Browsing by Author "Hildreth, Sherry B."

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    Comparative Metabolomics of Fruits and Leaves in a Hyperdiverse Lineage Suggests Fruits Are a Key Incubator of Phytochemical Diversification
    Schneider, Gerald F.; Salazar, Diego; Hildreth, Sherry B.; Helm, Richard F.; Whitehead, Susan R. (Frontiers, 2021-08-30)
    Interactions between plants and leaf herbivores have long been implicated as the major driver of plant secondary metabolite diversity. However, other plant-animal interactions, such as those between fruits and frugivores, may also be involved in phytochemical diversification. Using 12 species of Piper, we conducted untargeted metabolomics and molecular networking with extracts of fruits and leaves. We evaluated organ-specific secondary metabolite composition and compared multiple dimensions of phytochemical diversity across organs, including richness, structural complexity, and variability across samples at multiple scales within and across species. Plant organ identity, species identity, and the interaction between the two all significantly influenced secondary metabolite composition. Leaves and fruit shared a majority of compounds, but fruits contained more unique compounds and had higher total estimated chemical richness. While the relative levels of chemical richness and structural complexity across organs varied substantially across species, fruit diversity exceeded leaf diversity in more species than the reverse. Furthermore, the variance in chemical composition across samples was higher for fruits than leaves. By documenting a broad pattern of high phytochemical diversity in fruits relative to leaves, this study lays groundwork for incorporating fruit into a comprehensive and integrative understanding of the ecological and evolutionary factors shaping secondary metabolite composition at the whole-plant level.
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    The dynamic response of the Arabidopsis root metabolome to auxin and ethylene is not predicted by changes in the transcriptome
    Hildreth, Sherry B.; Foley, Evan E.; Muday, Gloria K.; Helm, Richard F.; Winkel, Brenda S. J. (Nature Research, 2020-01-20)
    While the effects of phytohormones on plant gene expression have been well characterized, comparatively little is known about how hormones influence metabolite profiles. This study examined the effects of elevated auxin and ethylene on the metabolome of Arabidopsis roots using a high-resolution 24 h time course, conducted in parallel to time-matched transcriptomic analyses. Mass spectrometry using orthogonal UPLC separation strategies (reversed phase and HILIC) in both positive and negative ionization modes was used to maximize identification of metabolites with altered levels. The findings show that the root metabolome responds rapidly to hormone stimulus and that compounds belonging to the same class of metabolites exhibit similar changes. The responses were dominated by changes in phenylpropanoid, glucosinolate, and fatty acid metabolism, although the nature and timing of the response was unique for each hormone. These alterations in the metabolome were not directly predicted by the corresponding transcriptome data, suggesting that post-transcriptional events such as changes in enzyme activity and/or transport processes drove the observed changes in the metabolome. These findings underscore the need to better understand the biochemical mechanisms underlying the temporal reconfiguration of plant metabolism, especially in relation to the hormone-metabolome interface and its subsequent physiological and morphological effects.
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    Exogenous Auxin Elicits Changes in the Arabidopsis thaliana Root Proteome in a Time-Dependent Manner.
    Slade, William O.; Ray, W. Keith; Hildreth, Sherry B.; Winkel, Brenda S. J.; Helm, Richard F. (MDPI, 2017-07-10)
    Auxin is involved in many aspects of root development and physiology, including the formation of lateral roots. Improving our understanding of how the auxin response is mediated at the protein level over time can aid in developing a more complete molecular framework of the process. This study evaluates the effects of exogenous auxin treatment on the Arabidopsis root proteome after exposure of young seedlings to auxin for 8, 12, and 24 h, a timeframe permitting the initiation and full maturation of individual lateral roots. Root protein extracts were processed to peptides, fractionated using off-line strong-cation exchange, and analyzed using ultra-performance liquid chromatography and data independent acquisition-based mass spectrometry. Protein abundances were then tabulated using label-free techniques and evaluated for significant changes. Approximately 2000 proteins were identified during the time course experiment, with the number of differences between the treated and control roots increasing over the 24 h time period, with more proteins found at higher abundance with exposure to auxin than at reduced abundance. Although the proteins identified and changing in levels at each time point represented similar biological processes, each time point represented a distinct snapshot of the response. Auxin coordinately regulates many physiological events in roots and does so by influencing the accumulation and loss of distinct proteins in a time-dependent manner. Data are available via ProteomeXchange with the identifier PXD001400.
