Development of a new high-performance liquid chromatography-electrospray ionization time-of-flight mass spectrometry method for the determination of low molecular mass organic acids in plant tissue extracts

Phenotyping methods to assess heat stress resilience in grapevine

Global warming has become an issue in recent years in viticulture, as increasing temperatures have a negative impact on grapevine (Vitis vinifera) production and on wine quality. Phenotyping for grapevine response to heat stress is, therefore, important to understand thermotolerance mechanisms, with the aim of improving field management strategies or developing more resilient varieties. Nonetheless, the choice of the phenotypic traits to be investigated is not trivial and depends mainly on the objectives of the study, but also on the number of samples and on the availability of instrumentation. Moreover, the grapevine literature reports few studies related to thermotolerance, generally assessing physiological responses, which highlights the need for more holistic approaches. In this context, the present review offers an overview of target traits that are commonly investigated in plant thermotolerance studies, with a special focus on grapevine, and of methods that can be employed to evaluate those traits. With the final goal of providing useful tools and references for future studies on grapevine heat stress resilience, advantages and limitations of each method are highlighted, and the available or possible implementations are described. In this way, the reader is guided in the choice of the best approaches in terms of speed, complexity, range of application, sensitivity, and specificity.

Mixed-mode chromatography-mass spectrometry enables targeted and untargeted screening of carboxylic acids in biological samples

Carboxylic acids are crucial metabolites in the tricarboxylic acid (TCA) cycle and thus participate in central carbon metabolism (CCM). Research dependent on the analysis of metabolites involved in central carbon metabolism requires fast separation and sensitive detection of carboxylic acids using liquid chromatography-mass spectrometry (LC-MS). However, successful separation of all carboxylic acids from the TCA cycle by liquid chromatography remains a challenging task because of their high polarity and thus low retention on the conventional reversed-phase columns. In this study, we tested a reversed-phase/anion exchange mixed-mode stationary phase (Waters BEH C₁₈ AX) using liquid chromatography-tandem mass spectrometry (LC-MS/MS). We developed and optimized a method that enables a 10 minute separation of all carboxylic acids from the TCA cycle and lactic acid without prior derivatization or addition of ion-pair reagents in the mobile phase. The developed method was validated for quantification of 8 acids in murine brown preadipocytes, 5 acids in human plasma and 6 acids in Arabidopsis thaliana leaves with limits of quantification ranging from 0.1 μM for malic acid to 10 μM for isocitric acid. Moreover, the mixed-mode chromatography enabled untargeted screening of medium- to long-chain fatty acids in murine brown preadipocytes, Arabidopsis thaliana, and human plasma, where 23 fatty acids were identified by using liquid chromatography with high-resolution mass spectrometry (HRMS).

Ion Chromatography and Related Techniques in Carboxylic Acids Analysis

Ion chromatography (IC) is a variant of high-performance liquid chromatography. Its most important applications include the determination of inorganic and some organic ions in different types of liquid samples. The development of new types of stationary phases with various separation mechanisms, sample preparation methods, and detection modes has extended ion chromatography applications to practically all ionic and ionogenic substances, as well as extending sample types to include gaseous and solid matrices. Carboxylic acids and their derivatives are examples of compounds that are becoming more frequently analyzed using ion chromatography and related techniques. Their occurrence in the environment can be natural or anthropogenic in origin and are broadly used in various industries and daily life. This article discusses the applications of ion chromatography and related techniques for the determination of carboxylic acids in different types of liquid, solid, and gaseous matrices. It also presents detailed methodologies and literature data on this subject from the last 15 years.

Unravelling the phytonutrients and antioxidant properties of European Vicia faba L. seeds

For the first time, seven European varieties of Vicia faba L. seeds were investigated for (1) their phytonutrients profile by HPLC-DAD-MS/MS, (2) total phenolic content (TPC), and (3) antioxidant capacity (DPPH and FRAP assays). A wide range of compounds were identified, namely alkaloids, organic acids, terpenoids, jasmonates, and phenolics; these two latter being the most abundant. TPC ranged between 2.62 and 4.3 mg (gallic acid equivalent) g^-1 dry weight, for V. faba major variety Belshi and V. faba minor variety Bauska, respectively. The DPPH radical scavenging capacity showed poor correlation (r = 0.550, P = .041) with TPC, suggesting the presence of other antioxidant sources than phenolics. Still, FRAP was positively correlated with TPC (r = 0.709, P < .01) and DPPH (r = 0.819, P < .01). These results elucidated the phytonutrients and antioxidant properties of V. faba L. seeds as functional food sources.

