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The signal transduction of the plant hormone cytokinin is mediated by a His-to-Asp phosphorelay. The canonical cytokinin receptor consists of an extra cytoplasmic hormone binding domain named cyclase/histidine kinase associated sensory extracellular (CHASE) and cytoplasmic histidine kinase and receiver domains. In addition to classical cytokinin receptors, a different type receptor—named CHASE domain receptor serine/threonine kinase (CHARK)—is also present in rice. It contains the same ligand binding domain as other cytokinin receptors but has a predicted Ser/Thr—instead of a His-kinase domain. Bioinformatic analysis indicates that CHARK is a retrogene and a product of trans-splicing. Here, we analyzed whether CHARK can function as a bona fide cytokinin receptor. A biochemical assay demonstrated its ability to bind cytokinin. Transient expression of CHARK in protoplasts increased their response to cytokinin. Expression of CHARK in an Arabidopsis receptor double mutant complemented its growth defects and restored the ability to activate cytokinin response genes, clearly demonstrating that CHARK functions as a cytokinin receptor. We propose that the CHARK gene presents an evolutionary novelty in the cytokinin signaling system.

Cold stress, including freezing stress and chilling stress, is one of the major environmental factors that limit the growth and productivity of plants. As a temperate dicot model plant species, Arabidopsis develops a capability to freezing tolerance through cold acclimation. The past decades have witnessed a deep understanding of mechanisms underlying cold stress signal perception, transduction, and freezing tolerance in Arabidopsis. In contrast, a monocot cereal model plant species derived from tropical and subtropical origins, rice, is very sensitive to chilling stress and has evolved a different mechanism for chilling stress signaling and response. In this review, the authors summarized the recent progress in our understanding of cold stress response mechanisms, highlighted the convergent and divergent mechanisms between Arabidopsis and rice plasma membrane cold stress perceptions, calcium signaling, phospholipid signaling, MAPK cascade signaling, ROS signaling, and ICE-CBF regulatory network, as well as light-regulated signal transduction system. Genetic engineering approaches of developing freezing tolerant Arabidopsis and chilling tolerant rice were also reviewed. Finally, the future perspective of cold stress signaling and tolerance in rice was proposed.

An excess of aluminum (Al) is a major factor limiting crop production in acidic soils. Secretion of organic acids (OAs) from the root apex of diverse plant species or genotypes via activation of anion channels has been recognized as the most important mechanism of Al exclusion. Citric, oxalic, and malic acids are the most effective OAs in detoxifying Al. In this review, we summarize biochemical properties of OAs secreted by plants. We also highlight the molecular mechanisms of Al signal perception, Al transport, signal regulators associated with OAs secretion, as well as interactions between Al and hormone signaling pathways. Based on a comprehensive understanding of the relationship between signal modulators and regulation of expression of relevant genes, a signal transduction model for Al-induced OAs secretion is proposed.

'Summary' 932 I. 'Introduction' 932 II. 'Major transcription factor families involved in plant immunity' 933 III. 'Regulation of transcriptional complexes' 934 IV. 'Transcription factor networks in phytohormone signaling' 938 V. 'Discussion and perspectives' 941 'Acknowledgements' 943 References 943 Summary Next to numerous abiotic stresses, plants are constantly exposed to a variety of pathogens within their environment. Thus, their ability to survive and prosper during the course of evolution was strongly dependent on adapting efficient strategies to perceive and to respond to such potential threats. It is therefore not surprising that modern plants have a highly sophisticated immune repertoire consisting of diverse signal perception and intracellular signaling pathways. This signaling network is intricate and deeply interconnected, probably reflecting the diverse lifestyles and infection strategies used by the multitude of invading phytopathogens. Moreover it allows signal communication between developmental and defense programs thereby ensuring that plant growth and fitness are not significantly retarded. How plants integrate and prioritize the incoming signals and how this information is transduced to enable appropriate immune responses is currently a major research area. An important finding has been that pathogen-triggered cellular responses involve massive transcriptional reprogramming within the host. Additional key observations emerging from such studies are that transcription factors (TFs) are often sites of signal convergence and that signal-regulated TFs act in concert with other context-specific TFs and transcriptional co-regulators to establish sensory transcription regulatory networks required for plant immunity.

