present a novel in planta screening approach that shortens the time from the actual screening process to the identification of a new modulator and simultaneously reduces space requirements and costs. The basic features of this screening approach are the creation of luciferase reporter plants which enable a non-invasive readout in a streamlined multiplate reader process, the transformation of those plants with an inducible, Gateway™-compatible expression vector, and a screening setup, in which whole plants at the seedling stage are screened in 96-multiwell plates in the first transformed generation without the use of an expensive charge-coupled device (CCD) camera system. The screening itself and the verification of candidates can be done in as little as 2-3 weeks. The screen enables the analysis of reporter gene activity upon different treatments. Primary positive plants can immediately be selected and grown further. In this study a fast, simple, cost- and space-efficient in planta screening system to detect novel mediators of a given transcriptional response was developed and successfully tested using the cytokinin signal transduction as a test case.

Light Controls Cytokinin Signaling via Transcriptional Regulation of Constitutively Active Sensor Histidine Kinase CKI1

In plants, the multistep phosphorelay (MSP) pathway mediates a range of regulatory processes, including those activated by cytokinins. The cross talk between cytokinin response and light has been known for a long time. However, the molecular mechanism underlying the interaction between light and cytokinin signaling remains elusive. In the screen for upstream regulators we identified a LONG PALE HYPOCOTYL (LPH) gene whose activity is indispensable for spatiotemporally correct expression of CYTOKININ INDEPENDENT1 (CKI1), encoding the constitutively active sensor His kinase that activates MSP signaling. lph is a new allele of HEME OXYGENASE1 (HY1) that encodes the key protein in the biosynthesis of phytochromobilin, a cofactor of photoconvertible phytochromes. Our analysis confirmed the light-dependent regulation of the CKI1 expression pattern. We show that CKI1 expression is under the control of phytochrome A (phyA), functioning as a dual (both positive and negative) regulator of CKI1 expression, presumably via the phyA-regulated transcription factors (TF) PHYTOCHROME INTERACTING FACTOR3 and CIRCADIAN CLOCK ASSOCIATED1. Changes in CKI1 expression observed in lph/hy1-7 and phy mutants correlate with misregulation of MSP signaling, changed cytokinin sensitivity, and developmental aberrations that were previously shown to be associated with cytokinin and/or CKI1 action. Besides that, we demonstrate a novel role of phyA-dependent CKI1 expression in the hypocotyl elongation and hook development during skotomorphogenesis. Based on these results, we propose that the light-dependent regulation of CKI1 provides a plausible mechanistic link underlying the well-known interaction between light- and cytokinin-controlled plant development.

Dynamic patterns of expression for genes regulating cytokinin metabolism and signaling during rice inflorescence development

Inflorescence development in cereals, including such important crops as rice, maize, and wheat, directly affects grain number and size and is a key determinant of yield. Cytokinin regulates meristem size and activity and, as a result, has profound effects on inflorescence development and architecture. To clarify the role of cytokinin action in inflorescence development, we used the NanoString nCounter system to analyze gene expression in the early stages of rice panicle development, focusing on 67 genes involved in cytokinin biosynthesis, degradation, and signaling. Results point toward key members of these gene families involved in panicle development and indicate that the expression of many genes involved in cytokinin action differs between the panicle and vegetative tissues. Dynamic patterns of gene expression suggest that subnetworks mediate cytokinin action during different stages of panicle development. The variation of expression during panicle development is greater among genes encoding proteins involved in cytokinin metabolism and negative regulators of the pathway than for the genes in the primary response pathway. These results provide insight into the expression patterns of genes involved in cytokinin action during inflorescence development in a crop of agricultural importance, with relevance to similar processes in other monocots. The identification of subnetworks of genes expressed at different stages of early panicle development suggests that manipulation of their expression could have substantial effects on inflorescence architecture.

Analysis of plant hormone profiles in response to moderate dehydration stress

Plant responses to dehydration stress are mediated by highly complex molecular systems involving hormone signaling and metabolism, particularly the major stress hormone abscisic acid (ABA) and ABA-dependent gene expression. To understand the roles of plant hormones and their interactions during dehydration, we analyzed the plant hormone profiles with respect to dehydration responses in Arabidopsis thaliana wild-type (WT) plants and ABA biosynthesis mutants (nced3-2). We developed a procedure for moderate dehydration stress, and then investigated temporal changes in the profiles of ABA, jasmonic acid isoleucine (JA-Ile), salicylic acid (SA), cytokinin (trans-zeatin, tZ), auxin (indole-acetic acid, IAA), and gibberellin (GA₄ ), along with temporal changes in the expression of key genes involved in hormone biosynthesis. ABA levels increased in a bi-phasic pattern (at the early and late phases) in response to moderate dehydration stress. JA-Ile levels increased slightly in WT plants and strongly increased in nced3-2 mutant plants at 72 h after the onset of dehydration. The expression profiles of dehydration-inducible genes displayed temporal responses in an ABA-dependent manner. The early phase of ABA accumulation correlated with the expression of touch-inducible genes and was independent of factors involved in the major ABA regulatory pathway, including the ABA-responsive element-binding (AREB/ABF) transcription factor. JA-Ile, SA, and tZ were negatively regulated during the late dehydration response phase. Transcriptome analysis revealed important roles for hormone-related genes in metabolism and signaling during dehydration-induced plant responses.

Temporal and spatial changes in gene expression, metabolite accumulation and phytohormone content in rice seedlings grown under drought stress conditions

In order to analyze the molecular mechanisms underlying the responses of plants to different levels of drought stress, we developed a soil matric potential (SMP)-based irrigation system that precisely controls soil moisture. Using this system, rice seedlings were grown under three different drought levels, denoted Md1, Md2 and Md3, with SMP values set to -9.8, -31.0 and -309.9 kPa, respectively. Although the Md1 treatment did not alter the visible phenotype, the Md2 treatment caused stomatal closure and shoot growth retardation (SGR). The Md3 treatment markedly induced SGR, without inhibition of photosynthesis. More severe drought (Sds) treatment, under which irrigation was terminated, resulted in the wilting of leaves and inhibition of photosynthesis. Metabolome analysis revealed the accumulation of primary sugars under Md3 and Sds and of most amino acids under Sds. The starch content was increased under Md3 and decreased under Sds. Transcriptome data showed that the expression profiles of associated genes supported the observed changes in photosynthesis and metabolites, suggesting that the time lag from SGR to inhibition of photosynthesis might lead to the accumulation of photosynthates under Md3, which can be used as osmolytes under Sds. To gain further insight into the observed SGR, transcriptome and hormonome analyses were performed in specific tissues. The results showed specific decreases in indole-3-acetic acid (IAA) and cytokinin levels in Md2-, Md3- and Sds-treated shoot bases, though the expression levels of hormone metabolism-related genes were not reflected in IAA and cytokinin contents. These observations suggest that drought stress affects the distribution or degradation of cytokinin and IAA molecules.

