Reconstitution of Cytokinin Signaling in Rice Protoplasts

The major components of the cytokinin (CK) signaling pathway have been identified from the receptors to their downstream transcription factors. However, since signaling proteins are encoded by multigene families, characterizing and quantifying the contribution of each component or their combinations to the signaling cascade have been challenging. Here, we describe a transient gene expression system in rice (Oryza sativa) protoplasts suitable to reconstitute CK signaling branches using the CK reporter construct TCSn:fLUC, consisting of a synthetic CK-responsive promoter and the firefly luciferase gene, as a sensitive readout of signaling output. We used this system to systematically test the contributions of CK signaling components, either alone or in various combinations, with or without CK treatment. The type-B response regulators (RRs) OsRR16, OsRR17, OsRR18, and OsRR19 all activated TCSn:fLUC strongly, with OsRR18 and OsRR19 showing the strongest induction by CK. Cotransfecting the reporter with OsHP01, OsHP02, OsHP05, or OsHK03 alone resulted in much weaker effects relative to those of the type-B OsRRs. When we tested combinations of OsHK03, OsHPs, and OsRRs, each combination exhibited distinct CK signaling activities. This system thus allows the rapid and high-throughput exploration of CK signaling in rice.

Two-component elements mediate interactions between cytokinin and salicylic acid in plant immunity

Recent studies have revealed an important role for hormones in plant immunity. We are now beginning to understand the contribution of crosstalk among different hormone signaling networks to the outcome of plant–pathogen interactions. Cytokinins are plant hormones that regulate development and responses to the environment. Cytokinin signaling involves a phosphorelay circuitry similar to two-component systems used by bacteria and fungi to perceive and react to various environmental stimuli. In this study, we asked whether cytokinin and components of cytokinin signaling contribute to plant immunity. We demonstrate that cytokinin levels in Arabidopsis are important in determining the amplitude of immune responses, ultimately influencing the outcome of plant–pathogen interactions. We show that high concentrations of cytokinin lead to increased defense responses to a virulent oomycete pathogen, through a process that is dependent on salicylic acid (SA) accumulation and activation of defense gene expression. Surprisingly, treatment with lower concentrations of cytokinin results in increased susceptibility. These functions for cytokinin in plant immunity require a host phosphorelay system and are mediated in part by type-A response regulators, which act as negative regulators of basal and pathogen-induced SA–dependent gene expression. Our results support a model in which cytokinin up-regulates plant immunity via an elevation of SA–dependent defense responses and in which SA in turn feedback-inhibits cytokinin signaling. The crosstalk between cytokinin and SA signaling networks may help plants fine-tune defense responses against pathogens.

Plant hormones play an important role in many aspects of a plant's life cycle, from the regulation of development to responses to constantly changing environmental conditions. In the past decade, the importance of hormones in plant immunity against a variety of pathogens has been uncovered. In this manuscript, we demonstrate that in the model plant species Arabidopsis thaliana components of the signaling system of the plant hormone cytokinin also mediate plant immunity. We demonstrate that this involves the type-A class of Arabidopsis response regulators in a process that occurs downstream of the plant defense hormone salicylic acid and involves a host two-component phosphorelay. Moreover, we show that the levels of cytokinin are important in determining the amplitude of plant immunity, ultimately influencing the outcome of plant–pathogen interactions. Finally, our results indicate that salicylic acid negatively regulates cytokinin signaling, which may serve to fine-tune the effects of cytokinin in plant immunity. Given the high energy costs of defense responses and the role of cytokinins in carbon partitioning and energy allocation, we hypothesize that the mechanisms uncovered here may help regulate the levels of energy that can be allocated into defense responses, an important aspect in the biology of plants.

Cytokinin signaling networks

Despite long-standing observations on diverse cytokinin actions, the discovery path to cytokinin signaling mechanisms was tortuous. Unyielding to conventional genetic screens, experimental innovations were paramount in unraveling the core cytokinin signaling circuitry, which employs a large repertoire of genes with overlapping and specific functions. The canonical two-component transcription circuitry involves His kinases that perceive cytokinin and initiate signaling, as well as His-to-Asp phosphorelay proteins that transfer phosphoryl groups to response regulators, transcriptional activators, or repressors. Recent advances have revealed the complex physiological functions of cytokinins, including interactions with auxin and other signal transduction pathways. This review begins by outlining the historical path to cytokinin discovery and then elucidates the diverse cytokinin functions and key signaling components. Highlights focus on the integration of cytokinin signaling components into regulatory networks in specific contexts, ranging from molecular, cellular, and developmental regulations in the embryo, root apical meristem, shoot apical meristem, stem and root vasculature, and nodule organogenesis to organismal responses underlying immunity, stress tolerance, and senescence.