Many tip-growing cells are capable of responding to guidance cues, during which cells precisely steer their growth toward the source of guidance signals. Though several players in signal perception have been identified, little is known about the downstream signaling that controls growth direction during guidance. Here, using combined modeling and experimental studies, we demonstrate that the growth guidance of Arabidopsis pollen tubes is regulated by the signaling network that controls tip growth. Tip-localized exocytosis plays a key role in this network by integrating guidance signals with the ROP1 Rho GTPase signaling and coordinating intracellular signaling with cell wall mechanics. This model reproduces the high robustness and responsiveness of pollen tube guidance and explains the connection between guidance efficiency and the parameters of the tip growth system. Hence, our findings establish an exocytosis-coordinated mechanism underlying the cellular pathfinding guided by signal gradients and the mechanistic linkage between tip growth and guidance., Tip-growing cells can find their growing path toward the source of attractive signals. Here, using experimental data and mathematical modeling, Luo et al. demonstrate that tip-localized exocytosis can integrate guidance cues with Rho GTPase signaling to control cell wall mechanics and direct tip growth in Arabidopsis pollen tubes.

Bacterial pathogens encounter several different environments during an infection, many of them possibly being detrimental. In order to sense its surroundings and adjust the gene expression accordingly, different regulatory schemes are undertaken. With these, the bacterium appropriately can differentiate between various environmental cues to express the correct virulence factor at the appropriate time and place. An attractive regulator device is RNA, which has an outstanding ability to alter its structure in response to external stimuli, such as metabolite concentration or alterations in temperature, to control its downstream gene expression. This review will describe the function of riboswitches and thermometers, with a particular emphasis on regulatory RNAs being important for bacterial pathogenicity. WIREs RNA 2017, 8:e1429. doi: 10.1002/wrna.1429 For further resources related to this article, please visit the WIREs website.

Signal perception and transduction is a key mechanism ensuring bacterial adaptation to environmental changes. By connecting two-component signal transduction systems and small regulatory RNAs, multiple stimuli can be integrated to fine-tune gene expression. The unique properties of small RNAs extend the flexibility, timing, and dynamics of two-component system signaling. Bacteria use networks of two-component systems and small RNAs to control social behavior such as virulence or quorum sensing.

Après avoir été longtemps dépendante des pesticides, l’agriculture mondiale est aujourd’hui frappée par un courant qui favorise des pratiques plus durables et plus respectueuses de l’environnement. Pour répondre à ces nouvelles exigences, les agriculteurs doivent se tourner vers l’exploitation et la rentabilisation des ressources naturelles par le biais de pratiques agricoles combinant la performance et la protection des cultures à un moindre coût écologique. Dans ce contexte, le développement de molécules biologiques capables de stimuler les défenses naturelles des végétaux (SDN) est une stratégie qui attire de plus en plus l’attention. Une molécule SDN est un éliciteur susceptible de déclencher une série d’évènements biochimiques menant à l’expression de la résistance chez la plante. La perception du signal par des récepteurs membranaires spécifiques et sa transduction par diverses voies de signalisation conduisent à la synthèse et à l’accumulation synchronisée de molécules défensives parmi lesquelles certaines jouent un rôle structural alors que d’autres exercent une fonction antimicrobienne directe. Les barrières structurales contribuent à retarder la progression de l’agent pathogène dans les tissus de la plante et à empêcher la diffusion de substances délétères telles des enzymes de dégradation des parois ou des toxines. Les mécanismes biochimiques incluent, entre autres, la synthèse de protéines de stress et d’inhibiteurs de protéases ainsi que la production de phytoalexines, des métabolites secondaires ayant un fort potentiel antimicrobien. Les progrès remarquables accomplis ces dernières années en termes de compréhension des mécanismes impliqués dans la résistance induite chez les plantes se traduisent aujourd’hui par la commercialisation d’un nombre de plus en plus important de SDN capables de stimuler le « système immunitaire » des plantes en mimant l’effet des agents pathogènes., For a long time, agriculture across the world was dependent on pesticides. Today, it is influenced by a movement that favours practices that are more sustainable and environmentally safe. To meet these new demands, producers wishing to exploit natural resources and make them profitable must turn to new agronomic practices that combine culture performance and protection at a low environmental cost. In this context, the development of biological molecules able to stimulate natural defence mechanisms in plants (or SDN, for “stimulateurs des défenses naturelles des plantes”) is a strategy that attracts much attention. A SDN molecule is an elicitor capable of initiating a series of biochemical events leading to the expression of disease resistance in plants. Signal perception by ultra-specific membrane receptors and its transduction by diverse signalling pathways lead to the synthesis and synchronized accumulation of defence molecules; some of these molecules play a structural role while others have a direct antimicrobial function. Structural barriers contribute to delaying pathogen ingress in plant tissues and prevent the diffusion of deleterious substances such as cell wall hydrolytic enzymes or toxins. Biochemical mechanisms include, among others, the synthesis of stress proteins and protease inhibitors as well as the production of phytoalexins, which are secondary metabolites with a strong antimicrobial potential. The remarkable advances made over the last few years as regards our understanding of the mechanisms involved in induced resistance in plants now translate into the marketing of an increasing number of SDN molecules able to stimulate a plant’s “immune system” by mimicking the effect of pathogens.

