Quantitative analysis of cytokinins in plants by liquid chromatography–single-quadrupole mass spectrometry

Can plant hormonomics be built on simple analysis? A review

The field of plant hormonomics focuses on the qualitative and quantitative analysis of the hormone complement in plant samples, akin to other omics sciences. Plant hormones, alongside primary and secondary metabolites, govern vital processes throughout a plant's lifecycle. While active hormones have received significant attention, studying all related compounds provides valuable insights into internal processes. Conventional single-class plant hormone analysis employs thorough sample purification, short analysis and triple quadrupole tandem mass spectrometry. Conversely, comprehensive hormonomics analysis necessitates minimal purification, robust and efficient separation and better-performing mass spectrometry instruments. This review summarizes the current status of plant hormone analysis methods, focusing on sample preparation, advances in chromatographic separation and mass spectrometric detection, including a discussion on internal standard selection and the potential of derivatization. Moreover, current approaches for assessing the spatiotemporal distribution are evaluated. The review touches on the legitimacy of the term plant hormonomics by exploring the current status of methods and outlining possible future trends.

Quantification of six types of cytokinins: Integration of an ultra-performance liquid chromatographic-electrospray tandem mass spectrometric method with antibody based immunoaffinity columns equally recognizing cytokinins in free base and nucleoside forms

Cytokinins (CTKs) exist in various types in plants. The accurate quantification of free base and nucleoside types of cytokinins are helpful for better understanding their physiological role. In the present study, antibodies against trans-zeatin riboside (tZR) and N⁶-isopentenyladenine riboside (iPR) antibodies with equal recognition to free base and nucleoside cytokinins were developed. The cross-reactivity of tZR mAb 3G101G7 with tZR, trans-zeatin (tZ), dihydrozeatin riboside (DHZR), dihydrozeatin (DHZ), iPR, and N⁶-isopentenyladenine (iP) was 100.0%, 95.7%, 19.1%, 18.0%, 1.1%, and 0.7%, and that of iPR mAb 5C82F1 with above-mentioned 6 types of cytokinins was 1.5%, 1.4%, 5.7%, 3.1%, 100.0% and 92.6%, respectively. The obtained antibodies were used to prepare two immunoaffinity columns (IAC). The elution efficiencies of tZR 3G101G7-IAC for tZ and tZR, DHZ and DHZR and of iPR 5C82F1-IAC for iP and iPR were almost no difference with the same loading amount on their corresponding IACs. Subsequently, six types of cytokinins in mepiquat chloride (MC)-treated cotton (Gossypium hirsutum L.) roots were determined by IACs combined with ultra-performance liquid chromatography-electrospray tandem mass spectrometry (UPLC-ESI-MS/MS). The contents of tZR, iPR and DHZR were increased by 9.3∼38.5%, 6.6∼23.5%, and 30.1∼110.0%, respectively, whereas those of tZ and iP were reduced by 5.3∼20.0% and 27.7∼32.1%, respectively. The decreased tZ and iP levels led to the ratio of auxin-to-active cytokinins increase to promote lateral root initiation in MC-treated cotton seeding. Integration of the IACs equally recognizing cytokinins in their free base and nucleoside forms with UPLC-ESI-MS/MS can accurately quantify different cytokinins in plant tissues.

Review: Isoprenoid and aromatic cytokinins in shoot branching

Shoot branching is an important event of plant development that defines growth and reproduction. The BRANCHED1 gene (BRC1/TB1/FC1) is crucial for this process. Within the phytohormones, cytokinins directly activate axillary buds to promote shoot branching. In addition, strigolactones and auxins inhibit bud outgrowth. This review addresses the involvement of aromatic and isoprenoid cytokinins in shoot branching. And how auxins and strigolactones contribute to regulating this process also. The results obtained by others and our working group with lemongrass (Cymbopogon citratus) show that cytokinins affect both shoot and root apical meristem development, consistent with other plant species. However, many questions remain about how cytokinins and strigolactones antagonistically regulate BRC1 gene expression. Additionally, many details of the interaction among cytokinins, auxins, and strigolactones need to be clarified. We will gain a more comprehensive scheme of bud outgrowth with these details.

The Effects of Exogenous Salicylic Acid on Endogenous Phytohormone Status in Hordeum vulgare L. under Salt Stress

Acclimation to salt stress in plants is regulated by complex signaling pathways involving endogenous phytohormones. The signaling role of salicylic acid (SA) in regulating crosstalk between endogenous plant growth regulators' levels was investigated in barley (Hordeum vulgare L. 'Ince'; 2n = 14) leaves and roots under salt stress. Salinity (150 and 300 mM NaCl) markedly reduced leaf relative water content (RWC), growth parameters, and leaf water potential (LWP), but increased proline levels in both vegetative organs. Exogenous SA treatment did not significantly affect salt-induced negative effects on RWC, LWP, and growth parameters but increased the leaf proline content of plants under 150 mM salt stress by 23.1%, suggesting that SA enhances the accumulation of proline, which acts as a compatible solute that helps preserve the leaf's water status under salt stress. Changes in endogenous phytohormone levels were also investigated to identify agents that may be involved in responses to increased salinity and exogenous SA. Salt stress strongly affected endogenous cytokinin (CK) levels in both vegetative organs, increasing the concentrations of CK free bases, ribosides, and nucleotides. Indole-3-acetic acid (IAA, auxin) levels were largely unaffected by salinity alone, especially in barley leaves, but SA strongly increased IAA levels in leaves at high salt concentration and suppressed salinity-induced reductions in IAA levels in roots. Salt stress also significantly increased abscisic acid (ABA) and ethylene levels; the magnitude of this increase was reduced by treatment with exogenous SA. Both salinity and SA treatment reduced jasmonic acid (JA) levels at 300 mM NaCl but had little effect at 150 mM NaCl, especially in leaves. These results indicate that under high salinity, SA has antagonistic effects on levels of ABA, JA, ethylene, and most CKs, as well as basic morphological and physiological parameters, but has a synergistic effect on IAA, which was well exhibited by principal component analysis (PCA).

Beyond transport: cytokinin ribosides are translocated and active in regulating the development and environmental responses of plants

Riboside type cytokinins are key components in cytokinin metabolism, transport, and sensitivity, making them important functional signals in plant growth and development and environmental stress responses. Cytokinin (CKs) are phytohormones that regulate multiple processes in plants and are critical for agronomy, as they are involved in seed filling and plant responses to biotic and abiotic stress. Among the over 30 identified CKs, there is uncertainty about the roles of many of the individual CK structural forms. Cytokinin free bases (CKFBs), have been studied in great detail, but, by comparison, roles of riboside-type CKs (CKRs) in CK metabolism and associated signaling pathways and their distal impacts on plant physiology remain largely unknown. Here, recent findings on CKR abundance, transport and localization, are summarized, and their importance in planta is discussed. The history of CKR analyses is reviewed, in the context of the determination of CK metabolic pathways, and research on CKR affinity for CK receptors, all of which yield essential insights into their functions. Recent studies suggest that CKR forms are a lot more than a group of transport CKs and, beyond this, they play important roles in plant development and responses to environmental stress. In this context, this review discusses the involvement of CKRs in plant development, and highlight the less anticipated functions of CKRs in abiotic stress tolerance. Based on this, possible mechanisms for CKR modes of action are proposed and experimental approaches to further uncover their roles and future biotechnological applications are suggested.

