A reverse-phase liquid chromatography/mass spectrometry method for the analysis of high-molecular-weight fructooligosaccharides

Discrimination of raffinose and planteose based on porous graphitic carbon chromatography in combination with mass spectrometry

Raffinose and planteose are non-reducing, isomeric trisaccharides present in many higher plants. Structurally, they differ in the linkage of α-D-galactopyranosyl to either glucose C(6) or to C (6') of fructose, respectively and thus differentiating each other is very challenging. The negative ion mode mass spectrometric analysis is shown to distinguish planteose and raffinose. However, to facilitate the robust identification of planteose in complex mixtures, herein, we have demonstrated the use of porous graphitic carbon (PGC) chromatography combined with QTOF-MS² analysis. The separation of planteose and raffinose was achieved on PGC, wherein both have recorded different retention time. Detection through MS² analysis revealed the specific fragmentation patterns for planteose and raffinose that are distinctive to each other. The applicability of this method on oligosaccharides pool extracted from different seeds showed clear separation of planteose that allowed unambiguous identification from complex mixtures. Therefore, we propose PGC-LC-MS/MS can be employed for sensitive, throughput screening of planteose from wider plant sources.

Bioactive polysaccharides and oligosaccharides from garlic (Allium sativum L.): Production, physicochemical and biological properties, and structure-function relationships

Garlic is a common food, and many of its biological functions are attributed to its components including functional carbohydrates. Garlic polysaccharides and oligosaccharides as main components are understudied but have future value due to the growing demand for bioactive polysaccharides/oligosaccharides from natural sources. Garlic polysaccharides have molecular weights of 1 × 10³ to 2 × 10⁶  Da, containing small amounts of pectins and fructooligosaccharides and large amounts of inulin-type fructans ((2→1)-linked β-d-Fruf backbones alone or with attached (2→6)-linked β-d-Fruf branched chains). This article provides a detailed review of research progress and identifies knowledge gaps in extraction, production, composition, molecular characteristics, structural features, physicochemical properties, bioactivities, and structure-function relationships of garlic polysaccharides/oligosaccharides. Whether the extraction processes, synthesis approaches, and modification methods established for other non-garlic polysaccharides are also effective for garlic polysaccharides/oligosaccharides (to preserve their desired molecular structures and bioactivities) requires verification. The metabolic processes of ingested garlic polysaccharides/oligosaccharides (as food ingredients/dietary supplements), their modes of action in healthy humans or populations with chronic conditions, and molecular/chain organization-bioactivity relationships remain unclear. Future research directions related to garlic polysaccharides/oligosaccharides are discussed.

A large-scale metabolomics study to harness chemical diversity and explore biochemical mechanisms in ryegrass

Perennial ryegrass (Lolium perenne) is integral to temperate pastoral agriculture, which contributes most of the milk and meat production worldwide. Chemical profiles and diversity of ryegrass offer several opportunities to harness specific traits and elucidate underlying biological mechanisms for forage improvement. We conducted a large-scale metabolomics study of perennial ryegrass comprising 715 genotypes, representing 118 populations from 21 countries. Liquid/gas chromatography–mass spectrometry based targeted and non-targeted techniques were used to analyse fructan oligosaccharides, lipids, fatty acid methyl esters, polar and semi-polar compounds. Fructan diversity across all genotypes was evaluated, high- and low-sugar groups identified, and fructan accumulation mechanisms explored. Metabolites differentiating the two groups were characterised, modules and pathways they represent deduced, and finally, visualisation and interpretation provided in a biological context. We also demonstrate a workflow for large-scale metabolomics studies from raw data through to statistical and pathway analysis. Raw files and metadata are available at the MetaboLights database.

Arvind Subbaraj et al. present the ryegrass metabolome, derived from 715 genotypes representing 118 populations of Lolium perenne from 21 countries. They analyze fructan diversity, identify high- and low-sugar groups, and explore biochemical modules and pathways that discriminate the phenotypes.

