Profiling of plant hormones by mass spectrometry

Organ-level distribution tandem mass spectrometry analysis of three structural types of brassinosteroids in rapeseed

Background

Brassinosteroids (BRs) are a class of naturally occurring steroidal phytohormones mediating a wide range of pivotal developmental and physiological functions throughout the plant's life cycle. Therefore, it is of great significance to determine the content and the distribution of BRs in plants.Regretfully, although a large number of quantitative methods for BRs by liquid chromatography-tandem mass spectrometry (LC-MS/MS) have been reported, the in planta distribution of BRs is still unclear because of their lower contents in plant tissues and the lack of effective ionizable groups in their chemical structures.

Methods

We stablished a novel analytical method of BRs based on C18 cartridge solid-phase extraction (SPE) purification, 4-(dimethylamino)-phenylboronic acid (DMAPBA) derivatization, and online valve-switching system coupled with ultra-high performance liquid chromatography-electro spray ionization-triple quadrupole mass spectrometry (UHPLC-ESI-MS/MS). This method has been used to quantify three structural types of BRs (epibrassinolide, epicastasterone, and 6-deoxo-24-epicastaster one) in different organs of Brassica napus L. (rapeseed).

Results

We obtained the contents of three structural types of BRs in various organ tissues of rapeseed. The contents of three BRs in rapeseed flowers were the highest, followed by tender pods. The levels of three BRs all decreased during the maturation of the organs. We outlined the spatial distribution maps of three BRs in rapeseed based on these results, so as to understand the spatial distribution of BRs at the visual level.

Conclusions

Our results provided useful information for the precise in situ localization of BRs in plants and the metabolomic research of BRs in future work. The in planta spatial distribution of BRs at the visual level has been studied for the first time.

Abscisic acid inhibits primary root growth by impairing ABI4-mediated cell cycle and auxin biosynthesis

Cell cycle progression and the phytohormones auxin and abscisic acid (ABA) play key roles in primary root growth, but how ABA mediates the transcription of cell cycle-related genes and the mechanism of crosstalk between ABA and auxin requires further research. Here, we report that ABA inhibits primary root growth by regulating the ABA INSENSITIVE4 (ABI4)-CYCLIN-DEPENDENT KINASE B2;2 (CDKB2;2)/CYCLIN B1;1 (CYCB1;1) module-mediated cell cycle as well as auxin biosynthesis in Arabidopsis (Arabidopsis thaliana). ABA induced ABI4 transcription in the primary root tip, and the abi4 mutant showed an ABA-insensitive phenotype in primary root growth. Compared with the wild type (WT), the meristem size and cell number of the primary root in abi4 increased in response to ABA. Further, the transcription levels of several cell-cycle positive regulator genes, including CDKB2;2 and CYCB1;1, were upregulated in abi4 primary root tips. Subsequent chromatin immunoprecipitation (ChIP)-seq, ChIP-qPCR, and biochemical analysis revealed that ABI4 repressed the expression of CDKB2;2 and CYCB1;1 by physically interacting with their promoters. Genetic analysis demonstrated that overexpression of CDKB2;2 or CYCB1;1 fully rescued the shorter primary root phenotype of ABI4-overexpression lines, and consistently, abi4/cdkb2;2-cr or abi4/cycb1;1-cr double mutations largely rescued the ABA-insensitive phenotype of abi4 with regard to primary root growth. The expression levels of DR5promoter-GFP and PIN1promoter::PIN1-GFP in abi4 primary root tips were significantly higher than those in WT after ABA treatment, with these changes being consistent with changes in auxin concentration and expression patterns of auxin biosynthesis genes. Taken together, these findings indicated that ABA inhibits primary root growth through ABI4-mediated cell cycle and auxin-related regulatory pathways.

Spatiotemporal plant hormone analysis from cryosections using laser microdissection-liquid chromatography-mass spectrometry

Laser microdissection (LMD) is used for isolating specific regions or single cells from a wide variety of tissue samples under direct microscopic observation. The LMD method enables the harvest of the cells of interest in a region or specific cells for several analyses, such as DNA/RNA analysis, proteomics, metabolomics, and other molecular analyses. Currently, LMD is used to study various biological events at the tissue or cellular level; it has been used in a wide range of research fields. In this report, we describe techniques for isolating different tissues/specific cells from cryosections of incised Arabidopsis flowering stems by LMD for spatiotemporal quantitative plant hormone analysis. The endogenous indole-3-acetic acid levels in the epidermis/cortex, vascular bundles, and pith of Arabidopsis flowering stems were approximately 19.0 pg mm^-3, 33.5 pg mm^-3, and 3.32 pg mm^-3, respectively, and these endogenous levels were altered spatiotemporally after incision. We also analyzed jasmonic acid from LMD-isolated cells and showed that the endogenous levels increased in the range of approximately 200-3,500 pg mm^-3 depending on the tissue and region at 1 h after incision and then decreased to less than 100 pg mm^-3 or undetectable levels at 24 h after incision. Quantitative analyses of phytohormones, including jasmonic acid-related molecules, gibberellin, abscisic acid, and cytokinins, could also be performed using the same cell samples. These results showed that spatiotemporal changes in plant hormones could be quantitatively and simultaneously analyzed by LMD-isolated cells from cryosections with positional information. The combination of quantitative analysis by liquid chromatography-mass spectrometry (LC-MS) and sampling by the LMD method provides a comprehensive and quantitative understanding of spatiotemporal changes in plant hormones in a region- and tissue-specific manner. Therefore, LMD-LC-MS methods will contribute to our understanding of the physiological events that control the process of plant growth and development.