Electrochemical biosensor for cytokinins based on the CHASE domain of Arabidopsis histidine kinases 4

In this study, An AHK4 CHASE domain was used to construct an electrochemical cytokinin biosensor using ferrocene as the electrochemical mediator. Upon addition of cytokinin, the binding of cytokinin and AHK4 led to dimerization, which blocked electron transfer between ferrocene and the electrode, and the redox peak current of ferrocene was gradually reduced. Cytokinin was detected by recording the change of the ferrocene redox peak current. The biosensor shows a linear range of 50-400 nM with a linear regression equation of i_p = 0.0086c + 0.732 (R² = 0.993) with i_p in μA and c in nM and a detection limit (LOD) of 1.5 nM (S/N = 3). The biosensor exhibits excellent performance that avoids interference of other types of plant hormones and was successfully applied to the detection of cytokinins in bean sprouts.

Characterization of the cytokinin sensor TCSv2 in arabidopsis and tomato

BACKGROUND

Hormones are crucial to plant life and development. Being able to follow the plants hormonal response to various stimuli and throughout developmental processes is an important and increasingly widespread tool. The phytohormone cytokinin (CK) has crucial roles in the regulation of plant growth and development.

RESULTS

Here we describe a version of the CK sensor Two Component signaling Sensor (TCS), referred to as TCSv2. TCSv2 has a different arrangement of binding motifs when compared to previous TCS versions, resulting in increased sensitivity in some examined tissues. Here, we examine the CK responsiveness and distribution pattern of TCSv2 in arabidopsis and tomato.

CONCLUSIONS

The increased sensitivity and reported expression pattern of TCSv2 make it an ideal TCS version to study CK response in particular hosts, such as tomato, and particular tissues, such as leaves and flowers.

Timing-dependent effects of salicylic acid treatment on phytohormonal changes, ROS regulation, and antioxidant defense in salinized barley (Hordeum vulgare L.)

Cross-talk between exogenous salicylic acid (SA) and endogenous phytohormone pathways affects the antioxidant defense system and its response to salt stress. The study presented here investigated the effects of SA treatment before and during salt stress on the levels of endogenous plant growth regulators in three barley cultivars with different salinity tolerances: Hordeum vulgare L. cvs. Akhisar (sensitive), Erginel (moderate), and Kalaycı (tolerant). The cultivars' relative leaf water contents, growth parameters, proline contents, chlorophyll a/b ratios, and lipid peroxidation levels were measured, along with the activities of enzymes involved in detoxifying reactive oxygen species (ROS) including superoxide-dismutase, peroxidase, catalase, ascorbate-peroxidase, and glutathione-reductase. In addition, levels of several endogenous phytohormones (indole-3-acetic-acid, cytokinins, abscisic acid, jasmonic acid, and ethylene) were measured. Barley is known to be more salt tolerant than related plant species. Accordingly, none of the studied cultivars exhibited changes in membrane lipid peroxidation under salt stress. However, they responded differently to salt-stress with respect to their accumulation of phytohormones and antioxidant enzyme activity. The strongest and weakest increases in ABA and proline accumulation were observed in Kalaycı and Akhisar, respectively, suggesting that salt-stress was more effectively managed in Kalaycı. The effects of exogenous SA treatment depended on both the timing of the treatment and the cultivar to which it was applied. In general, however, where SA helped mitigate salt stress, it appeared to do so by increasing ROS scavenging capacity and antioxidant enzyme activity. SA treatment also induced changes in phytohormone levels, presumably as a consequence of SA-phytohormone salt-stress cross-talk.

The Response of Maize to Inoculation with Arthrobacter sp. and Bacillus sp. in Phosphorus-Deficient, Salinity-Affected Soil

Salinity and phosphorus (P) deficiency are among the most serious soil factors constraining crop productivity. A proposed strategy for alleviating these stresses is supporting plants by inoculation with growth-promoting rhizobacteria (PGPR). Here, a comparison of the ability of two maize composite and two F1 hybrid varieties to tolerate a P deficiency in either a saline or a non-saline environment showed that the uptake of nutrients by all four entries was significantly reduced by the imposition of both soil salinity and P deficiency, and that their growth was compromised to a similar extent. Subsequently, the ameliorative effect of inoculation with three strains of either Arthrobacter sp. or Bacillus sp. in an environment, which suffered simultaneously from salinity and P deficiency, was investigated. Inoculation with each of the strains was found to limit the plants’ uptake of sodium cations, to increase their uptake of potassium cations, and to enhance their growth. The extent of the growth stimulation was more pronounced for the composite varieties than for the F1 hybrid ones, although the amount of biomass accumulated by the latter, whether the plants had been inoculated or not, was greater than that of the former varieties. When the bacterial strains were cultured in vitro, each of them was shown as able to produce the phytohormones auxin, abscisic acid, gibberellins, and cytokinins. The implication is that since the presence in the rhizospere of both Arthrobacter sp. and Bacillus sp. strains can support the growth of maize in salinity-affected and P deficient soils in a genotype-dependent fashion, it is important to not only optimize the PGPR strain used for inoculation, but also to select maize varieties which can benefit most strongly from an association with these bacteria.

Analytical methods in strigolactone research

Strigolactones (SLs) are important plant hormones that are produced via the carotenoid biosynthetic pathway and occur at extremely low concentrations in various plant species. They regulate root development, play important roles in symbioses between higher plants and mycorrhizal fungi, and stimulate germination of plant-parasitic Orobanche and Striga species. Chemical analysis is central to research on the biochemistry of SLs and their roles in developmental biology and plant physiology. Here we summarize key issues relating to the identification and quantification of SLs isolated from plant tissues and exudates. The advantages and drawbacks of different protocols used for strigolactone analysis are discussed, and guidelines for selecting a procedure that will minimize losses during isolation and purification prior to final analysis are proposed. Hyphenated techniques suitable for SL analysis such as GC-MS and LC-MS/MS are also discussed, and newer ambient techniques such as HR-DART-MS and DESI-MS are highlighted as tools with considerable potential in SL research. A key advantage of these methods is that they require only simply sample preparation.

Capillary Vibrating Sharp-Edge Spray Ionization (cVSSI) for Voltage-Free Liquid Chromatography-Mass Spectrometry

Here, we report a continuous flow-based ionization method, capillary vibrating sharp-edge spray ionization (cVSSI), that nebulizes liquid sample directly at the outlet of a capillary without using high-speed nebulization gas or a high electrical field. cVSSI is built upon the recently reported VSSI principle which nebulizes bulk liquid using vibrating sharp-edges. By attaching a short piece of fused silica capillary on top of the vibrating glass slide in VSSI, liquid is nebulized at the outlet of the capillary as the result of the vibration. Utilizing standard 360-μm OD/100-μm ID capillary, cVSSI works with a wide range of flow rates from 1 μL/min to 1 mL/min. The power consumption is as low as 130 mW. ESI-like MS spectra are obtained for small molecules, peptides, and proteins. Five orders of magnitude linear response for acetaminophen solution is achieved with a limit of detection (LOD) of 3 nM. cVSSI is also demonstrated to be compatible with LC-MS analysis. Two LC-MS applications are demonstrated with cVSSI: (1) separation and detection of a mixture of small molecules and (2) bottom-up proteomics using a protein digest. A mixture of nine common metabolites was appropriately separated and detected using LC-cVSSI-MS. In the bottom-up experiment, 78 peptides were detected using LC-cVSSI-MS/MS with a protein coverage of 100% for cytochrome c, which is comparable with the coverage obtained using LC-ESI-MS. cVSSI offers a means of interfacing LC or other continuous flow-based applications to mass spectrometers with the salient features of voltage-free, flexibility, small footprint, and low power consumption.