Cytokinins mediate resistance against Pseudomonas syringae in tobacco through increased antimicrobial phytoalexin synthesis independent of salicylic acid signaling

Cytokinins are phytohormones that are involved in various regulatory processes throughout plant development, but they are also produced by pathogens and known to modulate plant immunity. A novel transgenic approach enabling autoregulated cytokinin synthesis in response to pathogen infection showed that cytokinins mediate enhanced resistance against the virulent hemibiotrophic pathogen Pseudomonas syringae pv tabaci. This was confirmed by two additional independent transgenic approaches to increase endogenous cytokinin production and by exogenous supply of adenine- and phenylurea-derived cytokinins. The cytokinin-mediated resistance strongly correlated with an increased level of bactericidal activities and up-regulated synthesis of the two major antimicrobial phytoalexins in tobacco (Nicotiana tabacum), scopoletin and capsidiol. The key role of these phytoalexins in the underlying mechanism was functionally proven by the finding that scopoletin and capsidiol substitute in planta for the cytokinin signal: phytoalexin pretreatment increased resistance against P. syringae. In contrast to a cytokinin defense mechanism in Arabidopsis (Arabidopsis thaliana) based on salicylic acid-dependent transcriptional control, the cytokinin-mediated resistance in tobacco is essentially independent from salicylic acid and differs in pathogen specificity. It is also independent of jasmonate levels, reactive oxygen species, and high sugar resistance. The novel function of cytokinins in the primary defense response of solanaceous plant species is rather mediated through a high phytoalexin-pathogen ratio in the early phase of infection, which efficiently restricts pathogen growth. The implications of this mechanism for the coevolution of host plants and cytokinin-producing pathogens and the practical application in agriculture are discussed.

Cytokinins and plant immunity: old foes or new friends?

Cytokinins are plant growth promoting hormones involved in the specification of embryonic cells, maintenance of meristematic cells, shoot formation and development of vasculature. Cytokinins have also emerged as a major factor in plant-microbe interactions during nodule organogenesis and pathogenesis. Microbe-originated cytokinins confer abnormal hypersensitivity of cytokinins to plants, augmenting the sink activity of infected regions. However, recent findings have shed light on a distinct role of cytokinins in plant immune responses. Plant-borne cytokinins systemically induce resistance against pathogen infection. This resistance is orchestrated by endogenous cytokinin and salicylic acid signaling. Here, we discuss how plant- and pathogen-derived cytokinins inversely affect the plant defense response. In addition, we consider the molecular mechanisms underlying plant-derived cytokinin action in plant immunity.

Cytokinin profiling in plant tissues using ultra-performance liquid chromatography-electrospray tandem mass spectrometry

We have developed a simple, high-throughput batch immunoextraction (IAE) micropurification procedure for extracting a wide range of naturally occurring cytokinins (bases, ribosides, O- and N-glucosides, and nucleotides) from plant tissues in solutions that are compatible with ultra-performance liquid chromatography (UPLC), thereby facilitating sensitive subsequent analysis. The UPLC system was coupled to a tandem quadrupole mass spectrometer (MS/MS) equipped with an electrospray interface (ESI). Small (mg) amounts of tissues were purified by solid-phase extraction (SPE) followed by an immunoaffinity clean-up step and two fast chromatographic separations of most cytokinin metabolites (bases, ribosides, and 9-glucosides in the first, O-glucosides and nucleotides in the second). Using UPLC, the runs were up to 4-fold faster than in standard cytokinin analyses, and both retention times and injection volumes were less variable (RSDs, 0.15-0.3% and 1.0-5.5%, respectively). In multiple reaction monitoring (MRM) mode, the detection limit for most of the cytokinins analyzed was close to 1 fmol (5-25 fmol for O-glucosides and nucleotides) and the linear range spanned at least five orders of magnitude. The extraction and purification method was optimized using poplar (Populusxcanadensis Moench, cv Robusta) leaf samples, and the analytical accuracy was further validated using IAE-purified 10-day-old Arabidopsis thaliana plants spiked with 1 and 10 pmol of cytokinin derivatives. This approach can be used for rapid, sensitive qualitative and/or quantitative analysis of more than 50 natural cytokinins in minute amounts of plant tissues with high performance, robustness, and accuracy.

Role of human mast cells and basophils in bronchial asthma

Mast cells and basophils are the only cells expressing the tetrameric (alphabetagamma2) structure of the high affinity receptor for IgE (FcepsilonRI) and synthesizing histamine in humans. Human FcepsilonRI+ cells are conventionally considered primary effector cells of bronchial asthma. There is now compelling evidence that these cells differ immunologically, biochemically, and pharmacologically, which suggests that they might play distinct roles in the appearance and fluctuation of the asthma phenotype. Recent data have revealed the complexity of the involvement of human mast cells and basophils in asthma and have shed light on the control of recruitment and activation of these cells in different lung compartments. Preliminary evidence suggests that these cells might not always be detrimental in asthma but, under some circumstances, they might exert a protective effect by modulating certain aspects of innate and acquired immunity and allergic inflammation.