[Novel adsorption material for solid phase extraction in sample pretreatment of plant hormones]

Plant hormones (PHs) are of significance in plant growth, as they regulate the various processes related to plant growth, development, and resistance. Sensitive and precise quantitative analysis of PHs is a bottleneck in plant science research. Currently, liquid chromatography-tandem mass spectrometry is used for the accurate and efficient detection of PHs. Sample pretreatment is an indispensable step in the chromatography-mass spectrometry analysis of PHs because it directly affects the sensitivity and accuracy of subsequent detection methods. Among various pretreatment methods for PHs, solid phase extraction (SPE) is the most widely used. Various new types of SPE, such as dispersive SPE, magnetic SPE, and solid phase microextraction, have been developed by modifying the extraction cartridge. The choice of adsorption material is the key factor in the abovementioned SPE methods, which has a decisive effect on the extraction, purification, and enrichment effects of the target substance in the sample pretreatment process. Carbon-based materials, including carbon nanotubes, graphene, carbon and nitrogen compounds, as well as organic frameworks, including metal organic frameworks and covalent organic materials, are suitable adsorption materials because of their designable structure, large specific surface area, and good stability. Molecularly imprinted polymers and supramolecular compounds show specific molecular recognition based on host-guest interactions, which can significantly improve the selectivity of sample pretreatment methods. In this paper, SPE-related technology and the abovementioned types of functionalized adsorption materials in the pretreatment of PHs prevalent in the past five years have been reviewed. The related development trends are also summarized.

A Comprehensive Targeted Metabolomics Assay for Crop Plant Sample Analysis

Metabolomics plays an important role in various fields from health to agriculture. However, the comprehensive quantitative metabolomic analysis of plants and plant metabolites has not been widely performed. Liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS)-based plant metabolomics offers the sensitivity and breadth of coverage for both phenotyping and disease diagnosis of plants. Here, we report a high-coverage and quantitative MS-based assay for plant metabolite analysis. The assay detects and quantifies 206 primary and secondary plant metabolites, including many key plant hormones. In total, it measures 28 amino acids and derivatives, 27 organic acids, 20 biogenic amines and derivatives, 40 acylcarnitines, 90 phospholipids and C-6 sugars. All the analysis methods in this assay are based on LC-MS/MS techniques using both positive and negative-mode multiple reaction monitoring (MRM). The recovery rates of spiked plant samples at three different concentration levels (low, medium and high) ranged from 80% to 120%, with satisfactory precision values of less than 20%. This targeted plant metabolomic assay has been successfully applied to the analysis of large numbers of pine and spruce needle samples, canola root samples, as well as cannabis samples. Moreover, the assay was specifically developed in a 96-well plate format, which enables automated, high-throughput sample analysis. This assay has already been used to analyze over 1500 crop plant samples in less than two months.

Pinus pinaster Early Hormonal Defence Responses to Pinewood Nematode (Bursaphelenchus xylophilus) Infection

The pinewood nematode (PWN) is the causal agent of pine wilt disease, a pathology that affects conifer forests, mainly Pinus spp. PWN infection can induce the expression of phytohormone-related genes; however, changes at the early phytohormone level have not yet been explored. Phytohormones are low-abundance metabolites, and thus, difficult to quantify. Moreover, most methodologies focus mainly on Arabidopsis or crop species. This work aimed to validate a fast (run time 6.6 min) liquid chromatography-triple quadrupole tandem mass spectrometry (LC-QqQ-MS/MS) analytical method to quantify 14 phytohormones in Pinus pinaster stem tissues. This method was further applied to evaluate, for the first time, early phytohormone changes in susceptible and resistant phenotypes of P. pinaster 24, 48 and 72 h after inoculation (HAI) with PWN. A significant increase in salicylic acid (SA, 48 and 72 HAI) and jasmonic acid methyl ester (JA-ME, 72 HAI) was observed in susceptible phenotypes. Results indicate that the higher susceptibility of P. pinaster to PWN infection might result from an inefficient trigger of hypersensitive responses, with the involvement of JA and SA pathways. This work provides an important update in forest research, and adds to the current knowledge of Pinus spp. defence responses to PWN infection.

A Tobacco Syringe Agroinfiltration-Based Method for a Phytohormone Transporter Activity Assay Using Endogenous Substrates

Phytohormones are a group of small chemical molecules that play vital roles in plant development, metabolism, and stress responses. Phytohormones often have distinct biosynthesis and signaling perception sites, requiring long- or short-distance transportation. Unlike biosynthesis and signal transduction, phytohormone transport across cells and organs is poorly understood. The transporter activity assay is a bottleneck for the functional characterization of novel phytohormone transporters. In the present study, we report a tobacco syringe agroinfiltration and liquid chromatography tandem mass spectrometry (TSAL)-based method for performing a phytohormone transporter activity assay using endogenous hormones present in tobacco (Nicotiana benthamiana) leaves. A transporter activity assay using this method does not require isotope-labeled substrates and can be conveniently performed for screening multiple substrates by using endogenous hormones in tobacco leaves. The transporter activities of three known hormone transporters, namely AtABCG25 for abscisic acid, AtABCG16 for jasmonic acid, and AtPUP14 for cytokinin, were all successfully validated using this method. Using this method, cytokinins were found to be the preferred substrates of an unknown maize (Zea mays) transporter ZmABCG43. ZmABCG43 transporter activities toward cytokinins were confirmed in a cytokinin long-distance transport mutant atabcg14 through gene complementation. Thus, the TSAL method has the potential to be used for basic substrate characterization of novel phytohormone transporters or for the screening of novel transporters for a specific phytohormone on a large scale.

