MS has evolved as a critical component in metabolomics, which seeks to answer biological questions through large-scale qualitative and quantitative analyses of the metabolome. MSbased metabolomics techniques offer an excellent combination of sensitivity and selectivity, and they have become an indispensable platform in biology and metabolomics. In this minireview, various MS technologies used in metabolomics are briefly discussed, and future needs are suggested.Metabolomics is idealized as the large-scale, qualitative, and quantitative study of all metabolites in a given biological system. Unlike transcripts and proteins, the molecular identity of metabolites cannot be deduced from genomic information. Thus, the identification and quantification of metabolites must rely on sophisticated instrumentation such as MS, NMR spectroscopy, and laser-induced fluorescence detection. Each of these technologies has its own unique advantages and disadvantages. Optimal selection of a particular technology depends on the goals of the study and is usually a compromise among sensitivity, selectivity, and speed.NMR is highly selective and non-destructive and is generally accepted as the gold standard in metabolite structural elucidation, but it suffers from relatively lower sensitivity. Laser-induced fluorescence is one of the most sensitive techniques, but it lacks the chemical selectivity that is critical in structural identification. In contrast, MS offers a good combination of sensitivity and selectivity. Modern MS provides highly specific chemical information that is directly related to the chemical structure such as accurate mass, isotope distribution patterns for elemental formula determination, and characteristic fragment ions for structural elucidation or identification via spectral matching to authentic compound data. Moreover, the high sensitivity of MS allows detection and measurement of picomole to femtomole levels of many primary and secondary metabolites. These unique advantages make MS an important tool in metabolomics (1, 2).Modern MS offers an array of technologies that differ in operational principles and performance. Variations include ionization technique, mass analyzer technology, resolving power, and mass accuracy. The most common ionization techniques in metabolomics include electron ionization, electrospray ionization (ESI), 2 and atmospheric pressure chemical ionization (APCI). Other ionization techniques such as chemical ionization, MALDI, and, more recently, desorption ESI (DESI) (3) and extractive ESI (EESI) (4) have also been used. Mass analyzers with different resolving powers have also been used in metabolomics. These include ultrahigh and high resolution MS such as Fourier transform ion cyclotron resonance MS (FT-ICR-MS), orbitrap MS, and multipass TOF-MS. However, lower resolution instruments such as ion traps (both linear and three-dimensional quadrupoles) and single quadrupoles are utilized by many. Each of these mass analyzers has its own advantage and limitation. Selection of a specific MS pl...

The majority of flavonoids, such as anthocyanins, proanthocyanidins, and isoflavones, are stored in the central vacuole, but the molecular basis of flavonoid transport is still poorly understood. Here, we report the functional characterization of a multidrug and toxin extrusion transporter (MATE2), from Medicago truncatula. MATE 2 is expressed primarily in leaves and flowers. Despite its high similarity to the epicatechin 39-O-glucoside transporter MATE1, MATE2 cannot efficiently transport proanthocyanidin precursors. In contrast, MATE2 shows higher transport capacity for anthocyanins and lower efficiency for other flavonoid glycosides. Three malonyltransferases that are coexpressed with MATE2 were identified. The malonylated flavonoid glucosides generated by these malonyltransferases are more efficiently taken up into MATE2-containing membrane vesicles than are the parent glycosides. Malonylation increases both the affinity and transport efficiency of flavonoid glucosides for uptake by MATE2. Genetic loss of MATE2 function leads to the disappearance of leaf anthocyanin pigmentation and pale flower color as a result of drastic decreases in the levels of various flavonoids. However, some flavonoid glycoside malonates accumulate to higher levels in MATE2 knockouts than in wild-type controls. Deletion of MATE2 increases seed proanthocyanidin biosynthesis, presumably via redirection of metabolic flux from anthocyanin storage.

GC-MS-based metabolite profiling was used to analyse the response of Medicago truncatula cell cultures to elicitation with methyl jasmonate (MeJa), yeast elicitor (YE), or ultraviolet light (UV). Marked changes in the levels of primary metabolites, including several amino acids, organic acids, and carbohydrates, were observed following elicitation with MeJa. A similar, but attenuated response was observed following YE elicitation, whereas little response was observed following UV elicitation. MeJa induced the accumulation of the triterpene beta-amyrin, a precursor to the triterpene saponins, and LC-MS analysis confirmed the accumulation of triterpene saponins in MeJa-elicited samples. In addition, YE induced a slight, but significant accumulation of shikimic acid, an early precursor to the phenylpropanoid pathway, which was also demonstrated to be YE-inducible by LC-MS analyses. Correlation analyses of metabolite relationships revealed perturbation of the glycine, serine, and threonine biosynthetic pathway, and suggested the induction of threonine aldolase activity, an enzyme as yet uncharacterized from plants. Members of the branched chain amino acid pathway accumulated in a concerted fashion, with the strongest correlation being that between leucine and isoleucine (r2=0.941). While UV exposure itself had little effect on primary metabolites, the experimental procedure, as revealed by control treatments, induced changes in several metabolites which were similar to those following MeJa elicitation. Sucrose levels were lower in MJ- and YE-elicited samples compared with control samples, suggesting that a portion of the effects observed on the primary metabolic pool are a consequence of fundamental metabolic repartitioning of carbon resources rather than elicitor-specific induction. In addition, beta-alanine levels were elevated in all elicited samples, which, when viewed in the context of other elicitation responses, suggests the altered metabolism of coenzyme A and its esters, which are essential in secondary metabolism.

Pollen grains of land plants have evolved remarkably strong outer walls referred to as exine that protect pollen and interact with female stigma cells. Exine is composed of sporopollenin, and while the composition and synthesis of this biopolymer are not well understood, both fatty acids and phenolics are likely components. Here, we describe mutations in the Arabidopsis (Arabidopsis thaliana) LESS ADHESIVE POLLEN (LAP5) and LAP6 that affect exine development. Mutation of either gene results in abnormal exine patterning, whereas pollen of double mutants lacked exine deposition and subsequently collapsed, causing male sterility. LAP5 and LAP6 encode anther-specific proteins with homology to chalcone synthase, a key flavonoid biosynthesis enzyme. lap5 and lap6 mutations reduced the accumulation of flavonoid precursors and flavonoids in developing anthers, suggesting a role in the synthesis of phenolic constituents of sporopollenin. Our in vitro functional analysis of LAP5 and LAP6 using 4-coumaroyl-coenzyme A yielded bis-noryangonin (a commonly reported derailment product of chalcone synthase), while similar in vitro analyses using fatty acyl-coenzyme A as the substrate yielded medium-chain alkyl pyrones. Thus, in vitro assays indicate that LAP5 and LAP6 are multifunctional enzymes and may play a role in both the synthesis of pollen fatty acids and phenolics found in exine. Finally, the genetic interaction between LAP5 and an anther gene involved in fatty acid hydroxylation (CYP703A2) demonstrated that they act synergistically in exine production.