Root enhancement in cytokinin-deficient oilseed rape causes leaf mineral enrichment, increases the chlorophyll concentration under nutrient limitation and enhances the phytoremediation capacity

BACKGROUND

Cytokinin is a negative regulator of root growth, and a reduction of the cytokinin content or signalling causes the formation a larger root system in model plants, improves their growth under drought and nutrient limitation and causes increased accumulation of elements in the shoot. Roots are an important but understudied target of plant breeding. Here we have therefore explored whether root enhancement by lowering the cytokinin content can also be achieved in oilseed rape (Brassica napus L.) plants.

RESULTS

Transgenic plants overexpressing the CKX2 gene of Arabidopsis thaliana encoding a cytokinin-degrading cytokinin oxidase/dehydrogenase showed higher CKX activity and a strongly reduced cytokinin content. Cytokinin deficiency led to the formation of a larger root system under different growth conditions, which was mainly due to an increased number of lateral and adventitious roots. In contrast, shoot growth was comparable to wild type, which caused an enhanced root-to-shoot ratio. Transgenic plants accumulated in their leaves higher concentrations of macro- and microelements including P, Ca, Mg, S, Zn, Cu, Mo and Mn. They formed more chlorophyll under Mg- and S-deficiency and accumulated a larger amount of Cd and Zn from contaminated medium and soil.

CONCLUSIONS

These findings demonstrate the usefulness of ectopic CKX gene expression to achieve root enhancement in oilseed rape and underpin the functional relevance of a larger root system. Furthermore, the lack of major developmental consequences on shoot growth in cytokinin-deficient oilseed rape indicates species-specific differences of CKX gene and/or cytokinin action.

Bioactive molecules derived from smoke and seaweed Ecklonia maxima showing phytohormone-like activity in Spinacia oleracea L

Bioactive compounds such as karrikinolide (KAR1 from smoke) and eckol (from the seaweed Ecklonia maxima) show promising effects on several important crop plants. These plant growth-stimulating organic biomolecules, along with crude extracts (smoke-water and Kelpak® product prepared from Ecklonia maxima), were tested on spinach plants. Eckol sprayed at 10-6 M significantly increased all the growth and biochemical parameters examined compared to control spinach plants. All tested plant growth biostimulants significantly increased total chlorophyll, carotenoids and protein content of spinach leaves. The cytokinin profile of spinach plants was also determined. Cis-zeatin, dihydrozeatin and isopentenyladenine types of cytokinins were promoted by both smoke- and seaweed-based biostimulants. In comparison to the control plants, the level of free sinapic acid was greater in all spinach plants treated with these biostimulants. The application of these biostimulants can help spinach crop by improving growth, yield and nutritional quality; moreover, they are organic and cost-effective.

The interplay between cytokinins and light during senescence in detached Arabidopsis leaves

Light and cytokinins are known to be the key players in the regulation of plant senescence. In detached leaves, the retarding effect of light on senescence is well described; however, it is not clear to what extent is this effect connected with changes in endogenous cytokinin levels. We have performed a detailed analysis of changes in endogenous content of 29 cytokinin forms in detached leaves of Arabidopsis thaliana (wild-type and 3 cytokinin receptor double mutants). Leaves were kept under different light conditions, and changes in cytokinin content were correlated with changes in chlorophyll content, efficiency of photosystem II photochemistry, and lipid peroxidation. In leaves kept in darkness, we have observed decreased content of the most abundant cytokinin free bases and ribosides, but the content of cis-zeatin increased, which indicates the role of this cytokinin in the maintenance of basal leaf viability. Our findings underscore the importance of light conditions on the content of specific cytokinins, especially N⁶ -(Δ² -isopentenyl)adenine. On the basis of our results, we present a scheme summarizing the contribution of the main active forms of cytokinins, cytokinin receptors, and light to senescence regulation. We conclude that light can compensate the disrupted cytokinin signalling in detached leaves.

Circadian clock components control daily growth activities by modulating cytokinin levels and cell division-associated gene expression in Populus trees

Trees are carbon dioxide sinks and major producers of terrestrial biomass with distinct seasonal growth patterns. Circadian clocks enable the coordination of physiological and biochemical temporal activities, optimally regulating multiple traits including growth. To dissect the clock's role in growth, we analysed Populus tremula × P. tremuloides trees with impaired clock function due to down-regulation of central clock components. late elongated hypocotyl (lhy-10) trees, in which expression of LHY1 and LHY2 is reduced by RNAi, have a short free-running period and show disrupted temporal regulation of gene expression and reduced growth, producing 30-40% less biomass than wild-type trees. Genes important in growth regulation were expressed with an earlier phase in lhy-10, and CYCLIN D3 expression was misaligned and arrhythmic. Levels of cytokinins were lower in lhy-10 trees, which also showed a change in the time of peak expression of genes associated with cell division and growth. However, auxin levels were not altered in lhy-10 trees, and the size of the lignification zone in the stem showed a relative increase. The reduced growth rate and anatomical features of lhy-10 trees were mainly caused by misregulation of cell division, which may have resulted from impaired clock function.

Direct Determination of Six Cytokinin Nucleotide Monophosphates in Coconut Flesh by Reversed-Phase Liquid Chromatography-Tandem Mass Spectrometry

Coconut contains many uncharacterized cytokinins that have important physiological effects in plants and humans. In this work, a method based on liquid chromatography-tandem mass spectrometry was developed for identification and quantification of six cytokinin nucleotide monophosphates in coconut flesh. Excellent separation was achieved using a low-coverage C18 bonded-phase column with an acidic mobile phase, which greatly improved the retention of target compounds. To enable high-throughput analysis, a single-step solid-phase extraction using mixed-mode anion-exchange cartridges was employed for sample preparation. This proved to be an effective method to minimize matrix effects and ensure high selectivity. The limits of detection varied from 0.06 to 0.3 ng/mL, and the limits of quantification ranged from 0.2 to 1.0 ng/mL. The linearity was statistically verified over 2 orders of magnitude, giving a coefficient of determination (R²) greater than 0.9981. The mean recoveries were from 81 to 108%; the intraday precision (n = 6) was less than 11%; and the interday precision (n = 11) was within 14%. The developed method was applied to the determination of cytokinin nucleotide monophosphates in coconut flesh samples, and four of them were successfully identified and quantified. The results showed that trans-zeatin riboside-5'-monophosphate was the dominant cytokinin, with a concentration of 2.7-34.2 ng/g, followed by N⁶-isopentenyladenosine-5'-monophosphate (≤12.9 ng/g), while the concentrations of cis-zeatin riboside-5'-monophosphate and dihydrozeatin riboside-5'-monophosphate were less than 2.2 and 4.9 ng/g, respectively.