A stable isotope dilution method for a highly accurate analysis of karrikins

BACKGROUND

Karrikins (KARs) are recently described group of plant growth regulators with stimulatory effects on seed germination, seedling growth and crop productivity. So far, an analytical method for the simultaneous targeted profiling of KARs in plant tissues has not been reported.

RESULTS

We present a sensitive method for the determination of two highly biologically active karrikins (KAR₁ and KAR₂) in minute amounts of plant material (< 20 mg fresh weight). The developed protocol combines the optimized extraction and efficient single-step sample purification with ultra-high performance liquid chromatography-tandem mass spectrometry. Newly synthesized deuterium labelled KAR₁ was employed as an internal standard for the validation of KAR quantification using a stable isotope dilution method. The application of the matrix-matched calibration series in combination with the internal standard method yields a high level of accuracy and precision in triplicate, on average bias 3.3% and 2.9% RSD, respectively. The applicability of this analytical approach was confirmed by the successful analysis of karrikins in Arabidopsis seedlings grown on media supplemented with different concentrations of KAR₁ and KAR₂ (0.1, 1.0 and 10.0 µmol/l).

CONCLUSIONS

Our results demonstrate the usage of methodology for routine analyses and for monitoring KARs in complex biological matrices. The proposed method will lead to better understanding of the roles of KARs in plant growth and development.

Mass spectrometry-based forest tree metabolomics

Research in forest tree species has advanced slowly when compared with other agricultural crops and model organisms, mainly due to the long-life cycles, large genome sizes, and lack of genomic tools. Additionally, trees are complex matrices, and the presence of interferents (e.g., oleoresins and cellulose) challenges the analysis of tree tissues with mass spectrometry (MS)-based analytical platforms. In this review, advances in MS-based forest tree metabolomics are discussed. Given their economic and ecological significance, particular focus is given to Pinus, Quercus, and Eucalyptus forest tree species to better understand their metabolite responses to abiotic and biotic stresses in the current climate change scenario. Furthermore, MS-based metabolomics technologies produce large and complex datasets that require expertize to adequately manage, process, analyze, and store the data in dedicated repositories. To ensure that the full potential of forest tree metabolomics data are translated into new knowledge, these data should comply with the FAIR principles (i.e., Findable, Accessible, Interoperable, and Re-usable). It is essential that adequate standards are implemented to annotate metadata from forest tree metabolomics studies as is already required by many science and governmental agencies and some major scientific publishers. © 2019 John Wiley & Sons Ltd. Mass Spec Rev 40:126-157, 2021.

Recent developments and emerging trends of mass spectrometric methods in plant hormone analysis: a review

Plant hormones are naturally occurring small molecule compounds which are present at trace amounts in plant. They play a pivotal role in the regulation of plant growth. The biological activity of plant hormones depends on their concentrations in the plant, thus, accurate determination of plant hormone is paramount. However, the complex plant matrix, wide polarity range and low concentration of plant hormones are the main hindrances to effective analyses of plant hormone even when state-of-the-art analytical techniques are employed. These factors substantially influence the accuracy of analytical results. So far, significant progress has been realized in the analysis of plant hormones, particularly in sample pretreatment techniques and mass spectrometric methods. This review describes the classic extraction and modern microextraction techniques used to analyze plant hormone. Advancements in solid phase microextraction (SPME) methods have been driven by the ever-increasing requirement for dynamic and in vivo identification of the spatial distribution of plant hormones in real-life plant samples, which would contribute greatly to the burgeoning field of plant hormone investigation. In this review, we describe advances in various aspects of mass spectrometry methods. Many fragmentation patterns are analyzed to provide the theoretical basis for the establishment of a mass spectral database for the analysis of plant hormones. We hope to provide a technical guide for further discovery of new plant hormones. More than 140 research studies on plant hormone published in the past decade are reviewed, with a particular emphasis on the recent advances in mass spectrometry and sample pretreatment techniques in the analysis of plant hormone. The potential progress for further research in plant hormones analysis is also highlighted.

Experimental Design and Sample Preparation in Forest Tree Metabolomics

Appropriate experimental design and sample preparation are key steps in metabolomics experiments, highly influencing the biological interpretation of the results. The sample preparation workflow for plant metabolomics studies includes several steps before metabolite extraction and analysis. These include the optimization of laboratory procedures, which should be optimized for different plants and tissues. This is particularly the case for trees, whose tissues are complex matrices to work with due to the presence of several interferents, such as oleoresins, cellulose. A good experimental design, tree tissue harvest conditions, and sample preparation are crucial to ensure consistency and reproducibility of the metadata among datasets. In this review, we discuss the main challenges when setting up a forest tree metabolomics experiment for mass spectrometry (MS)-based analysis covering all technical aspects from the biological question formulation and experimental design to sample processing and metabolite extraction and data acquisition. We also highlight the importance of forest tree metadata standardization in metabolomics studies.