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    Identification of MOS9 as an interaction partner for chalcone synthase in the nucleus
    Watkinson, Jonathan I.; Bowerman, Peter A.; Crosby, Kevin C.; Hildreth, Sherry B.; Helm, Richard F.; Winkel, Brenda S. J. (PeerJ, 2018-09-19)
    Plant flavonoid metabolism has served as a platform for understanding a range of fundamental biological phenomena, including providing some of the early insights into the subcellular organization of metabolism. Evidence assembled over the past three decades points to the organization of the component enzymes as a membrane-associated complex centered on the entry-point enzyme, chalcone synthase (CHS), with flux into branch pathways controlled by competitive protein interactions. Flavonoid enzymes have also been found in the nucleus in a variety of plant species, raising the possibility of alternative, or moonlighting functions for these proteins in this compartment. Here, we present evidence that CHS interacts with MOS9, a nuclear-localized protein that has been linked to epigenetic control of R genes that mediate effector-triggered immunity. Overexpression of MOS9 results in a reduction of CHS transcript levels and a metabolite profile that substantially intersects with the effects of a null mutation in CHS. These results suggest that the MOS9–CHS interaction may point to a previously-unknown mechanism for controlling the expression of the highly dynamic flavonoid pathway.
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    Investigation of Protein-Protein Interactions among Nicotine Biosynthetic Enzymes and Characterization of a Nicotine Transporter
    Hildreth, Sherry B. (Virginia Tech, 2009-09-29)
    Alkaloids are a class of plant secondary metabolites produced in about 20% of plant families. Domesticated tobacco, Nicotiana tabacum produces nicotine as the predominant alkaloid. The biosynthesis of nicotine occurs exclusively in the roots of tobacco, yet accumulates in the leaves of tobacco where it is acts as a defense compound to deter insect herbivory. The research detailed in this dissertation addresses two aspects of nicotine physiology in tobacco: 1) an investigation of hypothesized protein-protein interactions among nicotine biosynthetic enzymes and 2) the characterization of a novel nicotine transporter. A hypothesized metabolic channel including the two nicotine biosynthetic enzymes putrescine N-methyltransferase (PMT), N-methylputrescine Oxidase (MPO) and the S-adenosylmethionine (SAM) recycling enzyme S-adenosylhomocysteine hydrolase (SAHH) has been proposed. To further explore this hypothesis, protein-protein interactions among nicotine biosynthetic enzymes PMT, MPO and SAHH were investigated using yeast two-hybrid assays and co-immunoprecipitation experiments. The yeast two-hybrid was conducted as both a directed screen to detect interactions between the hypothesized metabolic channel members and as a library screen to detect interactions between hypothesized metabolic channel members and proteins from a tobacco root cDNA library. Co-immunoprecipitation experiments were conducted using proteins produced in an in vitro transcription/ translation system and using native proteins from a tobacco root extract. The outcome of these experiments provided no further evidence of a nicotine metabolic channel and a discussion of the methods and outcomes of the experiments conducted is presented. The nicotine uptake permease, NUP1, was identified in tobacco roots and was shown to preferentially transport nicotine when expressed in Schizosaccharomyces pombe. This report presents the characterization of tobacco plants and hairy roots with diminished NUP1 transcripts created by using RNAi. The NUP1-RNAi hairy roots and plants showed a decreased level of nicotine and the hairy root cultures displayed an altered distribution of nicotine from the root to the culture medium. Additionally NUP1-GFP was used to determine that NUP1 localized to the plasma membrane of tobacco BY-2 protoplasts. Potential models for the role of NUP1 in nicotine physiology will be discussed.
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    Mutations that alter Arabidopsis flavonoid metabolism affect the circadian clock
    Hildreth, Sherry B.; Littleton, Evan S.; Clark, Leor C.; Puller, Gabrielle C.; Kojima, Shihoko; Winkel, Brenda S. J. (Society for Experimental Biology, 2022-02-26)
    Flavonoids are a well-known class of specialized metabolites that play key roles in plant development, reproduction, and survival. Flavonoids are also of considerable interest from the perspective of human health, as both phytonutrients and pharmaceuticals. RNA sequencing analysis of an Arabidopsis null allele for chalcone synthase (CHS), which catalyzes the first step in flavonoid metabolism, has uncovered evidence that these compounds influence the expression of genes associated with the plant circadian clock. Analysis of promoter-luciferase constructs further showed that the transcriptional activity of CCA1 and TOC1, two key clock genes, is altered in CHS-deficient seedlings across the day/night cycle. Similar findings for a mutant line lacking flavonoid 3′-hydroxylase (F3′H) activity, and thus able to synthesize mono- but not dihydroxylated B-ring flavonoids, suggests that the latter are at least partially responsible; this was further supported by the ability of quercetin to enhance CCA1 promoter activity in wild-type and CHS-deficient seedlings. The effects of flavonoids on circadian function were also reflected in photosynthetic activity, with chlorophyll cycling abolished in CHS- and F3′H-deficient plants. Remarkably, the same phenotype was exhibited by plants with artificially high flavonoid levels, indicating that neither the antioxidant potential nor the light-screening properties of flavonoids contribute to optimal clock function, as has recently also been demonstrated in animal systems. Collectively, the current experiments point to a previously unknown connection between flavonoids and circadian cycling in plants and open the way to better understanding of the molecular basis of flavonoid action.