Fructo-oligosaccharides: Production, Purification and Potential Applications

The nutritional and therapeutic benefits of prebiotics have attracted the keen interest of consumers and food processing industry for their use as food ingredients. Fructo-oligosaccharides (FOS), new alternative sweeteners, constitute 1-kestose, nystose, and 1-beta-fructofuranosyl nystose produced from sucrose by the action of fructosyltransferase from plants, bacteria, yeast, and fungi. FOS has low caloric values, non-cariogenic properties, and help gut absorption of ions, decrease levels of lipids and cholesterol and bifidus-stimulating functionality. The purified linear fructose oligomers are added to various food products like cookies, yoghurt, infant milk products, desserts, and beverages due to their potential health benefits. This review is focused on the various aspects of biotechnological production, purification and potential applications of fructo-oligosaccharides.

Metabolic changes and associated cytokinin signals in response to nitrate assimilation in roots and shoots of Lolium perenne

The efficiency of inorganic nitrogen (N) assimilation is a critical component of fertilizer use by plants and of forage production in Lolium perenne, an important pasture species worldwide. We present a spatiotemporal description of nitrate use efficiency in terms of metabolic responses and carbohydrate remobilization, together with components of cytokinin signal transduction following nitrate addition to N-impoverished plants. Perennial ryegrass (L. perenne cv. Grasslands Nui) plants were grown for 10 weeks in unfertilized soil and then treated with nitrate (5 mM) hydroponically. Metabolomic analysis by gas chromatography-mass spectrometry and liquid chromatography-tandem mass spectrometry revealed a dynamic interaction between N and carbon metabolism over a week-long time course represented by the relative abundance of amino acids, tricarboxylic acid intermediates and stored water-soluble carbohydrates (WSCs). The initial response to N addition was characterized by a rapid remobilization of carbon stores from the low-molecular weight WSC, along with an increase in N content and assimilation into free amino acids. Subsequently, the shoot became the main source of carbon through remobilization of a large pool of high-molecular weight WSC. Associated quantification of cytokinin levels and expression profiling of putative cytokinin response regulator genes by quantitative reverse transcription polymerase chain reaction support a role for cytokinin in the mediation of the response to N addition in perennial ryegrass. The presence of high levels of cis-zeatin-type cytokinins is discussed in the context of hormonal homeostasis under the stress of steady-state N deficiency.

Fructan synthesis, accumulation and polymer traits. II. Fructan pools in populations of perennial ryegrass (Lolium perenne L.) with variation for water-soluble carbohydrate and candidate genes were not correlated with biosynthetic activity and demonstrated constraints to polymer chain extension

Differences have been shown between ryegrass and fescue within the Festulolium subline introgression family for fructan synthesis, metabolism, and polymer-size traits. It is well-established that there is considerable variation for water-soluble carbohydrate and fructan content within perennial ryegrass. However there is much still to be discovered about the fructan polymer pool in this species, especially in regard to its composition and regulation. It is postulated that similar considerable variation for polymer traits may exist, providing useful polymers for biorefining applications. Seasonal effects on fructan content together with fructan synthesis and polymer-size traits have been examined in diverse perennial ryegrass material comprising contrasting plants from a perennial ryegrass F2 mapping family and from populations produced by three rounds of phenotypic selection. Relationships with copy number variation in candidate genes have been investigated. There was little evidence of any variation in fructan metabolism across this diverse germplasm under these conditions that resulted in substantial differences in the complement of fructan polymers present in leaf tissue at high water-soluble carbohydrate concentrations. The importance of fructan synthesis during fructan accumulation was unclear as fructan content and polymer characteristics in intact plants during the growing season did not reflect the capacity for de novo synthesis. However, the retention of fructan in environmental conditions favoring high sink/low source demand may be an important component of the high sugar trait and the roles of breakdown and turnover are discussed.