Effects of spraying nano-materials on the absorption of metal(loid)s in cucumber

This report investigates the spraying of nano-silica and fullerene on cucumber leaves to expose their ability to reduce the toxicity and uptake of metal(loid)s. Cucumber seedlings were randomly divided into six treatment groups: 10 mg/L nano-SiO2, 20 mg/L nano-SiO2, 10 mg/L Fullerene, 20 mg/L Fullerene, 5 mg/L Fullerene + 5 mg/L nano-SiO2, and 10 mg/L Fullerene + 10 mg/L nano-SiO2. Nano-silica-treated plants exhibited evidence of the potential mitigation of metal(loid)s poisoning. Specifically, results showed that 20 mg/L of nano-silica promoted Cd uptake by plants; comparatively, 10 mg/L of nano-silica did not significantly increase the silicon content in plants. Both low-concentration combined treatment and low-concentration fullerene groups inhibited metal(loid)s uptake by plants. Scanning electron microscopy (SEM) was then used to observe the surface morphology of cucumber leaves. Significant differences were observed on disease resistance in plants across the different nano-material conditions. Collectively, these findings suggest that both nano-silica materials and fullerene have the potential to control metal(loid)s toxicity in plants.

One-pot sample preparation approach for profiling spatial distribution of gibberellins in a single shoot of germinating cereal seeds

Sample preparation remains a bottleneck in the rapid and reliable quantification of gibberellins (GAs) for obtaining an insight into the physiological processes mediated by GAs. The challenges arise from not only the extremely low content of GAs in complex plant matrices, but the poor detectability of GAs by mass spectrometry (MS) in negative ion mode. In an effort to solve these urgent difficulties, we present a spatial-resolved analysis method to investigate the distribution of GAs in tiny plant tissues based on a simplified one-pot sample preparation approach coupled with ultrahigh-performance liquid chromatography-tandem MS. By integrating extraction and derivatization into one step, target GAs were effectively extracted from plant materials and simultaneously reacted with N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide, the sample preparation time was largely shortened, the probability of sample loss was minimized and the detection sensitivity of MS was also greatly improved compared with underivatized GAs. Under optimal conditions, the method was validated from the quantification linearity, limits of detection and limits of quantification in the presence of plant matrices, recoveries, and precision. With the proposed method, 15 endogenous GAs were detected and, among these, 11 GAs could be quantified in 0.50 mg fresh weight (FW) wheat shoot samples, and five GAs were quantified in only 0.15 mg FW developing seed samples of Arabidopsis thaliana. The distribution patterns of GAs along both the non-13-hydroxylation pathway and the early 13-hydroxylation pathway in a single shoot of germinating wheat, rice and maize seeds were finally profiled with a spatial resolution down to approximately 1 mm² .

Fall armyworm (Spodoptera frugiperda Smith) feeding elicits differential defense responses in upland and lowland switchgrass

Switchgrass (Panicum virgatum L.) is a low input, high biomass perennial grass being developed for the bioenergy sector. Upland and lowland cultivars can differ in their responses to insect herbivory. Fall armyworm [FAW; Spodoptera frugiperda JE Smith (Lepidoptera: Noctuidae)] is a generalist pest of many plant species and can feed on switchgrass as well. Here, in two different trials, FAW larval mass were significantly reduced when fed on lowland cultivar Kanlow relative to larvae fed on upland cultivar Summer plants after 10 days. Hormone content of plants indicated elevated levels of the plant defense hormone jasmonic acid (JA) and its bioactive conjugate JA-Ile although significant differences were not observed. Conversely, the precursor to JA, 12-oxo-phytodienoic acid (OPDA) levels were significantly different between FAW fed Summer and Kanlow plants raising the possibility of differential signaling by OPDA in the two cultivars. Global transcriptome analysis revealed a stronger response in Kanlow plant relative to Summer plants. Among these changes were a preferential upregulation of several branches of terpenoid and phenylpropanoid biosynthesis in Kanlow plants suggesting that enhanced biosynthesis or accumulation of antifeedants could have negatively impacted FAW larval mass gain on Kanlow plants relative to Summer plants. A comparison of the switchgrass-FAW RNA-Seq dataset to those from maize-FAW and switchgrass-aphid interactions revealed that key components of plant responses to herbivory, including induction of JA biosynthesis, key transcription factors and JA-inducible genes were apparently conserved in switchgrass and maize. In addition, these data affirm earlier studies with FAW and aphids that the cultivar Kanlow can provide useful genetics for the breeding of switchgrass germplasm with improved insect resistance.

Yeasts producing zeatin

The present paper describes the first screening study of the ability of natural yeast strains to synthesize in culture the plant-related cytokine hormone zeatin, which was carried out using HPLC-MS/MS. A collection of 76 wild strains of 36 yeast species (23 genera) isolated from a variety of natural substrates was tested for the production of zeatin using HPLC-MS/MS. Zeatin was detected in more than a half (55%) of studied strains and was more frequently observed among basidiomycetous than ascomycetous species. The amount of zeatin accumulated during the experiment varied among species and strains. Highest zeatin values were recorded for basidiomycete Sporobolomyces roseus and ascomycete Taphrina sp. that produced up to 8,850.0 ng and 5,166.4 ng of zeatin per g of dry biomass, respectively. On average, the ability to produce zeatin was more pronounced among species isolated from the arctic-alpine zone than among strains from tropical and temperate climates. Our study also demonstrated that epiphytic strains and pigmented yeast species, typically for phyllosphere, are able to more often produce a plant hormone zeatin than other yeasts.