Contrasting patterns of cytokinins between years in senescing aspen leaves

Cytokinins are plant hormones that typically block or delay leaf senescence. We profiled 34 different cytokinins/cytokinin metabolites (including precursors, conjugates and degradation products) in leaves of a free-growing mature aspen (Populus tremula) before and after the initiation of autumnal senescence over three consecutive years. The levels and profiles of individual cytokinin species, or classes/groups, varied greatly between years, despite the fact that the onset of autumn senescence was at the same time each year, and senescence was not associated with depletion of either active or total cytokinin levels. Levels of aromatic cytokinins (topolins) were low and changed little over the autumn period. Diurnal variations and weather-dependent variations in cytokinin content were relatively limited. We also followed the expression patterns of all aspen genes implicated as having roles in cytokinin metabolism or signalling, but neither the pattern of regulation of any group of genes nor the expression of any particular gene supported the notion that decreased cytokinin signalling could explain the onset of senescence. Based on the results from this tree, we therefore suggest that cytokinin depletion is unlikely to explain the onset of autumn leaf senescence in aspen.

Microscale magnetic microparticle-based immunopurification of cytokinins from Arabidopsis root apex

Cytokinins (CKs) are pivotal plant hormones that have crucial roles in plant growth and development. However, their isolation and quantification are usually challenging because of their extremely low levels in plant tissues (pmol g^-1 fresh weight). We have developed a simple microscale magnetic immunoaffinity-based method for selective one-step isolation of CKs from very small amounts of plant tissue (less than 0.1 mg fresh weight). The capacity of the immunosorbent and the effect of the complex plant matrix on the yield of the rapid one-step purification were tested using a wide range of CK concentrations. The total recovery range of the new microscale isolation procedure was found to be 30-80% depending on individual CKs. Immunoaffinity extraction using group-specific monoclonal CK antibodies immobilized onto magnetic microparticles was combined with a highly sensitive ultrafast mass spectrometry-based method with a detection limit close to one attomole. This combined approach allowed metabolic profiling of a wide range of naturally occurring CKs (bases, ribosides and N⁹ -glucosides) in 1.0-mm sections of the Arabidopsis thaliana root meristematic zone. The magnetic immunoaffinity separation method was shown to be a simple and extremely fast procedure requiring minimal amounts of plant tissue.

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.

Cytokinin, auxin and physiological polarity in the aquatic carnivorous plants Aldrovanda vesiculosa and Utricularia australis

BACKGROUND AND AIMS

The typical rootless linear shoots of aquatic carnivorous plants exhibit clear, steep polarity associated with very rapid apical shoot growth. The aim of this study was to determine how auxin and cytokinin contents are related to polarity and shoot growth in such plants.

METHODS

The main auxin and cytokinin metabolites in separated shoot segments and turions of two carnivorous plants, Aldrovanda vesiculosa and Utricularia australis, were analysed using ultra-high-performance liquid chromatography coupled with triple quad mass spectrometry.

KEY RESULTS

In both species, only isoprenoid cytokinins were identified. Zeatin cytokinins predominated in the apical parts, with their concentrations decreasing basipetally, and the trans isomer predominated in A. vesiculosa whereas the cis form was more abundant in U australis. Isopentenyladenine-type cytokinins, in contrast, increased basipetally. Conjugated cytokinin metabolites, the O-glucosides, were present at high concentrations in A. vesiculosa but only in minute amounts in U. australis. N(9)-glucoside forms were detected only in U. australis, with isopentenyladenine-9-glucoside (iP9G) being most abundant. In addition to free indole-3-acetic acid (IAA), indole-3-acetamide (IAM), IAA-aspartate (IAAsp), IAA-glutamate (IAGlu) and IAA-glycine (IAGly) conjugates were identified.

CONCLUSIONS

Both species show common trends in auxin and cytokinin levels, the apical localization of the cytokinin biosynthesis and basipetal change in the ratio of active cytokinins to auxin, in favour of auxin. However, our detailed study of cytokinin metabolic profiles also revealed that both species developed different regulatory mechanisms of active cytokinin content; on the level of their degradation, in U. australis, or in the biosynthesis itself, in the case of A. vesiculosa Results indicate that the rapid turnover of these signalling molecules along the shoots is essential for maintaining the dynamic balance between the rapid polar growth and development of the apical parts and senescence of the older, basal parts of the shoots.

Hormonal and epigenetic regulation during embryogenic tissue habituation in Cucurbita pepo L

Habituated embryogenic line of pumpkin contained more CKs and IAA, but less ABA than the non-habituated line. Pronounced hypomethylation correlated with the absence of 2,4-D, addition of 5-azaC, and the process of habituation. A comparative analysis between habituated and non-habituated embryogenic cultures of pumpkin (Cucurbita pepo L.) in relation to endogenous phytohormones, global DNA methylation, and developmental and regeneration capacities of the cultures was conducted. The analysis revealed more cytokinins (CKs) and indole-3-acetic acid (IAA), but less abscisic acid (ABA) in the habituated HEC line than in the non-habituated DEC line. Ribosides and ribotides were the most abundant CK forms in both HEC and DEC lines (75.9 and 57.6 %, respectively). HEC contained more free-base CKs (5.8 vs. 3.2 %), whereas DEC contained considerably more O-glycosides (39.1 vs. 18.3 %). Although prevalence of IAA was common for both lines, relative ratio of CKs and ABA differed between DEC and HEC lines. ABA was prevailing over CKs in DEC, while CKs prevailed over ABA in HEC line. Taking into account the importance of ABA for embryo maturation, the reduced endogenous ABA content in HEC line might be the reason for a 5-fold reduction in regeneration capacity compared to DEC. Both habituated and non-habituated embryogenic lines were highly methylated in the presence of 2,4-dichlorophenoxyacetic acid (2,4-D). Pronounced hypomethylation correlated with the absence of 2,4-D, addition of 5-azacytidine (5-azaC), but also with the process of habituation. The habituated line was resistant to the effect of hypomethylation drug 5-azaC and remained highly methylated even after the addition of 5-azaC. Also, 5-azaC did not change the developmental pattern in the habituated line, indicating the existence of separate mechanisms by which 2,4-D influences global DNA methylation in comparison to habituation-related global DNA methylation.

Biochemical Characterization of Putative Adenylate Dimethylallyltransferase and Cytokinin Dehydrogenase from Nostoc sp. PCC 7120

Cytokinins, a class of phytohormones, are adenine derivatives common to many different organisms. In plants, these play a crucial role as regulators of plant development and the reaction to abiotic and biotic stress. Key enzymes in the cytokinin synthesis and degradation in modern land plants are the isopentyl transferases and the cytokinin dehydrogenases, respectively. Their encoding genes have been probably introduced into the plant lineage during the primary endosymbiosis. To shed light on the evolution of these proteins, the genes homologous to plant adenylate isopentenyl transferase and cytokinin dehydrogenase were amplified from the genomic DNA of cyanobacterium Nostoc sp. PCC 7120 and expressed in Escherichia coli. The putative isopentenyl transferase was shown to be functional in a biochemical assay. In contrast, no enzymatic activity was detected for the putative cytokinin dehydrogenase, even though the principal domains necessary for its function are present. Several mutant variants, in which conserved amino acids in land plant cytokinin dehydrogenases had been restored, were inactive. A combination of experimental data with phylogenetic analysis indicates that adenylate-type isopentenyl transferases might have evolved several times independently. While the Nostoc genome contains a gene coding for protein with characteristics of cytokinin dehydrogenase, the organism is not able to break down cytokinins in the way shown for land plants.