Fructan synthesis, accumulation, and polymer traits. I. Festulolium chromosome substitution lines

The fructans found as storage carbohydrates in temperate forage grasses have a physiological role in regrowth and stress tolerance. They are also important for the nutritional value of fresh and preserved livestock feeds, and are potentially useful as feedstocks for biorefining. Seasonal variation in fructan content and the capacity for de novo fructan synthesis have been examined in a Festulolium monosomic substitution line family to investigate variation in the polymers produced by grasses in the ryegrass-fescue complex. There were significant differences between ryegrass and fescue. Fescue had low polymeric fructan content and a high oligomer/polymer ratio; synthesis of polymers longer than degree of polymerization 6 (DP6) from oligomers was slow. However, extension of polymer length from DP10/DP20 upward appeared to occur relatively freely, and, unlike ryegrass, fescue had a relatively even spread of polymer chain lengths above DP20. This included the presence of some very large polymers. Additionally fescue retained high concentrations of fructan, both polymeric and oligomeric, during conditions of low source/high sink demand. There were indications that major genes involved in the control of some of these traits might be located on fescue chromosome 3 opening the possibility to develop grasses optimized for specific applications.

Liquid chromatography/mass spectrometry analysis of branched fructans produced in vitro with 13C-labeled substrates

RATIONALE

Fructans are carbohydrates predominantly based on fructose which are generally considered to be soluble dietary fibers with health-promoting properties. It is known that the nutritional properties of fructans are affected by their structure. This study focused on structural determination of branched fructans, as the most important dietary fructans are branched graminan-type fructans.

METHODS

Branched fructans were synthesized enzymatically by incubation of a heterologously expressed sucrose:fructan 6-fructosyltransferase (6-SFT) from Pachysandra terminalis with native or (13)C-labeled substrates. Liquid chromatography/mass spectrometry (LC/MS) was used for the structural identification of branched fructans. The MS(2) fragmentation of these compounds is described for the first time. Analytes were charged by electrospray ionization in negative mode and a quadrupole mass analyzer was used for MS(2) analysis.

RESULTS

The MS(2) fragmentation patterns of branched and linear fructans were shown to differ and distinctive ion formation allowed differentiation between all branched fructan isomers formed. P. terminalis 6-SFT preferred extending the existing fructan branch rather than creating a new branch.

CONCLUSIONS

The MS(2) fragmentation patterns described in the current paper now allow rapid screening of large sample sets for the presence of branched, graminan-type fructans. Furthermore, the data enables the characterization of fructan-metabolizing enzymes by identification of the fructan structures produced by in vitro reactions as described here for P. terminalis 6-SFT.

A new high-throughput LC-MS method for the analysis of complex fructan mixtures

In this paper, a new liquid chromatography-mass spectrometry (LC-MS) method for the analysis of complex fructan mixtures is presented. In this method, columns with a trifunctional C18 alkyl stationary phase (T3) were used and their performance compared with that of a porous graphitized carbon (PGC) column. The separation of fructan isomers with the T3 phase improved clearly in comparison with the PGC phase, and retention times were lower and more stable. When the T3-based method was applied on a wheat grain extract, multiple fructan isomers could be discerned, even for fructans with a degree of polymerization of 10. This indicates that wheat grain fructans do not, or not only, have a simple linear structure. The presented method paves the way for elucidation of fructan structures in complex mixtures that contain many structural isomers.