The endoplasmic reticulum quality control (ER-QC) is a conserved mechanism in surveillance of secreted signaling factors during cell-to-cell communication in eukaryotes. Recent data show that the ER-QC plays important roles in diverse cell-to-cell signaling processes during immune response, vegetative and reproductive development in plants. Pollen tube guidance is a precisely guided cell-cell communication process between the male and female gametophytes during plant reproduction. Recently, the female signal has been identified as small secreted peptides, but how the pollen tube responds to this signal is still unclear. In this review, we intend to summarize the role of ER-QC in plants and discuss the recent advances regarding our understanding of the mechanism of pollen tube response to the female signals.

BACKGROUND AND AIMS The phytohormone abscisic acid (ABA) plays a vital role in various aspects of plant growth and development and in adaptation of plants to various environmental stresses. Cell response to ABA is initiated by ABA perception with a cell receptor. Recently, three distinct ABA receptors have been identified, opening a door to uncover the initial events of ABA signal transduction. The aim of this Botanical Briefing is to present a perspective of the ABA receptors identified. SCOPE This Briefing offers an introduction to the three ABA receptors identified and an analysis of the complexity and multiplicity of ABA receptors, and provides some viewpoints on future research.

Brassinoesteroides sao esteroides vegetais, essenciais para o crescimento e o desenvolvimento, que apresentam um oxigenio no carbono C-3 e oxigenios adicionais em um ou mais dos atomos de carbono C-2, C-6, C-22 e C-23. Nos ultimos anos, a aplicacao de tecnicas de genetica molecular possibilitou progresso significativo no entendimento da regulacao da via biossintetica e na identificacao de varios componentes da via de transducao de sinal de brassinoesteroides. Buscas em bases de dados eletronicas mostram duzias de registros para genes relacionados a brassinoesteroides nos ultimos doze meses, demonstrando os grandes esforcos desenvolvidos neste campo. Esta revisao destaca os recentes avancos na caracterizacao de genes e mutacoes que estao auxiliando na elucidacao dos mecanismos moleculares envolvidos na sintese/metabolismo, e percepcao e resposta de brassinoesteroides, com enfase especial no seu papel no alongamento de celulas vegetais. Aspectos do envolvimento de BRs na regulacao de proteinas que controlam o ciclo cellular tambem sao discutidos.

Gravity is a fundamental factor which affects all living organisms. Plant development is well adapted to gravity by directing roots downward and shoots upwards. For more than a century, plant biologists have been fascinated to describe the molecular mechanisms underlying the gravitropic response of plants. Important progress towards signal perception, transduction, and response has been made, but new tools are beginning to uncover the regulatory networks for gravitropic control. We summarise recent progress in study of gravitropism and discuss strategies to identify the molecular basis of the gravity response in Arabidopsis thaliana. This will put us on a road towards the molecular systems biology of the Arabidopsis gravitropic response.

Farmers have used metal compounds in phytosanitary treatments for more than a century; however, it has recently been suggested that plants absorb high concentrations of metals from the substrate as a self-defense mechanism against pathogens and herbivores. This metal defense hypothesis is among the most attractive proposals for the 'reason to be' of metal hyperaccumulator species. On a molecular basis, metal defense against biotic stress seems to imply common and/or complementary pathways of signal perception, signal transduction and metabolism. This does not imply a broad band of co-resistance to different stress types but reflects a continuous cross talk during the coevolution of plants, pathogens and herbivores competing in an environment where efficient metal ion acquisition and ion homeostasis are essential for survival.