Analytical method for the determination of organic acids in dilute acid pretreated biomass hydrolysate by liquid chromatography-time-of-flight mass spectrometry

BACKGROUND

For the development of lignocellulosic biofuels a common strategy to release hemicellulosic sugars and enhance the enzymatic digestibility of cellulose is the heat pretreatment of biomass with dilute acid. During this process, fermentation inhibitors such as 5-hydroxymethylfurfural, furfural, phenolics, and organic acids are formed and released into the so-called hydrolysate. The phenolic inhibitors have been studied fairly extensively, but fewer studies have focused on the analysis of the organic acids profile. For this purpose, a simple and fast liquid chromatography/mass spectrometry (LC/MS) method for the analysis of organic acids in the hydrolysate has been developed using an ion exchange column based on a polystyrene-divinylbenzene polymer frequently used in biofuel research. The application of the LC/MS method to a hydrolysate from Miscanthus has been evaluated.

RESULTS

The presented LC/MS method involving only simple sample preparation (filtration and dilution) and external calibration for the analysis of 24 organic acids present in dilute acid pretreated biomass hydrolysate is fast (12 min) and reasonably sensitive despite the small injection volume of 2 μL used. The lower limit of quantification ranged from 0.2 μg/mL to 2.9 μg/mL and the limit of detection from 0.03 μg/mL to 0.7 μg/mL. Analyte recoveries obtained from a spiked hydrolysate were in the range of 70 to 130% of the theoretical yield, except for glyoxylic acid, malic acid, and malonic acid, which showed a higher response due to signal enhancement. Relative standard deviations for the organic acids ranged from 0.4 to 9.2% (average 3.6%) for the intra-day experiment and from 2.1 to 22.8% (average 8.9%) for the inter-day (three-day) experiment.

CONCLUSION

We have shown that the analysis of the profile of 24 organic acids present in biomass hydrolysate can be achieved by a simple LC/MS method applying external calibration and minimal sample preparation. The organic acids eluted within only 12 min by isocratic elution, enabling high sample throughput. Repeatability (precision and accuracy) and recovery were sufficiently accurate for most of the organic acids tested, making the method suitable for their fast determination in hydrolysate. We envision that this method can be further expanded to a larger number of organic acids, including phenolic acids such as p-coumaric acid and ferulic acid and other molecules depending on the researchers' needs.

Changes in the concentration of organic acids in roots and leaves of carob-tree under Fe deficiency

Several fruit trees are able to cope with iron (Fe) deficiency when grown in calcareous soils in the Mediterranean region, although information regarding well adapted slow-growing species is scarce, and the mechanisms activated by these species are not described in the literature. A crucial issue related to tolerance is the need to transport Fe over relatively long distances inside the plant. To evaluate the possible role of organic acids in the movement of Fe in tolerant plants, we studied the concentration of low molecular weight organic acids in several organs of 1-year old carob plants grown for 55 days in nutrient solutions without Fe (0µM Fe) or with 1µM Fe and 10µM Fe. Roots, stems and leaves were harvested, and the biomass, Fe and organic acid contents quantified. Total leaf chlorophyll (Chl) was evaluated in young leaves over the experimental period and the activity of root ferric chelate-reductase (FC-R; EC 1.16.1.17) was determined after 35 days, when deficiency symptoms appeared. Iron chlorosis was observed only at the end of the experiment in plants grown in the absence of Fe, and these plants had a smaller DW of leaves and also significant greater activity of root FC-R. Iron deficiency (Fe0 and Fe1 treatments) induced significant changes in the concentrations of succinic, malic, citric and fumaric acids, which increased in roots, or in basal, middle and apical leaves. There were significant correlations between most organic acids (with the exceptions of 2-oxoglutaric and tartaric acids) and leaf Chl. Analysis of each type of leaf showed that more succinic and malic acids were present in young chlorotic leaves while the reverse was true for quinic acid. These changes in organic acids followed a root-to-foliage pathway that was similar in all leaf types and particularly evident in young chlorotic leaves. We hypothesised that it was associated with Fe transport from roots to aboveground tissues, as there were significant differences in Fe contents between treatments with and without Fe.