Analyzing Cytokinin Responses During Plant-Nematode Interactions

Cyst nematodes are obligate biotrophs that induce the formation of a hypertrophied and hypermetabolic syncytial-feeding site in roots of the host plants. Cytokinin signaling is activated at the site of infection and contributes significantly to the formation of syncytium. Here, we describe a protocol for visualizing cytokinin signaling activation in Arabidopsis upon infection with cyst nematode Heterodera schachtii using promoter reporter lines, TCSn:GFP and ARR5:GUS.

Quantification of Cytokinin Levels and Responses in Abiotic Stresses

Since their discovery in the 1950s, it has been established that cytokinins (CKs) play important regulatory roles in various physiological processes in plants. Only recently have CKs been also implicated in the response of plants to biotic and abiotic stresses. During the last years, several analytical methods have been developed to determine CK concentrations in plant tissues. Here we present a simple and robust method for CK extraction, purification and analysis in plant tissues, using ultrahigh-performance liquid chromatography coupled to high resolution mass spectrometry (U-HPLC-HRMS). The main advantage of this methodology is the simplicity of the purification protocol and the possibility to extend it to the analysis of other plant hormones and derivatives.

Should I fight or should I grow now? The role of cytokinins in plant growth and immunity and in the growth-defence trade-off

BACKGROUND

Perception and activation of plant immunity require a remarkable level of signalling plasticity and control. In Arabidopsis and other plant species, constitutive defence activation leads to resistance to a broad spectrum of biotrophic pathogens, but also frequently to stunted growth and reduced seed set. Plant hormones are important integrators of the physiological responses that influence the outcome of plant-pathogen interactions.

SCOPE

We review the mechanisms by which the plant hormone cytokinin regulates both plant growth and response to pathogens, and how cytokinins may connect these two processes, ultimately affecting the growth trade-offs observed in plant immunity.

Gain-of-Function Mutants of the Cytokinin Receptors AHK2 and AHK3 Regulate Plant Organ Size, Flowering Time and Plant Longevity

The phytohormone cytokinin is a regulator of numerous processes in plants. In Arabidopsis (Arabidopsis thaliana), the cytokinin signal is perceived by three membrane-located receptors named ARABIDOPSIS HISTIDINE KINASE2 (AHK2), AHK3, and AHK4/CRE1. How the signal is transmitted across the membrane is an entirely unknown process. The three receptors have been shown to operate mostly in a redundant fashion, and very few specific roles have been attributed to single receptors. Using a forward genetic approach, we isolated constitutively active gain-of-function variants of the AHK2 and AHK3 genes, named repressor of cytokinin deficiency2 (rock2) and rock3, respectively. It is hypothesized that the structural changes caused by these mutations in the sensory and adjacent transmembrane domains emulate the structural changes caused by cytokinin binding, resulting in domain motion propagating the signal across the membrane. Detailed analysis of lines carrying rock2 and rock3 alleles revealed how plants respond to locally enhanced cytokinin signaling. Early flowering time, a prolonged reproductive growth phase, and, thereby, increased seed yield suggest that cytokinin regulates various aspects of reproductive growth. In particular, it counteracts the global proliferative arrest, a correlative inhibition of maternal growth by seeds, an as yet unknown activity of the hormone.

Control of cytokinin and auxin homeostasis in cyanobacteria and algae

BACKGROUND AND AIMS

The metabolism of cytokinins (CKs) and auxins in vascular plants is relatively well understood, but data concerning their metabolic pathways in non-vascular plants are still rather rare. With the aim of filling this gap, 20 representatives of taxonomically major lineages of cyanobacteria and algae from Cyanophyceae, Xanthophyceae, Eustigmatophyceae, Porphyridiophyceae, Chlorophyceae, Ulvophyceae, Trebouxiophyceae, Zygnematophyceae and Klebsormidiophyceae were analysed for endogenous profiles of CKs and auxins and some of them were used for studies of the metabolic fate of exogenously applied radiolabelled CK, [3H]trans-zeatin (transZ) and auxin ([3H]indole-3-acetic acid (IAA)), and the dynamics of endogenous CK and auxin pools during algal growth and cell division.

METHODS

Quantification of phytohormone levels was performed by high-performance or ultrahigh-performance liquid chromatography-electrospray tandem mass spectrometry (HPLC-MS/MS, UHPLC-MS/MS). The dynamics of exogenously applied [3H]transZ and [3H]IAA in cell cultures were monitored by HPLC with on-line radioactivity detection.

KEY RESULTS

The comprehensive screen of selected cyanobacteria and algae for endogenous CKs revealed a predominance of bioactive and phosphate CK forms while O- and N-glucosides evidently did not contribute greatly to the total CK pool. The abundance of cis-zeatin-type CKs and occurrence of CK 2-methylthio derivatives pointed to the tRNA pathway as a substantial source of CKs. The importance of the tRNA biosynthetic pathway was proved by the detection of tRNA-bound CKs during the course of Scenedesmus obliquus growth. Among auxins, free IAA and its oxidation catabolite 2-oxindole-3-acetic acid represented the prevailing endogenous forms. After treatment with [3H]IAA, IAA-aspartate and indole-3-acetyl-1-glucosyl ester were detected as major auxin metabolites. Moreover, different dynamics of endogenous CKs and auxin profiles during S. obliquus culture clearly demonstrated diverse roles of both phytohormones in algal growth and cell division.

CONCLUSIONS

Our data suggest the existence and functioning of a complex network of metabolic pathways and activity control of CKs and auxins in cyanobacteria and algae that apparently differ from those in vascular plants.

Arabidopsis response regulator 22 inhibits cytokinin-regulated gene transcription in vivo

Cytokinin signaling in Arabidopsis is carried out by a two-component system (TCS) multi-step phosphorelay mechanism that involves three different protein families: histidine kinases (AHKs), phosphotransfer proteins (AHPs), and response regulators (ARRs) that are in turn, subdivided into A-, B- and C-type ARRs depending on their function and structure. Upon cytokinin perception, AHK proteins autophosphorylate; this phosphate is then transferred from the AHKs to the AHPs to finally reach the ARRs. When B-type ARRs are activated by phosphorylation, they function as transcription factors that regulate the expression of cytokinin-dependent genes such as the A-type ARRs, among many others. In cytokinin signaling, while A- and B-type ARR function is well understood, it is still unclear if C-type ARRs (ARR22 and ARR24) play a role in this mechanism. Here, we describe a novel method suitable to study TCS activity natively as an in vivo system. We also show that ARR22 inhibits gene transcription of an A-type ARR upon cytokinin treatment in vivo. Consequently, we propose that ARR22, by acting as a phosphatase on specific AHPs, disrupts the TCS phosphorelay and prevents B-type ARR phosphorylation, and thus their activation as transcription factors, explaining the observed deactivation of cytokinin-responsive genes.