The roots of most extant plants are able to become engaged in an interaction with a small group of fungi of the fungal order Glomales (Glomeromycota). This interaction-arbuscular mycorrhizal (AM) symbiosis-is the evolutionary precursor of most other mutualistic root-microbe associations. The molecular analysis of this interaction can elucidate basic principles regarding such associations. This review summarizes our present knowledge about cellular and molecular aspects of AM. Emphasis is placed on morphological changes in colonized cells, transfer of nutrients between both interacting partners, and plant defence responses. Similarities to and differences from other associations of plant and microorganisms are highlighted regarding defence reactions and signal perception.

Background The ColRS two-component system has been shown to contribute to the membrane functionality and stress tolerance of Pseudomonas putida as well as to the virulence of Pseudomonas aeruginosa and plant pathogenic Xanthomonas species. However, the conditions activating the ColRS pathway and the signal(s) sensed by ColS have remained unknown. Here we aimed to analyze the role of the ColRS system in metal tolerance of P. putida and to test whether ColS can respond to metal excess. Results We show that the ColRS system is necessary for P. putida to tolerate the excess of iron and zinc, and that it also contributes to manganese and cadmium tolerance. Excess of iron, zinc, manganese or cadmium activates ColRS signaling and as a result modifies the expression of ColR-regulated genes. Our data suggest that the genes in the ColR regulon are functionally redundant, as several loci have to be deleted to observe a significant decrease in metal tolerance. Site-directed mutagenesis of ColS revealed that excess of iron and, surprisingly, also zinc are sensed by a conserved ExxE motif in ColS’s periplasmic domain. While ColS is able to sense different metals, it still discriminates between the two oxidation states of iron, specifically responding to ferric and not ferrous iron. We propose a signal perception model involving a dimeric ColS, where each monomer donates one ExxE motif for metal binding. Conclusions Several transition metals are essential for living organisms in certain amounts, but toxic in excess. We show that ColRS is a sensor system which detects and responds to the excess of physiologically important metals such as zinc, iron and manganese. Thus, the ColRS system is an important factor for metal homeostasis and tolerance in P. putida.

The revised local curvature distribution model, which provides accurate computer simulations of the gravitropic response of mushroom stems, was found to produce accurate simulations of the gravitropic reaction of wheat (Triticum aestivum) coleoptiles. The key feature of the mathematical model that enables it to approach universality of application is the assumption that the stem has an autonomic straightening reaction (curvature compensation or 'autotropism'). In the model, the local bending rate for any segment of the organ is determined by the difference between the 'bending signal' (generated by the gravitropic signal perception system) and a 'straightening signal' (which is proportional to the local curvature of the segment). The model reveals three major differences between the gravitropic reactions of wheat coleoptiles and Coprinus mushroom stems. First, in Coprinus, the capacity for autonomic straightening is much more concentrated in the apical region of the stem. Second, local perception of the gravitropic signal, which is necessary for exact simulation in Coprinus, is not needed in wheat coleoptiles (the corresponding constant in the model can be set to zero). Third, the transmission rate of the gravitropic signal is about seven times faster in wheat coleoptiles than in the mushroom stem. Thus, we demonstrate that a single model, depending on the values given to its parameters, is able to simulate the spatial organization of the gravitropic reaction of wheat coleoptiles and Coprinus mushroom stems. The model promises to be a valuable predictive tool in guiding future research into the gravitropic reaction of axial organs of all types.

The functional, spatial and temporal complexity of pathogen defence in plants is becoming ever-more apparent. Functional complexity begins with the exogenous signals perceived from the pathogen, continues with the mechanisms of signal perception and signal transduction, and results in extensive ‘reprogramming' of cellular metabolism, involving large changes in gene activity. The spatial organization of these reactions is similarly complex and affects intracellular compartmentation, the fate of cells and, ultimately, the tissues that surround the infection site. The highly dynamic nature of the response adds temporal complexity. Thus, pathogen defence entails a major shift in metabolic activity, rather than altered expression of a few unique, defence-related genes. The observed complexity serves as a paradigm of the flexibility and plasticity of plant metabolism.