Metal species involved in long distance metal transport in plants

The mechanisms plants use to transport metals from roots to shoots are not completely understood. It has long been proposed that organic molecules participate in metal translocation within the plant. However, until recently the identity of the complexes involved in the long-distance transport of metals could only be inferred by using indirect methods, such as analyzing separately the concentrations of metals and putative ligands and then using in silico chemical speciation software to predict metal species. Molecular biology approaches also have provided a breadth of information about putative metal ligands and metal complexes occurring in plant fluids. The new advances in analytical techniques based on mass spectrometry and the increased use of synchrotron X-ray spectroscopy have allowed for the identification of some metal-ligand species in plant fluids such as the xylem and phloem saps. Also, some proteins present in plant fluids can bind metals and a few studies have explored this possibility. This study reviews the analytical challenges researchers have to face to understand long-distance metal transport in plants as well as the recent advances in the identification of the ligand and metal-ligand complexes in plant fluids.

Variation of antioxidants and secondary metabolites in nitrogen-deficient barley plants

Barley (Hordeum vulgare cv. Bojos) plants cultured in low nitrogen (N) containing Hoagland solution (20 mg/l) were exposed to N deficiency (-N) over 15 days. Plants revealed relatively high tolerance to total N deficit because shoot length was not altered and dry biomass was depleted by ca. 30% while root length increased by ca. 50% and dry biomass remained unaffected. Soluble proteins and free amino acids decreased more pronouncedly in the roots. Antioxidants (glutathione and ascorbic acid) decreased in the shoots but increased or were not affected in the roots. Ascorbate peroxidase and glutathione reductase activities were depleted in shoots and/or roots while guaiacol peroxidase activity was stimulated in the shoots. In accordance, fluorescence signal of reactive oxygen species (ROS) and nitric oxide was elevated in shoots but no extensive changes were observed in roots if +N and -N treatments are compared. At the level of phenolic metabolites, slight increase in soluble phenols and some phenolic acids and strong elevation of flavonoid homoorientin was found in the shoots but not in the roots. Fluorescence microscopy in terms of detection of phenols is also discussed. We also briefly discussed accuracy of quantification of some parameters owing to discrepancies in the literature. It is concluded that N deficiency induces increase in shoot phenolics but also elevates symptoms of oxidative stress while increase in root antioxidants probably contributes to ROS homeostasis aimed to maintain root development.

Genome-wide transcriptome analysis of Clavibacter michiganensis subsp. michiganensis grown in xylem mimicking medium

The interaction between Clavibacter michiganensis subsp. michiganensis with its host, the tomato plant (Solanum lycopersicum), is poorly understood and only few virulence factors are known. While studying of the bacteria in planta is time-consuming and difficult, the analysis in vitro would facilitate research. Therefore, a xylem mimicking medium (XMM) for C. michiganensis subsp. michiganensis was established in this study based on an apoplast medium for Xanthomonas campestris pv. vesicatoria. In contrast to the apoplast medium, XMM contains no sugars, but amino acids which serve as nitrogen and carbon source. As a result, growth in XMM induced transcriptional changes of genes encoding putative sugar, amino acid and iron uptake systems. In summary, mRNA levels of about 8% of all C. michiganensis subsp. michiganensis genes were changed when XMM-grown bacteria were compared to M9 minimal medium-grown cells. Almost no transcriptional changes of genes encoding hydrolytic enzymes were detected, leading to the idea that XMM reflects the situation in the beginning of infection and therefore allows the characterization of virulence factors in this early stage of infection. The addition of the plant wound substance acetosyringone to the XMM medium led to a change in transcript amount, including genes coding for proteins involved in protein transport, iron uptake and regulation processes.

Protein profile of Lupinus texensis phloem sap exudates: searching for Fe- and Zn-containing proteins

The aim of this study was to obtain a comprehensive overview of the phloem sap protein profile of Lupinus texensis, with a special focus on proteins binding Fe and Zn. L. texensis was chosen as model plant given the simplicity to obtain exudates from sieve elements. Protein profiling by 2DE revealed 249 spots, and 54 of them were unambiguously identified by MALDI-MS and ESI-MS/MS. The largest number of identified protein species belongs to protein modification/turnover and general metabolism (19-21%), followed by redox homeostasis (9%) and defense and cell structural components (7%). This protein profile is similar to that reported in other plant species, suggesting that the phloem sap proteome is quite conserved. Staining of 2DE gels for Fe-containing proteins and affinity chromatography experiments revealed the presence of two low molecular weight Fe-binding proteins in phloem sap: a metallothionein-like protein type 2B identified in the Fe-affinity chromatography, and a second protein identified with both Fe staining methods. This protein species had a molecular weight of 13.5 kDa, a pI of 5.6 and 51% homology to a phloem-specific protein from Medicago truncatula. Zinc affinity chromatography revealed four Zn-binding proteins in phloem sap, one belonging to the dehydrin family and three Zn finger proteins.