Pistil Transcriptome Analysis to Disclose Genes and Gene Products Related to Aposporous Apomixis in Hypericum perforatum L

Unlike sexual reproduction, apomixis encompasses a number of reproductive strategies, which permit maternal genome inheritance without genetic recombination and syngamy. The key biological features of apomixis are the circumvention of meiosis (i.e., apomeiosis), the differentiation of unreduced embryo sacs and egg cells, and their autonomous development in functional embryos through parthenogenesis, and the formation of viable endosperm either via fertilization-independent means or following fertilization with a sperm cell. Despite the importance of apomixis for breeding of crop plants and although much research has been conducted to study this process, the genetic control of apomixis is still not well understood. Hypericum perforatum is becoming an attractive model system for the study of aposporous apomixis. Here we report results from a global gene expression analysis of H. perforatum pistils collected from sexual and aposporous plant accessions for the purpose of identifying genes, biological processes and molecular functions associated with the aposporous apomixis pathway. Across two developmental stages corresponding to the expression of aposporous apomeiosis and parthenogenesis in ovules, a total of 224 and 973 unigenes were found to be significantly up- and down-regulated with a fold change ≥ 2 in at least one comparison, respectively. Differentially expressed genes were enriched for multiple gene ontology (GO) terms, including cell cycle, DNA metabolic process, and single-organism cellular process. For molecular functions, the highest scores were recorded for GO terms associated with DNA binding, DNA (cytosine-5-)-methyltransferase activity and heterocyclic compound binding. As deregulation of single components of the sexual developmental pathway is believed to be a trigger of the apomictic reproductive program, all genes involved in sporogenesis, gametogenesis and response to hormonal stimuli were analyzed in great detail. Overall, our data suggest that phenotypic expression of apospory is concomitant with the modulation of key genes involved in the sexual reproductive pathway. Furthermore, based on gene annotation and co-expression, we underline a putative role of hormones and key actors playing in the RNA-directed DNA methylation pathway in regulating the developmental changes occurring during aposporous apomixis in H. perforatum.

Methodological Advances in Auxin and Cytokinin Biology

The history of auxin and cytokinin biology including the initial discoveries by father-son duo Charles Darwin and Francis Darwin (1880), and Gottlieb Haberlandt (1919) is a beautiful demonstration of unceasing continuity of research. Novel findings are integrated into existing hypotheses and models and deepen our understanding of biological principles. At the same time new questions are triggered and hand to hand with this new methodologies are developed to address these new challenges.

Imaging TCSn::GFP, a Synthetic Cytokinin Reporter, in Arabidopsis thaliana

Cytokinins are classical plant hormones that control numerous developmental processes throughout the plant life cycle. Cytokinin-responsive cells activate transcription via a phospho-relay signaling network. Type-B nuclear RESPONSE REGULATOR (RR) proteins mediate transcriptional activation as the final step in the signaling cascade. They bind to promoters of immediate-early target genes via a conserved Myb-related DNA-binding domain. To monitor transcriptional activation in response to a cytokinin stimulus, we have constructed a synthetic promoter, TCS (two-component signaling sensor) that harbors the concatemerized binding motifs for activated type-B RR in an optimized configuration. Here, we describe our protocols for imaging TCSn::GFP expression in transgenic Arabidopsis plants. The use of the fluorescent reporter GFP allows the visualization of cytokinin-responding cells by fluorescent microscopy without the need for tissue processing steps, or staining reactions. This method is fast and with a low risk of artifacts. However, since cytokinin signaling integrates various environmental information including light, nutrient status, and biotic and abiotic stress, special care needs to be devoted to the control of growth conditions.

Cytokinins on the Move

Cytokinins are important signals that participate in different plant processes, and are well known for their strong influence in plant development. With the years, knowledge has been built about their effects, chemical nature, metabolism, and signaling mechanisms. However, one aspect about cytokinins that has been lagging behind is cytokinin transport. Recent reports are providing more information about how cytokinins are transported and how their transport is connected to their effects in development. This review provides a general overview of what is known about cytokinin transport, with a focus on the latest reports.

Characterization of the cytokinin-responsive transcriptome in rice

BACKGROUND

Cytokinin activates transcriptional cascades important for development and the responses to biotic and abiotic stresses. Most of what is known regarding cytokinin-regulated gene expression comes from studies of the dicotyledonous plant Arabidopsis thaliana. To expand the understanding of the cytokinin-regulated transcriptome, we employed RNA-Seq to analyze gene expression in response to cytokinin in roots and shoots of the monocotyledonous plant rice.

RESULTS

We identified over 4,600 and approximately 2,400 genes differentially expressed in response to cytokinin in roots and shoots respectively. There were some similarities in the sets of cytokinin-regulated genes identified in rice and Arabidopsis, including an up-regulation of genes that act to reduce cytokinin function. Consistent with this, we found that the preferred DNA-binding motif of a rice type-B response regulator is similar to those from Arabidopsis. Analysis of the genes regulated by cytokinin in rice revealed a large number of transcription factors, receptor-like kinases, and genes involved in protein degradation, as well as genes involved in development and the response to biotic stress. Consistent with the over-representation of genes involved in biotic stress, there is a substantial overlap in the genes regulated by cytokinin and those differentially expressed in response to pathogen infection, suggesting that cytokinin plays an integral role in the transcriptional response to pathogens in rice, including the induction of a large number of WRKY transcription factors.

CONCLUSIONS

These results begin to unravel the complex gene regulation after cytokinin perception in a crop of agricultural importance and provide insight into the processes and responses modulated by cytokinin in monocots.