Strigolactones belong to a newly identified class of plant hormones that are involved in the inhibition of shoot branching. Prior to this finding, strigolactones were proven to be root rhizosphere-signaling molecules that mediate plant–parasitic plant, and the symbiotic plant–AM fungi interactions. More recently, strigolactones were shown to have other biological functions as endogenous plant hormones in shoot development, root architecture, and seed germination (also in nonparasitic plants) and to regulate plant developmental processes in interaction with other signaling pathways (i.e., light and senescence signaling) or hormones. Gene discovery in the strigolactone biosynthesis and signal perception pathways is a key step in elucidating the mechanism and mode of action of the existing roles and discovering potential additional roles of strigolactones. Furthermore, insights into strigolactone and strigolactone-associated pathways will provide more knowledge for the control of parasitic weeds and improvement of crop yield. In this chapter, we outline different aspects of the roles that strigolactones play both in the rhizosphere and during plant development. Gene characterization in strigolactone pathways and strigolactone-related hormone cross-talk is also addressed.

Environmental stresses such as drought, high salinity, high temperature, and cold stresses are the major factors which cause significant losses to the yields of economically important crops. Sequence-specific binding transcription factors (TFs) are known to be involved in regulation of stress responses. In this chapter, we review our current knowledge about the functions of various TFs in stress responses, including the major DREB/CBF, AREB/ABF, and HSF TFs, and their signaling cascades. Understanding the mechanisms of how the TFs act as “master regulators” in the signal transduction networks involved in the conversion of stress signal perception to stress-responsive gene expression will enable us to develop stress-tolerant crops by genetic engineering of the major TFs.

Two-component systems (TCS) involving the His-Asp phosphotransfer are commonly utilized for signal transduction in prokaryotes in which the two essential components are a sensor histidine kinase (HK) receptor and a response regulator (RR). Despite great efforts in structural and functional characterization of signal perception mechanisms, the exact signaling mechanisms remain elusive for many TCSs. Mimicking the natural TCS signaling pathways, chimeric receptor kinases and response regulators have been constructed through the process of swapping modular domains of related TCSs. To design chimeras with new signaling pathways, domains from different proteins that have little relationship at the primary structural level but carrying desirable functional properties can be conjoined to engineer novel TCSs. These chimeras maintain the ability to respond to environmental stimulants by regulating protein phosphorylation to produce downstream output signals. Depending on the nature of external signals, chimeric TCSs can serve as a novel tool not only to examine the natural signaling mechanisms in TCSs, but also for industrial and clinical applications.

Cyanobacteria are primitive photosynthetic oxygen-evolving prokaryotes that appeared on the Earth when there was no ozone layer to protect them from damaging ultraviolet radiation (UVR). UVR has both direct and indirect effects on the cyanobacteria due to absorption by biomolecules and UVR-induced oxidative stress, respectively. However, these organisms have developed several lines of mitigation strategies/defense mechanisms such as avoidance, scavenging, screening, repair and programmed cell death to counteract the damaging effects of UVR. This review presents an update on the effects of UVR on cyanobacteria and the defense mechanisms employed by these prokaryotes to withstand UVR stress. In addition, recent developments in the field of molecular biology of UV-absorbing compounds such as mycosporine-like amino acids and scytonemin, are also added and the possible role of programmed cell death, signal perception as well their transduction under UVR stress is being discussed.

Two-component systems (TCS) regulate pathogenic commitment in many interactions and provide an opportunity for unique therapeutic intervention. The VirA/VirG TCS of Agrobacterium tummefaciens mediates inter-kingdom gene transfer in the development of host tumors and sets in motion the events that underlie the great success of this multi-host plant pathogen. Significant proof for the feasibility of interventions has now emerged with the discovery of a natural product that effectively “blinds” the pathogen to the host via inhibition of VirA/VirG signal transduction. Moreover, the emerging studies on the mechanism of signal perception have revealed general sites suitable for intervention of TCS signaling. Given the extensive functional homology, it should now be possible to transfer the models discovered for VirA/VirG broadly to other pathogenic interactions.

Plants release volatiles after herbivore attack in a highly regulated fashion. These compounds attract natural enemies and function as indirect defenses. Whether neighboring plants ‘eavesdrop’ on these volatile signals and tailor their defenses accordingly remains controversial. Recent laboratory studies have identified transcriptional changes that occur in plants in response to certain volatiles. These changes occur under conditions that enhance the probability of signal perception and response. Field studies have demonstrated repeatable increases in the herbivore resistance of plants growing downwind of damaged plants.