Metabolomic analysis of key central carbon metabolism carboxylic acids as their 3-nitrophenylhydrazones by UPLC/ESI-MS

Multiple hydroxy-, keto-, di-, and tri-carboxylic acids are among the cellular metabolites of central carbon metabolism (CCM). Sensitive and reliable analysis of these carboxylates is important for many biological and cell engineering studies. In this work, we examined 3-nitrophenylhydrazine as a derivatizing reagent and optimized the reaction conditions for the measurement of ten CCM-related carboxylic compounds, including glycolate, lactate, malate, fumarate, succinate, citrate, isocitrate, pyruvate, oxaloacetate, and α-ketoglutarate as their 3-nitrophenylhydrazones using LC/MS with ESI. With the derivatization protocol which we have developed, and using negative-ion multiple-reaction monitoring on a triple-quadrupole instrument, all of the carboxylates showed good linearity within a dynamic range of ca. 200 to more than 2000. The on-column LODs and LOQs were from high femtomoles to low picomoles. The analytical accuracies for eight of the ten analytes were determined to be between 89.5 to 114.8% (CV≤7.4%, n = 6). Using a QTOF instrument, the isotopic distribution patterns of these carboxylates, extracted from a (13) C-labeled mouse heart, were successfully determined by UPLC/MS with full-mass detection, indicating the possible utility of this analytical method for metabolic flux analysis. In summary, this work demonstrates an efficient chemical derivatization LC/MS method for metabolomic analysis of these key CCM intermediates in a biological matrix.

A sensitive LC-ESI-Q-TOF-MS method reveals novel phytosiderophores and phytosiderophore-iron complexes in barley

The direct analysis of phytosiderophores (PSs) and their metal complexes in plants is critical to understanding the biological functions of different PSs. Here we report on a rapid and highly sensitive liquid chromatography-electrospray ionization-quadrupole-time of flight-mass spectrometry (LC-ESI-Q-TOF-MS) method for the direct and simultaneous determination of free PSs and their ferric complexes in plants. In addition to previously reported PSs--deoxymugineic acid (DMA), mugineic acid (MA) and epihydroxymugineic acid (epi-HMA)--two more PSs, avenic acid (AVA) and hydroxyavenic acid (HAVA), were identified by this method in roots of Hordeum vulgare cv Himalaya and in root exudates under iron (Fe) deficiency. The two identified PSs could be responsible for Fe acquisition under Fe deficiency because of their relative abundance and ability to form ferric complexes in secreted root exudates. This LC-ESI-Q-TOF-MS method greatly facilitates the identification of free PSs and PS-Fe complexes in one plant sample.

Oxidative stress is a consequence, not a cause, of aluminum toxicity in the forage legume Lotus corniculatus

• Aluminum (Al) toxicity is a major limiting factor of crop production on acid soils, but the implication of oxidative stress in this process is controversial. A multidisciplinary approach was used here to address this question in the forage legume Lotus corniculatus. • Plants were treated with low Al concentrations in hydroponic culture, and physiological and biochemical parameters, together with semiquantitative metabolic and proteomic profiles, were determined. • The exposure of plants to 10 μM Al inhibited root and leaf growth, but had no effect on the production of reactive oxygen species or lipid peroxides. By contrast, exposure to 20 μM Al elicited the production of superoxide radicals, peroxide and malondialdehyde. In response to Al, there was a progressive replacement of the superoxide dismutase isoforms in the cytosol, a loss of ascorbate and consistent changes in amino acids, sugars and associated enzymes. • We conclude that oxidative stress is not a causative factor of Al toxicity. The increased contents in roots of two powerful Al chelators, malic and 2-isopropylmalic acids, together with the induction of an Al-activated malate transporter gene, strongly suggest that both organic acids are implicated in Al detoxification. The effects of Al on key proteins involved in cytoskeleton dynamics, protein turnover, transport, methylation reactions, redox control and stress responses underscore a metabolic dysfunction, which affects multiple cellular compartments, particularly in plants exposed to 20 μM Al.