Computational analyses of spectral trees from electrospray multi-stage mass spectrometry to aid metabolite identification

Mass spectrometry coupled with chromatography has become the major technical platform in metabolomics. Aided by peak detection algorithms, the detected signals are characterized by mass-over-charge ratio (m/z) and retention time. Chemical identities often remain elusive for the majority of the signals. Multi-stage mass spectrometry based on electrospray ionization (ESI) allows collision-induced dissociation (CID) fragmentation of selected precursor ions. These fragment ions can assist in structural inference for metabolites of low molecular weight. Computational investigations of fragmentation spectra have increasingly received attention in metabolomics and various public databases house such data. We have developed an R package “iontree” that can capture, store and analyze MS² and MS³ mass spectral data from high throughput metabolomics experiments. The package includes functions for ion tree construction, an algorithm (distMS2) for MS² spectral comparison, and tools for building platform-independent ion tree (MS²/MS³) libraries. We have demonstrated the utilization of the package for the systematic analysis and annotation of fragmentation spectra collected in various metabolomics platforms, including direct infusion mass spectrometry, and liquid chromatography coupled with either low resolution or high resolution mass spectrometry. Assisted by the developed computational tools, we have demonstrated that spectral trees can provide informative evidence complementary to retention time and accurate mass to aid with annotating unknown peaks. These experimental spectral trees once subjected to a quality control process, can be used for querying public MS² databases or de novo interpretation. The putatively annotated spectral trees can be readily incorporated into reference libraries for routine identification of metabolites.

Mathematical model of fructan biosynthesis and polymer length distribution in plants

BACKGROUND AND AIMS

There are many unresolved issues concerning the biochemistry of fructan biosynthesis. The aim of this paper is to address some of these by means of modelling mathematically the biochemical processes.

METHODS

A model has been constructed for the step-by-step synthesis of fructan polymers. This is run until a steady state is achieved for which a polymer distribution is predicted. It is shown how qualitatively different distributions can be obtained.

KEY RESULTS

It is demonstrated how a set of experimental results on polymer distribution can by simulated by a simple parameter adjustments.

CONCLUSIONS

Mathematical modelling of fructan biosynthesis can provide a useful tool for helping elucidate the details of the biosynthetic processes.

Metabolomics of forage plants: a review

BACKGROUND

Forage plant breeding is under increasing pressure to deliver new cultivars with improved yield, quality and persistence to the pastoral industry. New innovations in DNA sequencing technologies mean that quantitative trait loci analysis and marker-assisted selection approaches are becoming faster and cheaper, and are increasingly used in the breeding process with the aim to speed it up and improve its precision. High-throughput phenotyping is currently a major bottle neck and emerging technologies such as metabolomics are being developed to bridge the gap between genotype and phenotype; metabolomics studies on forages are reviewed in this article.

SCOPE

Major challenges for pasture production arise from the reduced availability of resources, mainly water, nitrogen and phosphorus, and metabolomics studies on metabolic responses to these abiotic stresses in Lolium perenne and Lotus species will be discussed here. Many forage plants can be associated with symbiotic microorganisms such as legumes with nitrogen fixing rhizobia, grasses and legumes with phosphorus-solubilizing arbuscular mycorrhizal fungi, and cool temperate grasses with fungal anti-herbivorous alkaloid-producing Neotyphodium endophytes and metabolomics studies have shown that these associations can significantly affect the metabolic composition of forage plants. The combination of genetics and metabolomics, also known as genetical metabolomics can be a powerful tool to identify genetic regions related to specific metabolites or metabolic profiles, but this approach has not been widely adopted for forages yet, and we argue here that more studies are needed to improve our chances of success in forage breeding.

CONCLUSIONS

Metabolomics combined with other '-omics' technologies and genome sequencing can be invaluable tools for large-scale geno- and phenotyping of breeding populations, although the implementation of these approaches in forage breeding programmes still lags behind. The majority of studies using metabolomics approaches have been performed with model species or cereals and findings from these studies are not easily translated to forage species. To be most effective these approaches should be accompanied by whole-plant physiology and proof of concept (modelling) studies. Wider considerations of possible consequences of novel traits on the fitness of new cultivars and symbiotic associations need also to be taken into account.