Volatile compounds emitted by diverse phytopathogenic microorganisms promote plant growth and flowering through cytokinin action

It is known that volatile emissions from some beneficial rhizosphere microorganisms promote plant growth. Here we show that volatile compounds (VCs) emitted by phylogenetically diverse rhizosphere and non-rhizhosphere bacteria and fungi (including plant pathogens and microbes that do not normally interact mutualistically with plants) promote growth and flowering of various plant species, including crops. In Arabidopsis plants exposed to VCs emitted by the phytopathogen Alternaria alternata, changes included enhancement of photosynthesis and accumulation of high levels of cytokinins (CKs) and sugars. Evidence obtained using transgenic Arabidopsis plants with altered CK status show that CKs play essential roles in this phenomenon, because growth and flowering responses to the VCs were reduced in mutants with CK-deficiency (35S:AtCKX1) or low receptor sensitivity (ahk2/3). Further, we demonstrate that the plant responses to fungal VCs are light-dependent. Transcriptomic analyses of Arabidopsis leaves exposed to A. alternata VCs revealed changes in the expression of light- and CK-responsive genes involved in photosynthesis, growth and flowering. Notably, many genes differentially expressed in plants treated with fungal VCs were also differentially expressed in plants exposed to VCs emitted by the plant growth promoting rhizobacterium Bacillus subtilis GB03, suggesting that plants react to microbial VCs through highly conserved regulatory mechanisms.

Abscisic acid and other plant hormones: Methods to visualize distribution and signaling

The exploration of plant behavior on a cellular scale in a minimal invasive manner is key to understanding plant adaptations to their environment. Plant hormones regulate multiple aspects of growth and development and mediate environmental responses to ensure a successful life cycle. To monitor the dynamics of plant hormone actions in intact tissue, we need qualitative and quantitative tools with high temporal and spatial resolution. Here, we describe a set of biological instruments (reporters) for the analysis of the distribution and signaling of various plant hormones. Furthermore, we provide examples of their utility for gaining novel insights into plant hormone action with a deeper focus on the drought hormone abscisic acid.

Arabidopsis Responds to Alternaria alternata Volatiles by Triggering Plastid Phosphoglucose Isomerase-Independent Mechanisms

Volatile compounds (VCs) emitted by phylogenetically diverse microorganisms (including plant pathogens and microbes that do not normally interact mutualistically with plants) promote photosynthesis, growth, and the accumulation of high levels of starch in leaves through cytokinin (CK)-regulated processes. In Arabidopsis (Arabidopsis thaliana) plants not exposed to VCs, plastidic phosphoglucose isomerase (pPGI) acts as an important determinant of photosynthesis and growth, likely as a consequence of its involvement in the synthesis of plastidic CKs in roots. Moreover, this enzyme plays an important role in connecting the Calvin-Benson cycle with the starch biosynthetic pathway in leaves. To elucidate the mechanisms involved in the responses of plants to microbial VCs and to investigate the extent of pPGI involvement, we characterized pPGI-null pgi1-2 Arabidopsis plants cultured in the presence or absence of VCs emitted by Alternaria alternata We found that volatile emissions from this fungal phytopathogen promote growth, photosynthesis, and the accumulation of plastidic CKs in pgi1-2 leaves. Notably, the mesophyll cells of pgi1-2 leaves accumulated exceptionally high levels of starch following VC exposure. Proteomic analyses revealed that VCs promote global changes in the expression of proteins involved in photosynthesis, starch metabolism, and growth that can account for the observed responses in pgi1-2 plants. The overall data show that Arabidopsis plants can respond to VCs emitted by phytopathogenic microorganisms by triggering pPGI-independent mechanisms.

Cytokinin acts through the auxin influx carrier AUX1 to regulate cell elongation in the root

Hormonal interactions are crucial for plant development. In Arabidopsis, cytokinins inhibit root growth through effects on cell proliferation and cell elongation. Here, we define key mechanistic elements in a regulatory network by which cytokinin inhibits root cell elongation in concert with the hormones auxin and ethylene. The auxin importer AUX1 functions as a positive regulator of cytokinin responses in the root; mutation of AUX1 specifically affects the ability of cytokinin to inhibit cell elongation but not cell proliferation. AUX1 is required for cytokinin-dependent changes of auxin activity in the lateral root cap associated with the control of cell elongation. Cytokinin regulates root cell elongation through ethylene-dependent and -independent mechanisms, both hormonal signals converging on AUX1 as a regulatory hub. An autoregulatory circuit is identified involving the control of ARR10 and AUX1 expression by cytokinin and auxin, this circuit potentially functioning as an oscillator to integrate the effects of these two hormones. Taken together, our results uncover several regulatory circuits controlling interactions of cytokinin with auxin and ethylene, and support a model in which cytokinin regulates shootward auxin transport to control cell elongation and root growth.

Nitric oxide-cytokinin interplay influences selenite sensitivity in Arabidopsis

Selenite oppositely modifies cytokinin and nitric oxide metabolism in Arabidopsis organs. A mutually negative interplay between the molecules exists in selenite-exposed roots; and their overproduction causes selenite insensitivity. Selenium-induced phytotoxicity is accompanied by developmental alterations such as primary root (PR) shortening. Growth changes are provoked by the modulation of hormone status and signalling. Cytokinin (CK) cooperates with the nitric oxide (NO) in many aspects of plant development; however, their interaction under abiotic stress has not been examined. Selenite inhibited the growth of Arabidopsis seedlings and reduced root meristem size through cell division arrest. The CK-dependent pARR5::GUS activity revealed the intensification of CK signalling in the PR tip, which may be partly responsible for the root meristem shortening. The selenite-induced alterations in the in situ expressions of cytokinin oxidases (AtCKX4::GUS, AtCKX5::GUS) are associated with selenite-triggered changes of CK signalling. In wild-type (WT) and NO-deficient nia1nia2 root, selenite led to the diminution of NO content, but CK overproducer ipt-161 and -deficient 35S:CKX2 roots did not show NO decrease. Exogenous NO (S-nitroso-N-acetyl-DL-penicillamine, SNAP) reduced the pARR5::GFP and pTCS::GFP expressions. Roots of the 35S:CKX and cyr1 plants suffered more severe selenite-triggered viability loss than the WT, while in ipt-161 and gsnor1-3 no obvious viability decrease was observed. Exogenous NO ameliorated viability loss, but benzyladenine intensified it. Based on the results, selenite impacts development by oppositely modifying CK signalling and NO level. In the root system, CK signalling intensifies which possibly contributes to the nitrate reductase-independent NO diminution. A mutually negative CK-NO interplay exists in selenite-exposed roots; however, overproduction of both molecules worsens selenite sensing. Hereby, we suggest novel regulatory interplay and role for NO and CK in abiotic stress signalling.