In vitro immunoregulatory effects of antidepressants in healthy volunteers

Major depression is accompanied by an activation of the inflammatory response system (IRS) and antidepressants may have immunoregulatory activities. This study was carried out to compare the effect of imipramine, mianserin and lithium on the in vitro production of Th1-like cytokines, such as IL-2, IFN-gamma, lymphotoxin and Th2-like cytokines such as IL-4, IL-10 as well as IL-12 and TGF-beta. Peripheral blood mononuclear cells (PBMC) of 16 healthy volunteers were stimulated with polyclonal activators (phytohemagglutinin with lipopolysaccharide PHA + LPS) with or without incubation with imipramine, mianserin (1 microM) or lithium (1 mM). Imipramine and mianserin exhibited similar activities enhancing unstimulated IFN-gamma and IL-10 production. In PHA + LPS-stimulated PBMC both antidepressants inhibited IFN-gamma, IL-2 and lymphotoxin production (Th1-like cytokines) as well as IL-12 and IL-4 production. Under the same in vitro conditions, both antidepressants stimulated production of negative immunoregulatory cytokines such as IL-10 and TGF-beta. Lithium differed significantly from imipramine and mianserin, as it enhanced IL-2, IFN-gamma, IL-10 and TGF-beta production and inhibited only IL-4. All three examined antidepressants reduced IFN-gamma/IL-10 ratio. None of the antidepressants at the used concentrations induced apoptosis in PBMC so those changes in cytokine production were not the result of selective killing of certain cell subpopulations. Imipramine and mianserin at high concentrations negatively influenced reactive oxygen species (ROS) production in neutrophils, however, at concentrations in the therapeutical range none of the antidepressants used influenced "oxidative burst" in neutrophils. The results indicate that antidepressants exert immunoregulatory effects on human leukocyte functions, especially on cytokine production.

Cytokinins in pathogenesis and disease resistance of Pyrenophora teres-barley and Dreschslera maydis-maize interactions during early stages of infection

Infection of Hordeum vulgare L. by Pyrenophora teres and of Zea mays by Dreschslera maydis were characterized by 'green island' formation, higher cytokinin levels and accumulation of metabolites in the infected areas. Higher cytokinin concentrations of the order 6-Y,Y-dimethylallylaminopurine > zeatinriboside > zeatin > dihydrozeatinriboside were detected at infection sites of susceptible hosts. By virtue of these cytokinins, infection sites may be acting as metabolic sinks helping proliferation of the pathogen. Existence of translocatory sinks at infection zones was confirmed from autoradiographic studies, where, accumulation of labeled metabolites was prominent at infection sites of susceptible hosts. Upon infection the lower cytokinin levels of resistant hosts decreased further with progress of infection. In the infected resistant hosts the concentrations of zeatin/zeatinriboside were the maximum among the four identified cytokinins. The pathogen is also capable of secreting cytokinins as evident from quantification of cytokinins in culture filtrate extracts using HPLC. Since detached leaves were used in the experiments the increase/decrease of various cytokinin levels may be attributed to pathogen influence. The increase in cytokinin levels in the susceptible host may be aiding the growth of the pathogen on one hand, while the decrease in the infected resistant host may signal the host to activate defenses against a potential pathogen at the early stage of infection.

Monoclonal antibodies against the plant cytokinin, dihydrozeatin riboside

Three monoclonal antibodies (all IgG1) were prepared against the plant cytokinin dihydrozeatin riboside (diH[9R]Z) and characterized for suitability in the isolation and quantification of dihydrozeatin-type phytohormones. These antibodies detected as little as 5-10 pg of the homologous cytokinin when used in both competitive and noncompetitive immunoassays and each had characteristic crossreactivity with structurally-related cytokinins. One antibody was used to quantify HPLC-purified diH[9R]Z recovered from wheat tissue. The same antibody also was linked to an insoluble support for use in affinity purification of a mixture of cytokinins. These antibodies will be useful for purification and quantification of dihydrozeatin and its 9-riboside and 9-glucoside in plant tissue.

New synthesis of immunogenic N6-isopent-2-enyladenosine-protein conjugate. Preparation, purification, and specificity of related antibodies

An original procedure which preserves the structure of the sugar ring is described to link a plant hormone as N6-isopent-2-enyladenosine [( 9R]iP) onto a protein carrier to prepare a more specific immunogen. This cytokinin is bound to bovine serum albumin (BSA) and ovalbumin by a five-step procedure. These [9R]iP-protein conjugates have a maximal absorption at 269 nm and show molar ratios of hormone bound to proteins in the range of 12:1 and 18:1 for BSA and ovalbumin, respectively. Polyclonal antibodies were raised in rabbits against [9R]iP-BSA and were purified by affinity chromatography. Titers and specificity of the antisera and purified antibodies were determined by ELISA and RIA. These antibodies are highly specific for [9R]iP and do not cross-react with zeatin and ribosylzeatin. An immunoaffinity matrix was prepared with a capacity of 1 microgram of [9R]iP/mL of gel.

Purification of antibodies for the cytokinin N6-(delta 2-isopentenyl) adenosine by affinity chromatography

Isopentenyl adenosine antibodies useful in the investigations of the "cytokinin" functions of isopentenyl adenosine were purified by affinity chromatography. Using different affinity columns, the antibodies were purified to near complete purity. Analyses of the purified proteins revealed the presence of isopentenyl adenosine binding proteins in normal rabbit serum, which presence supports a suggested role for isopentenyl adenosine and its related compounds in animal cell division in vivo.