The role of cis-zeatin-type cytokinins in plant growth regulation and mediating responses to environmental interactions

Cytokinins (CKs) are well-established as important phytohormonal regulators of plant growth and development. An increasing number of studies have also revealed the function of these hormones in plant responses to biotic and abiotic stresses. While the function of certain CK classes, including trans-zeatin and isopentenyladenine-type CKs, have been studied in detail, the role of cis-zeatin-type CKs (cZs) in plant development and in mediating environmental interactions is less well defined. Here we provide a comprehensive summary of the current knowledge about abundance, metabolism and activities of cZs in plants. We outline the history of their analysis and the metabolic routes comprising cZ biosynthesis and degradation. Further we provide an overview of changes in the pools of cZs during plant development and environmental interactions. We summarize studies that investigate the role of cZs in regulating plant development and defence responses to pathogen and herbivore attack and highlight their potential role as 'novel' stress-response markers. Since the functional roles of cZs remain largely based on correlative data and genetic manipulations of their biosynthesis, inactivation and degradation are few, we suggest experimental approaches using transgenic plants altered in cZ levels to further uncover their roles in plant growth and environmental interactions and their potential for crop improvement.

PHABULOSA controls the quiescent center-independent root meristem activities in Arabidopsis thaliana

Plant growth depends on stem cell niches in meristems. In the root apical meristem, the quiescent center (QC) cells form a niche together with the surrounding stem cells. Stem cells produce daughter cells that are displaced into a transit-amplifying (TA) domain of the root meristem. TA cells divide several times to provide cells for growth. SHORTROOT (SHR) and SCARECROW (SCR) are key regulators of the stem cell niche. Cytokinin controls TA cell activities in a dose-dependent manner. Although the regulatory programs in each compartment of the root meristem have been identified, it is still unclear how they coordinate one another. Here, we investigate how PHABULOSA (PHB), under the posttranscriptional control of SHR and SCR, regulates TA cell activities. The root meristem and growth defects in shr or scr mutants were significantly recovered in the shr phb or scr phb double mutant, respectively. This rescue in root growth occurs in the absence of a QC. Conversely, when the modified PHB, which is highly resistant to microRNA, was expressed throughout the stele of the wild-type root meristem, root growth became very similar to that observed in the shr; however, the identity of the QC was unaffected. Interestingly, a moderate increase in PHB resulted in a root meristem phenotype similar to that observed following the application of high levels of cytokinin. Our protoplast assay and transgenic approach using ARR10 suggest that the depletion of TA cells by high PHB in the stele occurs via the repression of B-ARR activities. This regulatory mechanism seems to help to maintain the cytokinin homeostasis in the meristem. Taken together, our study suggests that PHB can dynamically regulate TA cell activities in a QC-independent manner, and that the SHR-PHB pathway enables a robust root growth system by coordinating the stem cell niche and TA domain.

Plant roots are programmed to grow continuously into the soil, searching for nutrients and water. The iterative process of cell division, elongation, and differentiation contributes to root growth. The quiescent center (QC) is known to maintain the root meristem, and thus ensure root growth. In this study, we report a novel aspect of root growth regulation controlled independently of the QC by PHABULOSA (PHB). In shr mutant plants, PHB, which in the meristem is actively restricted to the central region of the stele by SHORTROOT (SHR) via miR165/6, suppresses root meristem activity leading to root growth arrest. A high concentration of PHB in the stele does this by modulating B-ARR activity through a QC-independent pathway. Accordingly, we observed a significant recovery of root meristem activity and growth in the shr phb double mutant, while the QC remained absent. However, the presence of QC may be required to sustain continuous root growth. On the basis of our results, we propose that SHR maintains root growth via two separate pathways: by modulating PHB levels in the root stele, and by maintaining the QC identity.

Proteasomal control of cytokinin synthesis protects Mycobacterium tuberculosis against nitric oxide

One of several roles of the Mycobacterium tuberculosis proteasome is to defend against host-produced nitric oxide (NO), a free radical that can damage numerous biological macromolecules. Mutations that inactivate proteasomal degradation in Mycobacterium tuberculosis result in bacteria that are hypersensitive to NO and attenuated for growth in vivo, but it was not known why. To elucidate the link between proteasome function, NO resistance, and pathogenesis, we screened for suppressors of NO hypersensitivity in a mycobacterial proteasome ATPase mutant and identified mutations in Rv1205. We determined that Rv1205 encodes a pupylated proteasome substrate. Rv1205 is a homolog of the plant enzyme LONELY GUY, which catalyzes the production of hormones called cytokinins. Remarkably, we report that an obligate human pathogen secretes several cytokinins. Finally, we determined that the Rv1205-dependent accumulation of cytokinin breakdown products is likely responsible for the sensitization of Mycobacterium tuberculosis proteasome-associated mutants to NO.

Detection of phytohormones in temperate forest fungi predicts consistent abscisic acid production and a common pathway for cytokinin biosynthesis

The phytohormones, abscisic acid and cytokinin, once were thought to be present uniquely in plants, but increasing evidence suggests that these hormones are present in a wide variety of organisms. Few studies have examined fungi for the presence of these "plant" hormones or addressed whether their levels differ based on the nutrition mode of the fungus. This study examined 20 temperate forest fungi of differing nutritional modes (ectomycorrhizal, wood-rotting, saprotrophic). Abscisic acid and cytokinin were present in all fungi sampled; this indicated that the sampled fungi have the capacity to synthesize these two classes of phytohormones. Of the 27 cytokinins analyzed by HPLC-ESI MS/MS, seven were present in all fungi sampled. This suggested the existence of a common cytokinin metabolic pathway in fungi that does not vary among different nutritional modes. Predictions regarding the source of isopentenyl, cis-zeatin and methylthiol CK production stemming from the tRNA degradation pathway among fungi are discussed.

Quo vadis plant hormone analysis?

Plant hormones act as chemical messengers in the regulation of myriads of physiological processes that occur in plants. To date, nine groups of plant hormones have been identified and more will probably be discovered. Furthermore, members of each group may participate in the regulation of physiological responses in planta both alone and in concert with members of either the same group or other groups. The ideal way to study biochemical processes involving these signalling molecules is 'hormone profiling', i.e. quantification of not only the hormones themselves, but also their biosynthetic precursors and metabolites in plant tissues. However, this is highly challenging since trace amounts of all of these substances are present in highly complex plant matrices. Here, we review advances, current trends and future perspectives in the analysis of all currently known plant hormones and the associated problems of extracting them from plant tissues and separating them from the numerous potentially interfering compounds.