An ionic liquid functionalized graphene adsorbent with multiple adsorption mechanisms for pipette-tip solid-phase extraction of auxins in soybean sprouts

A new ionic liquid functionalized graphene-pipette-tip solid-phase extraction method coupled with high-performance liquid chromatography was established for the simultaneous extraction and determination of three auxins in soybean sprouts. The graphene adsorbent, with multiple adsorption mechanisms, was first synthesized by functional modification of pentafluorobenzyl imidazolium bromide ionic liquid through thiol-ene click chemistry. The ionic liquid was applied to prevent the aggregation of graphene; it also imbued graphene with the ability for π-π interactions, ionic exchange, electrostatic interactions, as well as hydrogen bonding (which is stronger than the interaction between water and analytes), by augmenting the adsorption mechanisms between the adsorbent and analytes. Under optimized conditions, linearity was achieved in the ranges 0.03-5.00 µg/g for indole-3-acetic acid and 1-naphthaleneacetic acid and 0.09-5.00 µg/g for 2,4-dichlorophenoxyacetic acid, with a detection limit of 0.004-0.026 µg/g; this adsorbent has been successfully applied for the determination of auxins in soybean sprouts.

Standard Key Steps in Mass Spectrometry-Based Plant Metabolomics Experiments: Instrument Performance and Analytical Method Validation

Studies of the plant metabolome include the analysis of a wide range of chemical species with very diverse physicochemical properties requiring powerful analytical tools for the separation, characterization, and quantification of this vast compound diversity present in plant matrices. In quantitative metabolomics studies, major efforts are put into optimizing sample extraction and separation as well as instrument conditions to measure specific plant metabolites. Here, challenges in the use of mass spectrometry (MS) as a quantitative tool in plant metabolomics experiments are discussed, and an overview of the most critical steps in the development and validation of MS-based analytical methods is presented.

Spatial Profiling of Gibberellins in a Single Leaf Based on Microscale Matrix Solid-Phase Dispersion and Precolumn Derivatization Coupled with Ultraperformance Liquid Chromatography-Tandem Mass Spectrometry

A spatial-resolved analysis method for profiling of gibberellins (GAs) in a single leaf was developed on the basis of microscale sample preparation and precolumn derivatization coupled with ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The proposed microscale sample preparation was based on modified matrix solid-phase dispersion (MSPD) method, in which the plant sample (<1 mg) and C18 sorbent were ground together in one microcentrifuge tube, and then extraction solvent was added followed by centrifugation. In this protocol, the grinding, extraction, and purification were performed in one microcentrifuge tube without any sample transfer step, resulting in an obvious decrease in sample loss. Moreover, a new derivatization reagent, 3-bromopropyltrimethylammonium bromide (BPTAB), was used to further enhance the signal intensities of GAs on MS by 3-4 orders of magnitude, which was much higher than the reported derivatization reagents for GAs such as bromocholine bromide and 3-bromoactonyltrimethylammonium bromide. The present method showed high sensitivity (minimum detectable amount (MDA) of 10.1-72.3 amol for eight GAs) and low sample consumption (down to 0.30 mg FW). Under the optimized conditions, the distribution of GA₁₉ in a single Arabidopsis thaliana leaf was profiled with a spatial resolution of 2 × 2 mm².

Quantification of near-attomole gibberellins in floral organs dissected from a single Arabidopsis thaliana flower

There remains a methodological bottleneck in the quantification of ultra-trace plant hormones in very tiny plant organs at fresh weights below a milligram. The challenge becomes even more serious in the determination of endogenous gibberellins (GAs), which are a class of compounds that are difficult to separate and detect. Herein, a quantification method using ultra-high-performance liquid chromatography-tandem mass spectrometry was developed, combined with a derivatization technique in which GAs react with N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide in ethanol. The method was validated as capable of determining GAs in floral organs (about 80-250 μg) - pistil, stamens, petals, sepals and receptacle - which were dissected from only one flower of Arabidopsis thaliana. Substantially different abundance patterns of GAs were measured from the floral organs at floral stages 13, 14 and 15 along the non-13-hydroxylation pathway and the early 13-hydroxylation pathway in plants. This allows sub-flower-level insights into how GAs affect floral development. The method exhibited excellent limit of detection and limit of quantification down to 5.41 and 18.0 attomole, respectively, and offered a fairly wide linear range from 0.01 to 25 femtomole with linear coefficients above 0.9961. The precision of the method was evaluated with relative standard deviations below 10.6% for intra-day and 11.4% for inter-day assays, and recoveries ranged from 64.0% to 107%.

Quantification of Cytokinin Levels and Responses in Abiotic Stresses

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

Hormonal profiling: Development of a simple method to extract and quantify phytohormones in complex matrices by UHPLC-MS/MS

Plant growth regulators (PGRs) are very different chemical compounds that play essential roles in plant development and the regulation of physiological processes. They exert their functions by a mechanism called cross-talk (involving either synergistic or antagonistic actions) thus; it is for great interest to study as many PGRs as possible to obtain accurate information about plant status. Much effort has been applied to develop methods capable of analyze large numbers of these compounds but frequently excluding some chemical families or important PGRs within each family. In addition, most of the methods are specially designed for matrices easy to work with. Therefore, we wanted to develop a method which achieved the requirements lacking in the literature and also being fast and reliable. Here we present a simple, fast and robust method for the extraction and quantification of 20 different PGRs using UHPLC-MS/MS optimized in complex matrices.

Mass spectrometry as a quantitative tool in plant metabolomics

Metabolomics is a research field used to acquire comprehensive information on the composition of a metabolite pool to provide a functional screen of the cellular state. Studies of the plant metabolome include the analysis of a wide range of chemical species with very diverse physico-chemical properties, and therefore powerful analytical tools are required for the separation, characterization and quantification of this vast compound diversity present in plant matrices. In this review, challenges in the use of mass spectrometry (MS) as a quantitative tool in plant metabolomics experiments are discussed, and important criteria for the development and validation of MS-based analytical methods provided.This article is part of the themed issue 'Quantitative mass spectrometry'.