Shoot- and root-borne cytokinin influences arbuscular mycorrhizal symbiosis

The arbuscular mycorrhizal (AM) symbiosis is functionally important for the nutrition and growth of most terrestrial plants. Nearly all phytohormones are employed by plants to regulate the symbiosis with AM fungi, but the regulatory role of cytokinin (CK) is not well understood. Here, we used transgenic tobacco (Nicotiana tabacum) with a root-specific or constitutive expression of CK-degrading CKX genes and the corresponding wild-type to investigate whether a lowered content of CK in roots or in both roots and shoots influences the interaction with the AM fungus Rhizophagus irregularis. Our data indicates that shoot CK has a positive impact on AM fungal development in roots and on the root transcript level of an AM-responsive phosphate transporter gene (NtPT4). A reduced CK content in roots caused shoot and root growth depression following AM colonization, while neither the uptake of phosphorus or nitrogen nor the root transcript levels of NtPT4 were significantly affected. This suggests that root CK may restrict the C availability from the roots to the fungus thus averting parasitism by AM fungi. Taken together, our study indicates that shoot- and root-borne CK have distinct roles in AM symbiosis. We propose a model illustrating how plants may employ CK to regulate nutrient exchange with the ubiquitous AM fungi.

Transcriptomes of Eight Arabidopsis thaliana Accessions Reveal Core Conserved, Genotype- and Organ-Specific Responses to Flooding Stress

Climate change has increased the frequency and severity of flooding events, with significant negative impact on agricultural productivity. These events often submerge plant aerial organs and roots, limiting growth and survival due to a severe reduction in light reactions and gas exchange necessary for photosynthesis and respiration, respectively. To distinguish molecular responses to the compound stress imposed by submergence, we investigated transcriptomic adjustments to darkness in air and under submerged conditions using eight Arabidopsis (Arabidopsis thaliana) accessions differing significantly in sensitivity to submergence. Evaluation of root and rosette transcriptomes revealed an early transcriptional and posttranscriptional response signature that was conserved primarily across genotypes, although flooding susceptibility-associated and genotype-specific responses also were uncovered. Posttranscriptional regulation encompassed darkness- and submergence-induced alternative splicing of transcripts from pathways involved in the alternative mobilization of energy reserves. The organ-specific transcriptome adjustments reflected the distinct physiological status of roots and shoots. Root-specific transcriptome changes included marked up-regulation of chloroplast-encoded photosynthesis and redox-related genes, whereas those of the rosette were related to the regulation of development and growth processes. We identified a novel set of tolerance genes, recognized mainly by quantitative differences. These included a transcriptome signature of more pronounced gluconeogenesis in tolerant accessions, a response that included stress-induced alternative splicing. This study provides organ-specific molecular resolution of genetic variation in submergence responses involving interactions between darkness and low-oxygen constraints of flooding stress and demonstrates that early transcriptome plasticity, including alternative splicing, is associated with the ability to cope with a compound environmental stress.

Survey of Genes Involved in Biosynthesis, Transport, and Signaling of Phytohormones with Focus on Solanum lycopersicum

Phytohormones control the development and growth of plants, as well as their response to biotic and abiotic stress. The seven most well-studied phytohormone classes defined today are as follows: auxins, ethylene, cytokinin, abscisic acid, jasmonic acid, gibberellins, and brassinosteroids. The basic principle of hormone regulation is conserved in all plants, but recent results suggest adaptations of synthesis, transport, or signaling pathways to the architecture and growth environment of different plant species. Thus, we aimed to define the extent to which information from the model plant Arabidopsis thaliana is transferable to other plants such as Solanum lycopersicum. We extracted the co-orthologues of genes coding for major pathway enzymes in A. thaliana from the translated genomes of 12 species from the clade Viridiplantae. Based on predicted domain architecture and localization of the identified proteins from all 13 species, we inspected the conservation of phytohormone pathways. The comparison was complemented by expression analysis of (co-) orthologous genes in S. lycopersicum. Altogether, this information allowed the assignment of putative functional equivalents between A. thaliana and S. lycopersicum but also pointed to some variations between the pathways in eudicots, monocots, mosses, and green algae. These results provide first insights into the conservation of the various phytohormone pathways between the model system A. thaliana and crop plants such as tomato. We conclude that orthologue prediction in combination with analysis of functional domain architecture and intracellular localization and expression studies are sufficient tools to transfer information from model plants to other plant species. Our results support the notion that hormone synthesis, transport, and response for most part of the pathways are conserved, and species-specific variations can be found.

Cytokinins Are Initial Targets of Light in the Control of Bud Outgrowth

Bud outgrowth is controlled by environmental and endogenous factors. Through the use of the photosynthesis inhibitor norflurazon and of masking experiments, evidence is given here that light acts mainly as a morphogenic signal in the triggering of bud outgrowth and that initial steps in the light signaling pathway involve cytokinins (CKs). Indeed, in rose (Rosa hybrida), inhibition of bud outgrowth by darkness is suppressed solely by the application of CKs. In contrast, application of sugars has a limited effect. Exposure of plants to white light (WL) induces a rapid (after 3-6 h of WL exposure) up-regulation of CK synthesis (RhIPT3 and RhIPT5), of CK activation (RhLOG8), and of CK putative transporter RhPUP5 genes and to the repression of the CK degradation RhCKX1 gene in the node. This leads to the accumulation of CKs in the node within 6 h and in the bud at 24 h and to the triggering of bud outgrowth. Molecular analysis of genes involved in major mechanisms of bud outgrowth (strigolactone signaling [RwMAX2], metabolism and transport of auxin [RhPIN1, RhYUC1, and RhTAR1], regulation of sugar sink strength [RhVI, RhSUSY, RhSUC2, and RhSWEET10], and cell division and expansion [RhEXP and RhPCNA]) reveal that, when supplied in darkness, CKs up-regulate their expression as rapidly and as intensely as WL Additionally, up-regulation of CKs by WL promotes xylem flux toward the bud, as evidenced by Methylene Blue accumulation in the bud after CK treatment in the dark. Altogether, these results suggest that CKs are initial components of the light signaling pathway that controls the initiation of bud outgrowth.

Cytokinin Regulates the Etioplast-Chloroplast Transition through the Two-Component Signaling System and Activation of Chloroplast-Related Genes

One of the classical functions of the plant hormone cytokinin is the regulation of plastid development, but the underlying molecular mechanisms remain elusive. In this study, we employed a genetic approach to evaluate the role of cytokinin and its signaling pathway in the light-induced development of chloroplasts from etioplasts in Arabidopsis (Arabidopsis thaliana). Cytokinin increases the rate of greening and stimulates ultrastructural changes characteristic for the etioplast-to-chloroplast transition. The steady-state levels of metabolites of the tetrapyrrole biosynthesis pathway leading to the production of chlorophyll are enhanced by cytokinin. This effect of cytokinin on metabolite levels arises due to the modulation of expression for chlorophyll biosynthesis genes such as HEMA1, GUN4, GUN5, and CHLM Increased expression of HEMA1 is reflected in an enhanced level of the encoded glutamyl-tRNA reductase, which catalyzes one of the rate-limiting steps of chlorophyll biosynthesis. Mutant analysis indicates that the cytokinin receptors ARABIDOPSIS HIS KINASE2 (AHK2) and AHK3 play a central role in this process. Furthermore, the B-type ARABIDOPSIS RESPONSE REGULATOR1 (ARR1), ARR10, and ARR12 play an important role in mediating the transcriptional output during etioplast-chloroplast transition. B-type ARRs bind to the promotors of HEMA1 and LHCB6 genes, indicating that cytokinin-dependent transcription factors directly regulate genes of chlorophyll biosynthesis and the light harvesting complex. Together, these results demonstrate an important role for the cytokinin signaling pathway in chloroplast development, with the direct transcriptional regulation of chlorophyll biosynthesis genes as a key aspect for this hormonal control.