Isopentenyltransferase-1 (IPT1) knockout in Physcomitrella together with phylogenetic analyses of IPTs provide insights into evolution of plant cytokinin biosynthesis

The moss Physcomitrella patens is part of an early divergent clade of land plants utilizing the plant hormone cytokinin for growth control. The rate-limiting step of cytokinin biosynthesis is mediated by isopentenyltransferases (IPTs), found in land plants either as adenylate-IPTs or as tRNA-IPTs. Although a dominant part of cytokinins in flowering plants are synthesized by adenylate-IPTs, the Physcomitrella genome only encodes homologues of tRNA-IPTs. This study therefore looked into the question of whether cytokinins in moss derive from tRNA exclusively. Targeted gene knockout of ipt1 (d|ipt1) along with localization studies revealed that the chloroplast-bound IPT1 was almost exclusively responsible for the A37 prenylation of tRNA in Physcomitrella. Ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS)-based cytokinin profiling demonstrated that the total amount of all free cytokinins in tissue was almost unaffected. However, the knockout plants showed increased levels of the N (6) -isopentenyladenine (iP)- and trans-zeatin (tZ)-type cytokinins, considered to provide active forms, while cis-zeatin (cZ)-type cytokinins were reduced. The data provide evidence for an additional and unexpected tRNA-independent cytokinin biosynthetic pathway in moss. Comprehensive phylogenetic analysis indicates a diversification of tRNA-IPT-like genes in bryophytes probably related to additional functions.

Overexpression of the cytosolic cytokinin oxidase/dehydrogenase (CKX7) from Arabidopsis causes specific changes in root growth and xylem differentiation

Degradation of the plant hormone cytokinin is catalyzed by cytokinin oxidase/dehydrogenase (CKX) enzymes. The Arabidopsis thaliana genome encodes seven CKX proteins which differ in subcellular localization and substrate specificity. Here we analyze the CKX7 gene, which to the best of our knowledge has not yet been studied. pCKX7:GUS expression was detected in the vasculature, the transmitting tissue and the mature embryo sac. A CKX7-GFP fusion protein localized to the cytosol, which is unique among all CKX family members. 35S:CKX7-expressing plants developed short, early terminating primary roots with smaller apical meristems, contrasting with plants overexpressing other CKX genes. The vascular bundles of 35S:CKX7 primary roots contained only protoxylem elements, thus resembling the wol mutant of the CRE1/AHK4 receptor gene. We show that CRE1/AHK4 activity is required to establish the CKX7 overexpression phenotype. Several cytokinin metabolites, in particular cis-zeatin (cZ) and N-glucoside cytokinins, were depleted stronger in 35S:CKX7 plants compared with plants overexpressing other CKX genes. Interestingly, enhanced protoxylem formation together with reduced primary root growth was also found in the cZ-deficient tRNA isopentenyltransferase mutant ipt2,9. However, different cytokinins were similarly efficient in suppressing 35S:CKX7 and ipt2,9 vascular phenotypes. Therefore, we hypothesize that the pool of cytosolic cytokinins is particularly relevant in the root procambium where it mediates the differentiation of vascular tissues through CRE1/AHK4. Taken together, the distinct consequences of CKX7 overexpression indicate that the cellular compartmentalization of cytokinin degradation and substrate preference of CKX isoforms are relevant parameters that define the activities of the hormone.

Role of LONELY GUY genes in indeterminate nodulation on Medicago truncatula

LONELY GUY (LOG) genes encode cytokinin riboside 5'-monophosphate phosphoribohydrolases and are directly involved in the activation of cytokinins. To assess whether LOG proteins affect the influence of cytokinin on nodulation, we studied two LOG genes of Medicago truncatula. Expression analysis showed that MtLOG1 and MtLOG2 were upregulated during nodulation in a CRE1-dependent manner. Expression was mainly localized in the dividing cells of the nodule primordium. In addition, RNA interference revealed that MtLOG1 is involved in nodule development and that the gene plays a negative role in lateral root development. Ectopic expression of MtLOG1 resulted in a change in cytokinin homeostasis, triggered cytokinin-inducible genes and produced roots with enlarged vascular tissues and shortened primary roots. In addition, those 35S:LOG1 roots also displayed fewer nodules than the wild-type. This inhibition in nodule formation was local, independent of the SUPER NUMERIC NODULES gene, but coincided with an upregulation of the MtCLE13 gene, encoding a CLAVATA3/EMBRYO SURROUNDING REGION peptide. In conclusion, we demonstrate that in M. truncatula LOG proteins might be implicated in nodule primordium development and lateral root formation.

A highly selective biosensor with nanomolar sensitivity based on cytokinin dehydrogenase

We have developed a N6-dimethylallyladenine (cytokinin) dehydrogenase-based microbiosensor for real-time determination of the family of hormones known as cytokinins. Cytokinin dehydrogenase from Zea mays (ZmCKX1) was immobilised concurrently with electrodeposition of a silica gel film on the surface of a Pt microelectrode, which was further functionalized by free electron mediator 2,6-dichlorophenolindophenol (DCPIP) in supporting electrolyte to give a bioactive film capable of selective oxidative cleavage of the N6- side chain of cytokinins. The rapid electron shuffling between freely diffusible DCPIP and the FAD redox group in ZmCKX1 endowed the microbiosensor with a fast response time of less than 10 s. The immobilised ZmCKX1 retained a high affinity for its preferred substrate N6-(Δ2-isopentenyl) adenine (iP), and gave the miniaturized biosensor a large linear dynamic range from 10 nM to 10 µM, a detection limit of 3.9 nM and a high sensitivity to iP of 603.3 µAmM-1cm-2 (n = 4, R2 = 0.9999). Excellent selectivity was displayed for several other aliphatic cytokinins and their ribosides, including N6-(Δ2-isopentenyl) adenine, N6-(Δ2-isopentenyl) adenosine, cis-zeatin, trans-zeatin and trans-zeatin riboside. Aromatic cytokinins and metabolites such as cytokinin glucosides were generally poor substrates. The microbiosensors exhibited excellent stability in terms of pH and long-term storage and have been used successfully to determine low nanomolar cytokinin concentrations in tomato xylem sap exudates.

Endogenous cytokinin profiles of tissue-cultured and acclimatized 'Williams' bananas subjected to different aromatic cytokinin treatments

Endogenous cytokinin (CK) levels of in vitro-cultured and greenhouse-acclimatized 'Williams' bananas treated with six aromatic CKs were quantified using UPLC-MS/MS. The underground parts had higher endogenous CK levels than the aerial parts. Control plantlets had more isoprenoid CKs while the aromatic-type CKs were predominant in all other regenerants. Following acclimatization of the control and 10 μM CK regenerants, there was a rapid decline in both isoprenoid and aromatic CK in the greenhouse-grown plants. Apart from the control and 6-(3-Methoxybenzylamino)-9-tetrahydropyran-2-ylpurine (MemTTHP) treatment with higher level of isoprenoid CK, aromatic CK remain the predominant CK-type across all CK treatments. The most abundant CK forms were meta-topolin (mT) and benzyladenine (BA) in the micropropagated and acclimatized plants, respectively. Micropropagated plantlets had cis-Zeatin (cZ) as the major isoprenoid CK-type which was in turn replaced by isopentenyladenine (iP) upon acclimatization. On a structural and functional basis, 9-glucoside, a deactivation/detoxicification product was the most abundant and mainly located in the underground parts (micropropagation and acclimatization). The results establish the wide variation in metabolic products of the tested aromatic CKs during micropropagation and acclimatization. The findings are discussed with the possible physiological roles of the various CK constituents on the growth and development of banana plants.