Progress and development of analytical methods for gibberellins

Gibberellins, as a group of phytohormones, exhibit a wide variety of bio-functions within plant growth and development, which have been used to increase crop yields. Many analytical procedures, therefore, have been developed for the determination of the types and levels of endogenous and exogenous gibberellins. As plant tissues contain gibberellins in trace amounts (usually at the level of nanogram per gram fresh weight or even lower), the sample pre-treatment steps (extraction, pre-concentration, and purification) for gibberellins are reviewed in details. The primary focus of this comprehensive review is on the various analytical methods designed to meet the requirements for gibberellins analyses in complex matrices with particular emphasis on high-throughput analytical methods, such as gas chromatography, liquid chromatography, and capillary electrophoresis, mostly combined with mass spectrometry. The advantages and drawbacks of the each described analytical method are discussed. The overall aim of this review is to provide a comprehensive and critical view on the different analytical methods nowadays employed to analyze gibberellins in complex sample matrices and their foreseeable trends.

Paclobutrazol Residue Determination in Potato and Soil Using Low Temperature Partition Extraction and Ultrahigh Performance Liquid Chromatography Tandem Mass Spectrometry

A simple, accurate, and highly sensitive analytical method was developed for determining the paclobutrazol residue in potato and soil, the dynamics dissipation in soil. Extraction was carried out by low temperature partitioning and analyzed by ultrahigh performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS). For a favor extraction yield, the parameters such as temperature and solvent were optimized. The result showed that sample would be easily frozen and separated using acetonitrile under -20°C for 10 min. The limit of detection (LOD) was 0.5 μg/kg, and the limit of quantification (LOQ) was 2 and 5 μg/kg for potato and soil, respectively. The influence of paclobutrazol residue in potato was evaluated. The possible contamination of paclobutrazol from surface can be rinsed by distilled water or peeled off, but the paclobutrazol in potato harvest comes mainly from absorption and transport, which could not be removed by peeling. The half-life of paclobutrazol in soil was 20.64 days, and the residue was below 0.22 mg/kg on 50th day after spraying. According to the risk assessment with Need Maximum Daily Intake (NEDI) and Acceptable Daily Intake (ADI), a Maximum Residue Limit (MRL) of paclobutrazol in potato was recommended as 1.0 mg/kg.

Identification of jasmonic acid-associated microRNAs and characterization of the regulatory roles of the miR319/TCP4 module under root-knot nematode stress in tomato

MicroRNAs (miRNAs) are important transcriptional and post-transcriptional modulators of gene expression that play crucial roles in the responses to diverse stresses. To explore jasmonic acid (JA)-dependent miRNA-mediated regulatory networks that are responsive to root-knot nematode (RKN), two small RNA libraries were constructed from wild-type (WT) and JA mutant (spr2) plants. A total of 263 known miRNAs and 441 novel miRNAs were significantly regulated under RKN stress in the two libraries. The spatio-temporal expression of candidate miRNAs and their corresponding targets were analysed by qRT-PCR under RKN stress. A clear negative correlation was observed between miR319 and its target TEOSINTE BRANCHED1/CYCLOIDEA/PRO-LIFERATING CELL FACTOR 4 (TCP4) in leaf, stem, and root under RKN stress, implying that the miR319/TCP4 module is involved in the systemic defensive response. Reverse genetics demonstrated that the miR319/TCP4 module affected JA synthetic genes and the endogenous JA level in leaves, thereby mediating RKN resistance. These results suggested that the action of miR319 in serving as a systemic signal responder and regulator that modulated the RKN systemic defensive response was mediated via JA. The potential cross-talk between miR319/TCP4 and miR396/GRF (GROWTH RESPONDING FACTOR) in roots under RKN invasion is discussed, and a predictive model regarding miR319/TCP4-mediated RKN resistance is proposed.

Simultaneous determination of plant hormones in peach based on dispersive liquid-liquid microextraction coupled with liquid chromatography-ion trap mass spectrometry

Fruit development is influenced greatly by endogenous hormones including salicylic acid (SA) and abscisic acid (ABA). Mass spectrometry with high sensitivity has become a routine technology to analyze hormones. However, pretreatment of plant samples remains a difficult problem. Thus, dispersive liquid-liquid microextraction (DLLME) was used to concentrate trace plant hormones before liquid chromatography-ion trap mass spectrometry (LC-ITMS) analysis. Standard curves were linear within the ranges of 0.5-50, 0.2-20ng/mL for SA and ABA, respectively. The correlation coefficients were greater than 0.9995 with recoveries above 87.5%. The limits of detection were 0.2ng/mL for SA and 0.1ng/mL for ABA in spiked water solution, respectively (injection 20μL). The successful analysis of SA and ABA in fruit samples indicated our DLLME-LC-ITMS approach was efficient, allowing reliable quantification of both two compounds from very small amounts of plant material. Moreover, this research revealed the relationship between SA and ABA content and development of peach fruit at different growth stages.