Analysis of high-molecular-weight fructan polymers in crude plant extracts by high-resolution LC-MS

The main water-soluble carbohydrates in temperate forage grasses are polymeric fructans. Fructans consist of fructose chains of various chain lengths attached to sucrose as a core molecule. In grasses, fructans are a complex mixture of a large number of isomeric oligomers with a degree of polymerisation ranging from 3 to >100. Accurate monitoring and unambiguous peak identification requires chromatographic separation coupled to mass spectrometry. The mass range of ion trap mass spectrometers is limited, and we show here how monitoring selected multiply charged ions can be used for the detection and quantification of individual isomers and oligomers of high mass, particularly those of high degree of polymerization (DP > 20) in complex plant extracts. Previously reported methods using linear ion traps with low mass resolution have been shown to be useful for the detection of fructans with a DP up to 49. Here, we report a method using high-resolution mass spectrometry (MS) using an Exactive Orbitrap MS which greatly improves the signal-to-noise ratio and allows the detection of fructans up to DP = 100. High-sugar (HS) Lolium perenne cultivars with high concentrations of these fructans have been shown to be of benefit to the pastoral agricultural industry because they improve rumen nitrogen use efficiency and reduce nitrous oxide emissions from pastures. We demonstrate with our method that these HS grasses not only contain increased amounts of fructans in leaf blades but also accumulate fructans with much higher DP compared to cultivars with normal sugar levels.

Analysis of high-molecular-weight fructan polymers in crude plant extracts by high-resolution LC-MS

The main water-soluble carbohydrates in temperate forage grasses are polymeric fructans. Fructans consist of fructose chains of various chain lengths attached to sucrose as a core molecule. In grasses, fructans are a complex mixture of a large number of isomeric oligomers with a degree of polymerisation ranging from 3 to >100. Accurate monitoring and unambiguous peak identification requires chromatographic separation coupled to mass spectrometry. The mass range of ion trap mass spectrometers is limited, and we show here how monitoring selected multiply charged ions can be used for the detection and quantification of individual isomers and oligomers of high mass, particularly those of high degree of polymerization (DP > 20) in complex plant extracts. Previously reported methods using linear ion traps with low mass resolution have been shown to be useful for the detection of fructans with a DP up to 49. Here, we report a method using high-resolution mass spectrometry (MS) using an Exactive Orbitrap MS which greatly improves the signal-to-noise ratio and allows the detection of fructans up to DP = 100. High-sugar (HS) Lolium perenne cultivars with high concentrations of these fructans have been shown to be of benefit to the pastoral agricultural industry because they improve rumen nitrogen use efficiency and reduce nitrous oxide emissions from pastures. We demonstrate with our method that these HS grasses not only contain increased amounts of fructans in leaf blades but also accumulate fructans with much higher DP compared to cultivars with normal sugar levels.

Seasonal changes in the metabolic fingerprint of 21 grass and legume cultivars studied by nuclear magnetic resonance-based metabolomics

A nuclear magnetic resonance (NMR)-based approach was introduced for metabolic fingerprinting of 21 grass and legume cultivars in the present study. Applying principal component analysis (PCA) on the fingerprints obtained on water extracts, it was possible to elucidate the variation between cultivars and the magnitude of changes in the metabolic fingerprint between the spring growth and the second regrowth. Consequently, the potential of the method for tracking differences and changes related to cultivar and season was demonstrated. In addition, partial least-squares (PLS) regressions revealed correlations between the NMR fingerprints and the value of the grasses as animal feed evaluated as concentration of sugars, neutral detergent fibres (NDF) (R = 0.82), indigestible neutral detergent fibres (iNDF) (R = 0.90), and in vitro organic matter digestibility (IVOMD) (R = 0.75). The correlations between these parameters and the NMR fingerprint could mainly be ascribed to differences in spectral intensities from signals assigned to malic acid (2.40 and 4.70 ppm), choline (3.27 ppm), and glucose (5.24 ppm), and the biochemical rationale for this relation is discussed.