Overexpression of cytokinin dehydrogenase genes in barley (Hordeum vulgare cv. Golden Promise) fundamentally affects morphology and fertility

Barley is one of the most important cereal crops grown worldwide. It has numerous applications, but its utility could potentially be extended by genetically manipulating its hormonal balances. To explore some of this potential we identified gene families of cytokinin dehydrogenases (CKX) and isopentenyl transferases, enzymes that respectively irreversibly degrade and synthesize cytokinin (CK) plant hormones, in the raw sequenced barley genome. We then examined their spatial and temporal expression patterns by immunostaining and qPCR. Two CKX-specific antibodies, anti-HvCKX1 and anti-HvCKX9, predominantly detect proteins in the aleurone layer of maturing grains and leaf vasculature, respectively. In addition, two selected CKX genes were used for stable, Agrobacterium tumefaciens-mediated transformation of the barley cultivar Golden Promise. The results show that constitutive overexpression of CKX causes morphological changes in barley plants and prevents their transition to flowering. In all independent transgenic lines roots proliferated more rapidly and root-to-shoot ratios were higher than in wild-type plants. Only one transgenic line, overexpressing CKX under the control of a promoter from a phosphate transporter gene, which is expressed more strongly in root tissue than in aerial parts, yielded progeny. Analysis of several T1-generation plants indicates that plants tend to compensate for effects of the transgene and restore CK homeostasis later during development. Depleted CK levels during early phases of development are restored by down-regulation of endogenous CKX genes and reinforced de novo biosynthesis of CKs.

Proteome and metabolome profiling of cytokinin action in Arabidopsis identifying both distinct and similar responses to cytokinin down- and up-regulation

In plants, numerous developmental processes are controlled by cytokinin (CK) levels and their ratios to levels of other hormones. While molecular mechanisms underlying the regulatory roles of CKs have been intensely researched, proteomic and metabolomic responses to CK deficiency are unknown. Transgenic Arabidopsis seedlings carrying inducible barley cytokinin oxidase/dehydrogenase (CaMV35S>GR>HvCKX2) and agrobacterial isopentenyl transferase (CaMV35S>GR>ipt) constructs were profiled to elucidate proteome- and metabolome-wide responses to down- and up-regulation of CK levels, respectively. Proteome profiling identified >1100 proteins, 155 of which responded to HvCKX2 and/or ipt activation, mostly involved in growth, development, and/or hormone and light signalling. The metabolome profiling covered 79 metabolites, 33 of which responded to HvCKX2 and/or ipt activation, mostly amino acids, carbohydrates, and organic acids. Comparison of the data sets obtained from activated CaMV35S>GR>HvCKX2 and CaMV35S>GR>ipt plants revealed unexpectedly extensive overlaps. Integration of the proteomic and metabolomic data sets revealed: (i) novel components of molecular circuits involved in CK action (e.g. ribosomal proteins); (ii) previously unrecognized links to redox regulation and stress hormone signalling networks; and (iii) CK content markers. The striking overlaps in profiles observed in CK-deficient and CK-overproducing seedlings might explain surprising previously reported similarities between plants with down- and up-regulated CK levels.

A simple sample preparation approach based on hydrophilic solid-phase extraction coupled with liquid chromatography-tandem mass spectrometry for determination of endogenous cytokinins

Cytokinins (CKs), a vital family of phytohormones, play important roles in the regulation of shoot and root development. However, the quantification of CKs in plant samples is frequently affected by the complex plant matrix. In the current study, we developed a simple, rapid and efficient hydrophilic interaction chromatography-solid phase extraction (HILIC-SPE) method for CKs purification. CKs were extracted by acetonitrile (ACN) followed by HILIC-SPE (silica as sorbents) purification. The extraction solution of plant samples could be directly applied to HILIC-SPE without solvent evaporation step, which simplified the analysis process. Moreover, with HILIC chromatographic retention mechanism, the hydrophobic co-extracted impurities were efficiently removed. Subsequently, CKs were separated by RPLC, orthogonal to the HILIC pretreatment process, and detected by tandem mass spectrometry. The method exhibits high specificity and recovery yield (>77.0%). Good linearities were obtained for all eight CKs ranging from 0.002 to 100ngmL(-1) with correlation coefficients (r) higher than 0.9927. The limits of detection (LODs, signal/noise=5) for the CKs were between 1.0 and 12.4pgmL(-1). Reproducibility of the method was evaluated by intra-day and inter-day measurements and the results showed that relative standard deviations (RSDs) were less than 10.5%. Employing this method, we successfully quantified six CKs in 20mg Oryza sativa leaves and the method was also successfully applied to Brassica napus (flower and leaves).

High cytokinin levels induce a hypersensitive-like response in tobacco

BACKGROUND AND AIMS

Cytokinins are positive regulators of shoot development. However, it has previously been demonstrated that efficient activation of the cytokinin biosynthesis gene ipt can cause necrotic lesions and wilting in tobacco leaves. Some plant pathogens reportedly use their ability to produce cytokinins in disease development. In response to pathogen attacks, plants can trigger a hypersensitive response that rapidly kills cells near the infection site, depriving the pathogen of nutrients and preventing its spread. In this study, a diverse set of processes that link ipt activation to necrotic lesion formation were investigated in order to evaluate the potential of cytokinins as signals and/or mediators in plant defence against pathogens.

METHODS

The binary pOp-ipt/LhGR system for dexamethasone-inducible ipt expression was used to increase endogenous cytokinin levels in transgenic tobacco. Changes in the levels of cytokinins and the stress hormones salicylic, jasmonic and abscisic acid following ipt activation were determined by ultra-performance liquid chromatography-electrospray tandem mass spectrometry (UPLC-MS/MS). Trends in hydrogen peroxide content and lipid peroxidation were monitored using the potassium iodide and malondialdehyde assays. The subcellular distribution of hydrogen peroxide was investigated using 3,3'-diaminobenzidine staining. The dynamics of transcripts related to photosynthesis and pathogen response were analysed by reverse transcription followed by quantitative PCR. The effects of cytokinins on photosynthesis were deciphered by analysing changes in chlorophyll fluorescence and leaf gas exchange.

KEY RESULTS

Plants can produce sufficiently high levels of cytokinins to trigger fast cell death without any intervening chlorosis - a hallmark of the hypersensitive response. The results suggest that chloroplastic hydrogen peroxide orchestrates the molecular responses underpinning the hypersensitive-like response, including the inhibition of photosynthesis, elevated levels of stress hormones, oxidative membrane damage and stomatal closure.

CONCLUSIONS

Necrotic lesion formation triggered by ipt activation closely resembles the hypersensitive response. Cytokinins may thus act as signals and/or mediators in plant defence against pathogen attack.