Analysis of phytohormones in vermicompost using a novel combinative sample preparation strategy of ultrasound-assisted extraction and solid-phase extraction coupled with liquid chromatography-tandem mass spectrometry

Vermicompost (VC), a widely used premium organic fertilizer, is the by-product of symbiotic interactions between earthworms and microorganisms living within them. It has been postulated that phytohormones are plausible "magic compounds" in VC that are responsible for making them such good fertilizers. Thus, a novel approach involving ultrasound-assisted extraction (UAE) and solid-phase extraction (SPE) was developed as a fast and efficient sample preparation method to screen for different classes of phytohormones in VC by liquid chromatography-tandem mass spectrometric (LC-MS/MS) analysis. Nine phytohormones from three different classes, including trans-zeatin (tZ), kinetin (K), N(6)-[2-isopentyl]adenine (iP), N(6)-benzyladenine (BA), N(6)-isopentenyladenosine (iPR), indole-3-acetic acid (IAA), 4-[3-indolyl]butyric acid (IBA), 1-naphthaleneacetic acid (NAA) and (+)-abscisic acid (ABA), were simultaneously screened. The extraction parameters influencing UAE efficiency were optimized to provide comparable recovery to the conventional mix-stirring (MSt) method. The optimized UAE method was subsequently applied on the analysis of phytohormones in VC, i.e. phytohormone extract was further pre-concentrated and purified using C18 and MCX SPE cartridges prior to LC-MS/MS analysis. The following phytohormones, namely iP, iPR and IAA, were detected and quantified to be 0.49, 0.53, 79.78ngg(-1), respectively; tZ was found to be below the limit of quantitation. Recoveries of 10.2%, 9.1%, 18.9% and 0.3% for tZ, iP, iPR and IAA were obtained. This is one of the few reported works for the successful detection and quantitation of cytokinins and auxins in VC, that provided the key empirical evidence to explain the growth efficacy of applying VC in promoting plant growth. Additionally, this pioneering work could potentially be applicable for the analysis of other types of organic fertilizers such as composts and activated composted materials awaiting phytohormone analyzes for quality assessment and control.

Simultaneous quantification of phytohormones in fermentation extracts of Botryodiplodia theobromae by liquid chromatography-electrospray tandem mass spectrometry

Fermentation broth and biomass from three strains of Botryodiplodia theobromae were characterized by high performance liquid chromatography-electrospray tandem mass spectrometry (HPLC-ESI-MS/MS) method, in order to quantify different phytohormones and to identify amino acid conjugates of jasmonic acid (JA) present in fermentation broths. A liquid-liquid extraction with ethyl acetate was used as sample preparation. The separation was carried out on a C18 reversed-phase HPLC column followed by analysis via ESI-MS/MS. The multiple reaction monitoring mode was used for quantitative measurement. For the first time, indole-3-acetic acid, indole-3-propionic acid, indole-3-butyric acid and JA were identified and quantified in the ethyl acetate extracts from the biomass, after the separation of mycelium from supernatant. The fermentation broths showed significantly higher levels of JA in relation to the other phytohormones. This is the first report of the presence of gibberellic acid, abscisic acid, salicylic acid and the cytokinins zeatin, and zeatin riboside in fermentation broths of Botryodiplodia sp. The presence of JA-serine and JA-threonine conjugates in fermentation broth was confirmed using HPLC-ESI tandem mass spectrometry in negative ionization mode, while the occurrence of JA-glycine and JA-isoleucine conjugates was evidenced with the same technique but with positive ionization. The results demonstrated that the used HPLC-ESI-MS/MS method was effective for analysing phytohormones in fermentation samples.

Simultaneous determination of 2-naphthoxyacetic acid and indole-3-acetic acid by first derivation synchronous fluorescence spectroscopy

A simple, rapid, sensitive and selective method for simultaneously determining 2-naphthoxyacetic acid (BNOA) and Indole-3-Acetic Acid (IAA) in mixtures has been developed using derivation synchronous fluorescence spectroscopy based on their synchronous fluorescence. The synchronous fluorescence spectra were obtained with Δλ=100 nm in a pH 8.5 NaH2PO4-NaOH buffer solution, and the detected wavelengths of quantitative analysis were set at 239 nm for BNOA and 293 nm for IAA respectively. The over lapped fluorescence spectra were well separated by the synchronous derivative method. Under optimized conditions, the limits of detection (LOD) were 0.003 μg/mL for BNOA and 0.012 μg/mL for IAA. This method is simple and expeditious, and it has been successfully applied to the determination of 2-naphthoxyacetic acid and indole-3-acetic acid in fruit juice samples with satisfactory results. The samples were only filtrated through a 0.45 μm membrane filter, which was free from the tedious separation procedures. The obtaining recoveries were in the range of 83.88-87.43% for BNOA and 80.76-86.68% for IAA, and the relative standard deviations were all less than 5.0%. Statistical comparison of the results with high performance liquid chromatography Mass Spectrometry (HPLC-MS) method revealed good agreement and proved that there were no significant difference in the accuracy and precision between these two methods.