Howard R. G. Clarke,’ John A. Leigh,” and Carl J. Douglast *Department of Microbiology University of Washington Seattle, Washington 98195 tDepartment of Botany University of British Columbia Vancouver, British Columbia V8T 124 Canada Plants coexist with a great number of microorganisms. Plant-microbe interactions can be beneficial, as in symbi- otic Rhizobium-legume and mycorrhizal-root associa- tions, but in other cases, microbes represent potential pathogens with the ability to parasitize plants and cause disease. Early in the interactions between plants and mi- crobes, signals are produced that elicit discrete responses in the respective partners, the first steps in the cascade that determines the outcomeof the symbiosisor pathogen- esis or the expression of disease resistance. The sixth International Symposium on Molecular Plant-Microbe In- teractions (University of Washington, July 11-16, 1992) brought together a diverse group of investigators focusing on the genetics, physiology, and molecular biology of bac- teria, fungi, and plants as they relate to plant-microbe interactions. Like other organisms, signal generation, perception, and transduction are important in plant growth and devel- opment. The sessile nature of plants requires that they perceive and respond to many environmental signals as well as to endogenous signals. In contrast with some other prokaryotic and eukaryotic systems, however, the molecu- lar basis of signal perception and transduction and the nature of some of the signaling molecules are just begin- ning to be elucidated (reviewed by Becraft and Freeling, 1992). Genetic and molecular approaches are being used to identify genes involved in response to growth and devel- opmental regulators, as well as to external stimuli such as light, gravity, wounding, touch, water stress, and interac- tions with microbes. In most cases, however, the receptors and signaling mechanisms mediating these responses are unknown or ill defined. Signal generation, perception, and transduction in many of the microbes that interact with plants are understood in greater detail and parallel find- ings from other prokaryotic and eukaryotic microbes. Plant-microbe interactions offer opportunities for the elu- cidation of basic mechanisms governing signal perception and transduction in plants, as well as of mechanisms by which microbes respond to plant signals. This review will highlight recent discoveries regarding the molecular interactions between plants and microor

During the past decade there has appeared a large number of papers devoted to the effects of blue light on higher plants, algae, and fungi (see Senger, 1980, 1984, 1987; Senger and Schmidt, 1986). Despite intensive efforts, however, a specific blue light photoreceptor has yet to be rigorously characterized from these organisms. Indeed there is some evidence (Briggs and Iino, 1983) that there may be more than one blue light photoreceptor of physiological consequence, and there is strong evidence for an additional photoreceptor which absorbs in the UV-B region of the spectrum as well (Beggs et al., 1983; Wellmann, 1983). The absence of knowledge of the exact chemical nature of the photoreceptor(s) makes it difficult to say much either about the nature of signal perception or about early steps in the subsequent transduction chain leading to a physiological response.

Cells can react to environmental changes by transduction of extracellular signals, to produce intracellular responses. Membrane-impermeable signal molecules are recognized by receptors, which are localized on the plasma membrane of the cell. Binding of a ligand can result in the stimulation of an intrinsic enzymatic activity of its receptor or the modulation of a transducing protein. The modulation of one or more intracellular transducing proteins can finally lead to the activation or inhibition of a so-called ‘effector protein’. In many instances, this also results in altered gene expression. Phosphorylation by protein kinases is one of the most common and important regulatory mechanisms in signal transmission. This review discusses the non-channel transmembrane receptors and their downstream signaling, with special focus on the role of protein kinases.

Background: The mechanisms of signal perception and transmission in the ‘two-component' autokinase transmitters/response regulators are poorly understood, especially considering the vast number of such systems now known. Virulence induction from the tumor-inducing (Ti) plasmid of Agrobacterium tumefaciens represents one of the best understood systems with regard to the chemistry of the activating signal, and yet the existing data does not support a receptor-mediated perception event for the xenognostic phenols. Results : Here we provide the first conclusive evidence that a specific receptor must be involved in xenognostic phenol perception, detail structural requirements of the xenognosins necessary for perception by this receptor, and develop a genetic strategy that demonstrates critical components of the phenol recognition system are not encoded on the Ti plasmid. Conclusions: Although the basic elements of the two-component system required for phenol-mediated induction of virulence gene expression are encoded on the Ti plasmid, they are dependent on the chromosomal background for even the very first stage of signal perception. This discovery suggests a curious evolutionary history, and also provides functional insight into the mechanisms of two-component signal detection and transmission in general.