Plant development regulated by cytokinin sinks

Morphogenetic signals control the patterning of multicellular organisms. Cytokinins are mobile signals that are perceived by subsets of plant cells. We found that the responses to cytokinin signaling during Arabidopsis development are constrained by the transporter PURINE PERMEASE 14 (PUP14). In our experiments, the expression of PUP14 was inversely correlated to the cytokinin signaling readout. Loss of PUP14 function allowed ectopic cytokinin signaling accompanied by aberrant morphogenesis in embryos, roots, and the shoot apical meristem. PUP14 protein localized to the plasma membrane and imported bioactive cytokinins, thus depleting apoplastic cytokinin pools and inhibiting perception by plasma membrane-localized cytokinin sensors to create a sink for active ligands. We propose that the spatiotemporal cytokinin sink patterns established by PUP14 determine the cytokinin signaling landscape that shapes the morphogenesis of land plants.

Hormones and nitrate: a two-way connection

During their sessile mode of life, plants need to endure variations in their environment such as a drastic variability in the nutrient concentration in soil solution. It is almost trivial to say that such fluctuations in the soil modify plant growth, development and phase transitions. However, the signaling pathways underlying the connections between nitrogen related signaling and hormonal signaling controlling growth are still poorly documented. This review is meant to present how nitrate/nitrogen controls hormonal pathways. Furthermore, it is very interesting to highlight the increasing evidence that the hormonal signaling pathways themselves seem to feed back control of the nitrate/nitrogen transport and assimilation to adapt nutrition to growth. This thus defines a feed-forward cycle that finely coordinates plant growth and nutrition.

Function of the Golgi-located phosphate transporter PHT4;6 is critical for senescence-associated processes in Arabidopsis

The phosphate transporter PHT4;6 locates to the trans-Golgi compartment, and its impaired activity causes altered intracellular phosphate compartmentation, leading to low cytosolic Pi levels, a blockage of Golgi-related processes such as protein glycosylation and hemicellulose biosynthesis, and a dwarf phenotype. However, it was unclear whether altered Pi homeostasis in pht4;6 mutants causes further cellular problems, typically associated with limited phosphate availability. Here we report that pht4;6 mutants exhibit a markedly increased disposition to induce dark-induced senescence. In control experiments, in which pht4;6 mutants and wild-type plants developed similarly, we confirmed that accelerated dark-induced senescence in mutants is not a 'pleiotropic' process associated with the dwarf phenotype. In fact, accelerated dark-induced senescence in pht4;6 mutants correlates strongly with increased levels of toxic NH4 (+) and higher sensitivity to ammonium, which probably contribute to the inability of pht4;6 mutants to recover from dark treatment. Experiments with modified levels of either salicylic acid (SA) or trans-zeatin (tZ) demonstrate that altered concentrations of these compounds in pht4;6 plants act as major cellular mediators for dark-induced senescence. This conclusion gained further support from the notion that the expression of the pht4;6 gene is, in contrast to genes coding for major phosphate importers, substantially induced by tZ. Taken together, our findings point to a critical function of PHT4;6 to control cellular phosphate levels, in particular the cytosolic Pi availability, required to energize plant primary metabolism for proper plant development. Phosphate and its allocation mediated by PHT4;6 is critical to prevent onset of dark-induced senescence.

The Yin-Yang of Cytokinin Homeostasis and Drought Acclimation/Adaptation

Increasing evidence has shown that cytokinins (CKs) regulate plant drought acclimation/adaptation through a multistep phosphorelay pathway. Recent progress has allowed us to suggest a yin-and-yang-type relationship between CK homeostasis and acclimation/adaptation responses that modulates plant fitness and yields stability under drought through a complex network involving crosstalk with abscisic acid (ABA).

Circadian Stress Regimes Affect the Circadian Clock and Cause Jasmonic Acid-Dependent Cell Death in Cytokinin-Deficient Arabidopsis Plants

The circadian clock helps plants measure daylength and adapt to changes in the day-night rhythm. We found that changes in the light-dark regime triggered stress responses, eventually leading to cell death, in leaves of Arabidopsis thaliana plants with reduced cytokinin levels or defective cytokinin signaling. Prolonged light treatment followed by a dark period induced stress and cell death marker genes while reducing photosynthetic efficiency. This response, called circadian stress, is also characterized by altered expression of clock and clock output genes. In particular, this treatment strongly reduced the expression of CIRCADIAN CLOCK ASSOCIATED1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY). Intriguingly, similar changes in gene expression and cell death were observed in clock mutants lacking proper CCA1 and LHY function. Circadian stress caused strong changes in reactive oxygen species- and jasmonic acid (JA)-related gene expression. The activation of the JA pathway, involving the accumulation of JA metabolites, was crucial for the induction of cell death, since the cell death phenotype was strongly reduced in the jasmonate resistant1 mutant background. We propose that adaptation to circadian stress regimes requires a normal cytokinin status which, acting primarily through the AHK3 receptor, supports circadian clock function to guard against the detrimental effects of circadian stress.

Plant cytokinin signalling

Cytokinin is an essential plant hormone that is involved in a wide range of plant growth and developmental processes which are controlled through its signalling pathway. Cytokinins are a class of molecules that are N(6)-substituted adenine derivatives, such as isopentenyl adenine, and trans- and cis-zeatin, which are common in most plants. The ability to perceive and respond to cytokinin occurs through a modified bacterial two-component pathway that functions via a multi-step phosphorelay. This cytokinin signalling process is a crucial part of almost all stages of plant life, from embryo patterning to apical meristem regulation, organ development and eventually senescence. The cytokinin signalling pathway involves the co-ordination of three types of proteins: histidine kinase receptors to perceive the signal, histidine phosphotransfer proteins to relay the signal, and response regulators to provide signal output. This pathway contains both positive and negative elements that function in a complex co-ordinated manner to control cytokinin-regulated plant responses. Although much is known about how this cytokinin signal is perceived and initially regulated, there are still many avenues that need to be explored before the role of cytokinin in the control of plant processes is fully understood.