Ozone-induced dissociation of conjugated lipids reveals significant reaction rate enhancements and characteristic odd-electron product ions

Ozone-induced dissociation (OzID) is an alternative ion activation method that relies on the gas phase ion-molecule reaction between a mass-selected target ion and ozone in an ion trap mass spectrometer. Herein, we evaluated the performance of OzID for both the structural elucidation and selective detection of conjugated carbon-carbon double bond motifs within lipids. The relative reactivity trends for [M + X](+) ions (where X = Li, Na, K) formed via electrospray ionization (ESI) of conjugated versus nonconjugated fatty acid methyl esters (FAMEs) were examined using two different OzID-enabled linear ion-trap mass spectrometers. Compared with nonconjugated analogues, FAMEs derived from conjugated linoleic acids were found to react up to 200 times faster and to yield characteristic radical cations. The significantly enhanced reactivity of conjugated isomers means that OzID product ions can be observed without invoking a reaction delay in the experimental sequence (i.e., trapping of ions in the presence of ozone is not required). This possibility has been exploited to undertake neutral-loss scans on a triple quadrupole mass spectrometer targeting characteristic OzID transitions. Such analyses reveal the presence of conjugated double bonds in lipids extracted from selected foodstuffs. Finally, by benchmarking of the absolute ozone concentration inside the ion trap, second order rate constants for the gas phase reactions between unsaturated organic ions and ozone were obtained. These results demonstrate a significant influence of the adducting metal on reaction rate constants in the fashion Li > Na > K.

Simultaneous determination of 24 or more acidic and alkaline phytohormones in femtomole quantities of plant tissues by high-performance liquid chromatography-electrospray ionization-ion trap mass spectrometry

Phytohormones act at relatively low concentrations as major regulatory factors of plant growth and development, and cross talk of phytohormones is currently of great interest throughout the plant science community. To meet this demand, a method that is capable of simultaneously analyzing diverse plant hormones is essential. This paper introduces a high-performance liquid chromatographic separation technique coupled with sensitive and selective ion trap mass spectrometry to simultaneously determine 24 or more acidic and alkaline phytohormones, including auxin, cis- and trans-abscisic acid, 11 cytokinins, and 10 gibberellins, in a single injection of sample. A binary solid-phase extraction using Oasis MCX cartridges for cations and Oasis MAX cartridges for anions was used to prepurify more than 24 acidic and alkaline phytohormones from a single plant extract. The method showed good linearity for all 24 phytohormones with R(2) values ranging from 0.9903 to 0.9997. Limits of detection for most of the phytohormones were in the femtomole range with some extending into the sub-femtomole range. This method was applied to hundreds of plant samples comprising different tissues from various plants, including herbaceous, woody climbing, and woody plants to demonstrate feasibility and to validate the methodology.

Ultra-high performance liquid chromatography coupled to high resolution Orbitrap mass spectrometry for metabolomic profiling of the endogenous phytohormonal status of the tomato plant

Phytohormones are key signalling biomolecules and are of particular interest because of their regulating role in numerous physiological and developmental plant processes. Since the plant response to a given stimulus results amongst others from the complex interaction between phytohormones, there is a mounting interest for multiple phytohormone analysis. Therefore, with the primary aim of profiling the hormonal status of the tomato plant, a generic extraction protocol and an U-HPLC-Orbitrap-MS analysis were developed and validated for both tomato fruit and leaf tissue. To this end, eight phytohormones were considered, i.e. gibberellic acid, indol-3-acetic acid, abscisic acid, jasmonic acid, salicylic acid, zeatin, N6-benzyladenine and epibrassinolide, representing the major hormonal classes. The sample pre-treatment involved liquid extraction with a buffer of methanol, ultrapure water and formic acid (75:20:5, v/v/v), after which the extract was purified by means of an Amicon® Ultra centrifugal unit. Subsequently, analytes were chromatographically separated on a sub-2 μm particles Nucleodur Gravity C18 column and detected by an Exactive™ high-resolution mass spectrometer. Validation of the analytical method demonstrated that linearity (≥0.99), precision (CV≤15%) and mean corrected recovery (between 80% and 110%) performed well for the majority of the eight targeted phytohormones. In addition, the generic nature of the extraction protocol and the full scan approach of the Orbitrap mass spectrometer allowed metabolomic profiling of the hormonal status of the tomato plant.

Ultrasensitive determination of jasmonic acid in plant tissues using high-performance liquid chromatography with fluorescence detection

An ultrasensitive and selective high-performance liquid chromatographic method for the volatile signaling hormone, jasmonic acid, has been developed based on precolumn derivatization with 1,3,5,7-tetramethyl-8-aminozide-difluoroboradiaza-s-indacene (BODIPY-aminozide). The derivatization reaction was carried out at 60 °C for 30 min in the presence of phosphoric acid. The formed jasmonic acid derivative was eluted using a mobile phase of methanol/pH 6.50 ammonium formate buffer/tetrahydrofuran (67:30:3, v/v/v) in 10 min on a C(18) column and detected with fluorescence detection at excitation and emission wavelengths of 495 and 505 nm, respectively. The detection limit (signal-to-noise ratio = 4) reached 1.14 × 10(-10) M or 2.29 fmol per injection (20 μL), which is the lowest of the existing methods. The proposed method has been successfully applied to the direct determination of trace jasmonic acid in the crude extracts of soybean leaves from soybean mosaic virus-infected and normal plants with recoveries of 95-104%.

Analytical methods for tracing plant hormones

Plant hormones play important roles in regulating numerous aspects of plant growth, development, and response to stress. In the past decade, more analytical methods for the accurate identification and quantitative determination of trace plant hormones have been developed to better our understanding of the molecular mechanisms of plant hormones. As sample preparation is often the bottleneck in analysis of plant hormones in biological samples, this review firstly discusses sample preparation techniques after a brief introduction to the classes, roles, and methods used in the analysis of plant hormones. The analytical methods, especially chromatographic techniques and immuno-based methods, are reviewed in detail, and their corresponding advantages, limitations, applications, and prospects are also discussed. This review mainly covers reports published from 2000 to the present on methods for the analysis of plant hormones.