Enhanced drought and heat stress tolerance of tobacco plants with ectopically enhanced cytokinin oxidase/dehydrogenase gene expression

Responses to drought, heat, and combined stress were compared in tobacco (Nicotiana tabacum L.) plants ectopically expressing the cytokinin oxidase/dehydrogenase CKX1 gene of Arabidopsis thaliana L. under the control of either the predominantly root-expressed WRKY6 promoter or the constitutive 35S promoter, and in the wild type. WRKY6:CKX1 plants exhibited high CKX activity in the roots under control conditions. Under stress, the activity of the WRKY6 promoter was down-regulated and the concomitantly reduced cytokinin degradation coincided with raised bioactive cytokinin levels during the early phase of the stress response, which might contribute to enhanced stress tolerance of this genotype. Constitutive expression of CKX1 resulted in an enlarged root system, a stunted, dwarf shoot phenotype, and a low basal level of expression of the dehydration marker gene ERD10B. The high drought tolerance of this genotype was associated with a relatively moderate drop in leaf water potential and a significant decrease in leaf osmotic potential. Basal expression of the proline biosynthetic gene P5CSA was raised. Both wild-type and WRKY6:CKX1 plants responded to heat stress by transient elevation of stomatal conductance, which correlated with an enhanced abscisic acid catabolism. 35S:CKX1 transgenic plants exhibited a small and delayed stomatal response. Nevertheless, they maintained a lower leaf temperature than the other genotypes. Heat shock applied to drought-stressed plants exaggerated the negative stress effects, probably due to the additional water loss caused by a transient stimulation of transpiration. The results indicate that modulation of cytokinin levels may positively affect plant responses to abiotic stress through a variety of physiological mechanisms.

Auxin and cytokinin relationships in 24 microalgal strains(1)

Endogenous auxins and cytokinins were quantitated in 24 axenic microalgal strains from the Chlorophyceae, Trebouxiophyceae, Ulvophyceae, and Charophyceae. These strains were in an exponential growth phase, being harvested on day 4. Acutodesmus acuminatus Mosonmagyaróvár Algal Culture Collection-41 (MACC) produced the highest biomass and Chlorococcum ellipsoideum MACC-712 the lowest biomass. The auxins, indole-3-acetic acid (IAA) and indole-3-acetamide (IAM) were present in all microalgal strains. No other auxin conjugates were detected. IAA and IAM concentrations varied greatly, ranging from 0.50 to 71.49 nmol IAA · g(-1) DW and 0.18 to 99.83 nmol IAM · g(-1) DW, respectively. In 19 strains, IAA occurred in higher concentrations than IAM. Nineteen cytokinins were identified in the microalgal strains. Total cytokinin concentrations varied, ranging from 0.29 nmol · g(-1) DW in Klebsormidium flaccidum MACC-692 to 21.40 nmol · g(-1) DW in Stigeoclonium nanum MACC-790. The general trend was that cis-zeatin types were the predominant cytokinins; isopentenyladenine-type cytokinins were present in moderate concentrations, while low levels of trans-zeatin-type and very low levels of dihydrozeatin-type cytokinins were detected. Ribotides were generally the main cytokinin conjugate forms present with the cytokinin free bases and ribosides present in similar but moderate levels. The levels of O-glucosides were low. Only one N-glucoside was detected, being present in nine strains in very low concentrations. In 15 strains, the auxin content was 2- to 4-fold higher than the cytokinin content.

Endogenous cytokinin and auxin profiles during in vitro organogenesis from vegetative buds of Pinus radiata adult trees

In Pinus radiata D. Don, the transition from the juvenile to the mature phase is characterized by a reduction in the tree's organogenic potential, which is usually reverted in breeding programs by reinvigoration procedures to enable vegetative propagation. In this work, we have determined the best culture conditions for in vitro reinvigoration of radiata pine buds, tested different cytokinin (CK) types [N⁶-benzyladenine (BA), meta-topolin (mT) and trans-zeatin] and concentrations (25 and 50 µM), and studied the effect of culture conditions on endogenous CK and indole-3-acetic acid (IAA) levels at different stages of the organogenic process. To this end, the levels of 43 CKs and IAA were determined in P. radiata buds before and during the reinvigoration process. When BA or mT was applied to the induction medium, we did not observe any significant increase or decrease in endogenous isoprenoid CK content. We also report for the first time the presence of O-glucosides in non-treated P. radiata explants from the field and remark the importance of O-glucosides as storage forms.

Endogenous cytokinin profiles and their relationships to between-family differences during adventitious caulogenesis in Pinus pinea cotyledons

Caulogenesis in mature stone pine (Pinus pinea L.) cotyledons is promoted, to varying degrees depending on genotype, by exogenous application of the cytokinin (CK) benzyladenine (BA). In the present study, endogenous CK profiles of cotyledons from open-pollinated plants and two families of stone pine with widely differing organogenic capacities were monitored during caulogenesis and linked to previously characterized BA uptake and induction phases. Changes in levels of free bases, ribosides, ribotides and glucosides of both isoprenoid and aromatic CKs were followed. Before BA application, the pool of endogenous CKs in all sets of cotyledons was dominated by isoprenoid ribotides, but 1h after BA exposure, aromatic CKs (mainly active free bases and ribosides of topolins) accounted for more than 90% of the pool. BA N-glucosides were also observed, levels of which (and topolins) rose from 2d until the end of the (six-day) culture period. The CK profiles of the two selected pine families also differed, although the general trends were similar. During the first 6h, levels of BA and meta-topolin were highest in cotyledons from the family with the strongest caulogenic responses, while levels of ribotides and aromatic glucosides were highest in cotyledons from the other family.

Physiological responses and endogenous cytokinin profiles of tissue-cultured 'Williams' bananas in relation to roscovitine and an inhibitor of cytokinin oxidase/dehydrogenase (INCYDE) treatments

The effect of supplementing either meta-topolin (mT) or N(6)-benzyladenine (BA) requiring cultures with roscovitine (6-benzylamino-2-[1(R)-(hydroxymethyl)propyl]amino-9-isopropylpurine), a cyclin-dependent kinase (CDK) and N-glucosylation inhibitor, and INCYDE (2-chloro-6-(3-methoxyphenyl)aminopurine), an inhibitor of cytokinin (CK) degradation, on the endogenous CK profiles and physiology of banana in vitro was investigated. Growth parameters including multiplication rate and biomass were recorded after 42 days. Endogenous CK levels were quantified using UPLC-MS/MS while the photosynthetic pigment and phenolic contents were evaluated spectrophotometrically. The highest regeneration rate (93 %) was observed in BA + roscovitine while mT + INCYDE plantlets produced most shoots. Treatment with BA + roscovitine had the highest shoot length and biomass. Although not significant, there was a higher proanthocyanidin level in BA + roscovitine treatments compared to the control (BA). The levels of total phenolics and flavonoids were significantly higher in mT + roscovitine treatment than in the mT-treated regenerants. The presence of roscovitine and/or INCYDE had no significant effect on the photosynthetic pigments of the banana plantlets. Forty-seven aromatic and isoprenoid CKs categorized into nine CK-types were detected at varying concentrations. The presence of mT + roscovitine and/or INCYDE increased the levels of O-glucosides while 9-glucosides were higher in the presence of BA. Generally, the underground parts had higher CK levels than the aerial parts; however, the presence of INCYDE increased the level of CK quantified in the aerial parts. From a practical perspective, the use of roscovitine and INCYDE in micropropagation could be crucial in the alleviation of commonly observed in vitro-induced physiological abnormalities.