The Putative O-Linked N-Acetylglucosamine Transferase SPINDLY Inhibits Class I TCP Proteolysis to Promote Sensitivity to Cytokinin

Arabidopsis (Arabidopsis thaliana) SPINDLY (SPY) is a putative serine and threonine O-linked N-acetylglucosamine transferase (OGT). While SPY has been shown to suppress gibberellin signaling and to promote cytokinin (CK) responses, its catalytic OGT activity was never demonstrated and its effect on protein fate is not known. We previously showed that SPY interacts physically and functionally with TCP14 and TCP15 to promote CK responses. Here, we aimed to identify how SPY regulates TCP14/15 activities and how these TCPs promote CK responses. We show that SPY activity is required for TCP14 stability. Mutation in the putative OGT domain of SPY (spy-3) stimulated TCP14 proteolysis by the 26S proteasome, which was reversed by mutation in CULLIN1 (CUL1), suggesting a role for SKP, CUL1, F-box E3 ubiquitin ligase in TCP14 proteolysis. TCP14 proteolysis in spy-3 suppressed all TCP14 misexpression phenotypes, including the enhanced CK responses. The increased CK activity in TCP14/15-overexpressing flowers resulted from increased sensitivity to the hormone and not from higher CK levels. TCP15 overexpression enhanced the response of the CK-induced synthetic promoter pTCS to CK, suggesting that TCP14/15 affect early steps in CK signaling. We propose that posttranslational modification of TCP14/15 by SPY inhibits their proteolysis and that the accumulated proteins promote the activity of the CK phosphorelay cascade in developing Arabidopsis leaves and flowers.

Homodimerization of Ehd1 Is Required to Induce Flowering in Rice

In plants, flowering time is elaborately controlled by various environment factors. Ultimately, florigens such as FLOWERING LOCUS T (FT) or FT-like molecules induce flowering. In rice (Oryza sativa), Early heading date 1 (Ehd1) is a major inducer of florigen gene expression. Although Ehd1 is highly homologous to the type-B response regulator (RR) family in the cytokinin signaling pathway, its precise molecular mechanism is not well understood. In this study, we showed that the C-terminal portion of the protein containing the GARP DNA-binding (G) domain can promote flowering when overexpressed. We also observed that the N-terminal portion of Ehd1, carrying the receiver (R) domain, delays flowering by inhibiting endogenous Ehd1 activity. Ehd1 protein forms a homomer via a 16-amino acid region in the inter domain between R and G. From the site-directed mutagenesis analyses, we demonstrated that phosphorylation of the Asp-63 residue within the R domain induces the homomerization of Ehd1, which is crucial for Ehd1 activity. A type-A RR, OsRR1, physically interacts with Ehd1 to form a heterodimer. In addition, OsRR1-overexpressing plants show a late-flowering phenotype. Based on these observations, we conclude that OsRR1 inhibits Ehd1 activity by binding to form an inactive complex.

Cytokinin Synthesis, Signaling, and Function--Advances and New Insights

The plant hormones referred to as cytokinins are chemical signals that control numerous developmental processes throughout the plant life cycle, including gametogenesis, root meristem specification, vascular development, shoot and root growth, meristem homeostasis, senescence, and more. In addition, they mediate responses to environmental cues such as light, stress, and nutrient conditions. The core mechanistics of cytokinin metabolism and signaling have been elucidated, but more layers of regulation, additional functions, and interactions with other signals are continuously discovered and described. In this chapter, we recapitulate the highlights of over 100 years of cytokinin research covering its isolation, the elucidation of phosphorelay signaling, and how cytokinin functions in various developmental contexts including its interaction with other pathways. Additionally, given cytokinin's paracrine signaling mechanism, we postulate that cellular exporters for cytokinins exist.

The SHORT-ROOT-like gene PtSHR2B is involved in Populus phellogen activity

SHORT-ROOT (SHR) is a GRAS transcription factor first characterized for its role in the specification of the stem cell niche and radial patterning in Arabidopsis thaliana (At) roots. Three SHR-like genes have been identified in Populus trichocarpa (Pt). PtSHR1 shares high similarity with AtSHR over the entire length of the coding sequence. The two other Populus SHR-like genes, PtSHR2A and PtSHR2B, are shorter in their 5' ends when compared with AtSHR. Unlike PtSHR1, that is expressed throughout the cambial zone of greenhouse-grown Populus trees, PtSHR2Bprom:uidA expression was detected in the phellogen. Additionally, PtSHR1 and PtSHR2B expression patterns markedly differ in the shoot apex and roots of in vitro plants. Transgenic hybrid aspen expressing PtSHR2B under the 35S constitutive promoter showed overall reduced tree growth while the proportion of bark increased relative to the wood. Reverse transcription-quantitative PCR (RT-qPCR) revealed increased transcript levels of cytokinin metabolism and response-related genes in the transgenic plants consistent with an increase of total cytokinin levels. This was confirmed by cytokinin quantification by LC-MS/MS. Our results indicate that PtSHR2B appears to function in the phellogen and therefore in the regulation of phellem and periderm formation, possibly acting through modulation of cytokinin homeostasis. Furthermore, this work points to a functional diversification of SHR after the divergence of the Populus and Arabidopsis lineages. This finding may contribute to selection and breeding strategies of cork oak in which, unlike Populus, the phellogen is active throughout the entire tree lifespan, being at the basis of a highly profitable cork industry.

Arabidopsis type B cytokinin response regulators ARR1, ARR10, and ARR12 negatively regulate plant responses to drought

In this study, we used a loss-of-function approach to elucidate the functions of three Arabidopsis type B response regulators (ARRs)--namely ARR1, ARR10, and ARR12--in regulating the Arabidopsis plant responses to drought. The arr1,10,12 triple mutant showed a significant increase in drought tolerance versus WT plants, as indicated by its higher relative water content and survival rate on drying soil. This enhanced drought tolerance of arr1,10,12 plants can be attributed to enhanced cell membrane integrity, increased anthocyanin biosynthesis, abscisic acid (ABA) hypersensitivity, and reduced stomatal aperture, but not to altered stomatal density. Further drought-tolerance tests of lower-order double and single mutants indicated that ARR1, ARR10, and ARR12 negatively and redundantly control plant responses to drought, with ARR1 appearing to bear the most critical function among the three proteins. In agreement with these findings, a comparative genome-wide analysis of the leaves of arr1,10,12 and WT plants under both normal and dehydration conditions suggested a cytokinin (CK) signaling-mediated network controlling plant adaptation to drought via many dehydration/drought- and/or ABA-responsive genes that can provide osmotic adjustment and protection to cellular and membrane structures. Expression of all three ARR genes was repressed by dehydration and ABA treatments, inferring that plants down-regulate these genes as an adaptive mechanism to survive drought. Collectively, our results demonstrate that repression of CK response, and thus CK signaling, is one of the strategies plants use to cope with water deficit, providing novel insight for the design of drought-tolerant plants by genetic engineering.

Centering the Organizing Center in the Arabidopsis thaliana Shoot Apical Meristem by a Combination of Cytokinin Signaling and Self-Organization

Plants have the ability to continously generate new organs by maintaining populations of stem cells throught their lives. The shoot apical meristem (SAM) provides a stable environment for the maintenance of stem cells. All cells inside the SAM divide, yet boundaries and patterns are maintained. Experimental evidence indicates that patterning is independent of cell lineage, thus a dynamic self-regulatory mechanism is required. A pivotal role in the organization of the SAM is played by the WUSCHEL gene (WUS). An important question in this regard is that how WUS expression is positioned in the SAM via a cell-lineage independent signaling mechanism. In this study we demonstrate via mathematical modeling that a combination of an inhibitor of the Cytokinin (CK) receptor, Arabidopsis histidine kinase 4 (AHK4) and two morphogens originating from the top cell layer, can plausibly account for the cell lineage-independent centering of WUS expression within SAM. Furthermore, our laser ablation and microsurgical experiments support the hypothesis that patterning in SAM occurs at the level of CK reception and signaling. The model suggests that the interplay between CK signaling, WUS/CLV feedback loop and boundary signals can account for positioning of the WUS expression, and provides directions for further experimental investigation.

CHASE domain-containing receptors play an essential role in the cytokinin response of the moss Physcomitrella patens

While the molecular basis for cytokinin action is quite well understood in flowering plants, little is known about the cytokinin signal transduction in early diverging land plants. The genome of the bryophyte Physcomitrella patens (Hedw.) B.S. encodes three classical cytokinin receptors, the CHASE domain-containing histidine kinases, CHK1, CHK2, and CHK3. In a complementation assay with protoplasts of receptor-deficient Arabidopsis thaliana as well as in cytokinin binding assays, we found evidence that CHK1 and CHK2 receptors can function in cytokinin perception. Using gene targeting, we generated a collection of CHK knockout mutants comprising single (Δchk1, Δchk2, Δchk3), double (Δchk1,2, Δchk1,3, Δchk2,3), and triple (Δchk1,2,3) mutants. Mutants were characterized for their cytokinin response and differentiation capacities. While the wild type did not grow on high doses of cytokinin (1 µM benzyladenine), the Δchk1,2,3 mutant exhibited normal protonema growth. Bud induction assays showed that all three cytokinin receptors contribute to the triggering of budding, albeit to different extents. Furthermore, while the triple mutant showed no response in this bioassay, the remaining mutants displayed budding responses in a diverse manner to different types and concentrations of cytokinins. Determination of cytokinin levels in mutants showed no drastic changes for any of the cytokinins; thus, in contrast to Arabidopsis, revealing only small impacts of cytokinin signaling on homeostasis. In summary, our study provides a first insight into the molecular action of cytokinin in an early diverging land plant and demonstrates that CHK receptors play an essential role in bud induction and gametophore development.

Efficient and Inducible Use of Artificial MicroRNAs in Marchantia polymorpha

We describe the efficient use of artificial microRNAs (amiRs) in Marchantia polymorpha using both endogenous and heterologous primary microRNA (pri-miR) hairpin backbones. Targeting of two transcription factor genes, MpARF1 and MpRR-B, mediating different hormonal responses, demonstrated that amiRs can create specific and reproducible physiological and morphological defects, facilitating interpretation of gene function. A third amiR was designed to target a gene encoding a component of the Polycomb recessive complex 2, MpE(z), and constitutive expression of this amiR results in sporeling lethality. Adaptation of an estrogen-inducible system allowed analysis of the phenotypic effects of induction of this amiR during other stages of the life cycle. We discuss the advantages and challenges of the use of amiRs as a tool for reverse genetic analysis in M. polymorpha.

Structural Aspects of Multistep Phosphorelay-Mediated Signaling in Plants

The multistep phosphorelay (MSP) is a central signaling pathway in plants integrating a wide spectrum of hormonal and environmental inputs and controlling numerous developmental adaptations. For the thorough comprehension of the molecular mechanisms underlying the MSP-mediated signal recognition and transduction, the detailed structural characterization of individual members of the pathway is critical. In this review we describe and discuss the recently known crystal and nuclear magnetic resonance structures of proteins acting in MSP signaling in higher plants, focusing particularly on cytokinin and ethylene signaling in Arabidopsis thaliana. We discuss the range of functional aspects of available structural information including determination of ligand specificity, activation of the receptor via its autophosphorylation, and downstream signal transduction through the phosphorelay. We compare the plant structures with their bacterial counterparts and show that although the overall similarity is high, the differences in structural details are frequent and functionally important. Finally, we discuss emerging knowledge on molecular recognition mechanisms in the MSP, and mention the latest findings regarding structural determinants of signaling specificity in the Arabidopsis MSP that could serve as a general model of this pathway in all higher plants.

Gene expression and functional analyses in brassinosteroid-mediated stress tolerance

The plant hormone brassinosteroid (BR) plays essential roles in plant growth and development, while also controlling plant stress responses. This dual ability of BR is intriguing from a mechanistic point of view and as a viable solution for stabilizing crop yields under the changing climatic conditions. Here we report a time course analysis of BR responses under both stress and no-stress conditions, the results of which establish that BR incorporates many stress-related features even under no-stress conditions, which are then accompanied by a dynamic stress response under unfavourable conditions. Found within the BR transcriptome were distinct molecular signatures of two stress hormones, abscisic acid and jasmonic acid, which were correlated with enhanced endogenous levels of the two hormones in BR-treated seedlings. The marked presence of genes related to protein metabolism and modification, defence responses and calcium signalling highlights the significance of their associated mechanisms and roles in BR processes. Functional analysis of loss-of-function mutants of a subset of genes selected from the BR transcriptome identified abiotic stress-related roles for ACID PHOSPHATASE5 (ACP5), WRKY33, JACALIN-RELATED LECTIN1-3 (JAC-LEC1-3) and a BR-RESPONSIVE-RECEPTOR-LIKE KINASE (BRRLK). Overall, the results of this study provide a clear link between the molecular changes impacted by BR and its ability to confer broad-range stress tolerance, emphasize the importance of post-translational modification and protein turnover as BR regulatory mechanisms and demonstrate the BR transcriptome as a repertoire of new stress-related regulatory and structural genes.