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    A Role for Inositol Pyrophosphates in the Metabolic Adaptations to Low Phosphate in Arabidopsis
    Land, Eric S.; Cridland, Caitlin A.; Craige, Branch; Dye, Anna; Hildreth, Sherry B.; Helm, Richard F.; Gillaspy, Glenda E.; Perera, Imara Y. (MDPI, 2021-09-04)
    Phosphate is a major plant macronutrient and low phosphate availability severely limits global crop productivity. In Arabidopsis, a key regulator of the transcriptional response to low phosphate, phosphate starvation response 1 (PHR1), is modulated by a class of signaling molecules called inositol pyrophosphates (PP-InsPs). Two closely related diphosphoinositol pentakisphosphate enzymes (AtVIP1 and AtVIP2) are responsible for the synthesis and turnover of InsP8, the most implicated molecule. This study is focused on characterizing Arabidopsis vip1/vip2 double mutants and their response to low phosphate. We present evidence that both local and systemic responses to phosphate limitation are dampened in the vip1/vip2 mutants as compared to wild-type plants. Specifically, we demonstrate that under Pi-limiting conditions, the vip1/vip2 mutants have shorter root hairs and lateral roots, less accumulation of anthocyanin and less accumulation of sulfolipids and galactolipids. However, phosphate starvation response (PSR) gene expression is unaffected. Interestingly, many of these phenotypes are opposite to those exhibited by other mutants with defects in the PP-InsP synthesis pathway. Our results provide insight on the nexus between inositol phosphates and pyrophosphates involved in complex regulatory mechanisms underpinning phosphate homeostasis in plants.
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    Sinorhizobium meliloti Chemoreceptor McpV Senses Short-Chain Carboxylates via Direct Binding
    Compton, K. Karl; Hildreth, Sherry B.; Helm, Richard F.; Scharf, Birgit E. (2018-12)
    Sinorhizobium meliloti is a soil-dwelling endosymbiont of alfalfa that has eight chemoreceptors to sense environmental stimuli during its free-living state. The functions of two receptors have been characterized, with McpU and McpX serving as general amino acid and quaternary ammonium compound sensors, respectively. Both receptors use a dual Cache (calcium channels and chemotaxis receptors) domain for ligand binding. We identified that the ligand-binding periplasmic region (PR) of McpV contains a single Cache domain. Homology modeling revealed that McpVPR is structurally similar to a sensor domain of a chemoreceptor with unknown function from Anaeromyxobacter dehalogenans, which crystallized with acetate in its binding pocket. We therefore assayed McpV for carboxylate binding and S. meliloti for carboxylate sensing. Differential scanning fluorimetry identified 10 potential ligands for McpVPR. Nine of these are monocarboxylates with chain lengths between two and four carbons. We selected seven compounds for capillary assay analysis, which established positive chemotaxis of the S. meliloti wild type, with concentrations of peak attraction at 1 mM for acetate, propionate, pyruvate, and glycolate, and at 100 mM for formate and acetoacetate. Deletion of mcpV or mutation of residues essential for ligand coordination abolished positive chemotaxis to carboxylates. Using microcalorimetry, we determined that dissociation constants of the seven ligands with McpVPR were in the micromolar range. An McpVPR variant with a mutation in the ligand coordination site displayed no binding to isobutyrate or propionate. Of all the carboxylates tested as attractants, only glycolate was detected in alfalfa seed exudates. This work examines the relevance of carboxylates and their sensor to the rhizobium-legume interaction.
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    The unusual cell wall of the Lyme disease spirochaete Borrelia burgdorferi is shaped by a tick sugar
    DeHart, Tanner G.; Kushelman, Mara R.; Hildreth, Sherry B.; Helm, Richard F.; Jutras, Brandon L. (Springer Nature, 2021-11-24)
    Peptidoglycan—a mesh sac of glycans that are linked by peptides—is the main component of bacterial cell walls. Peptidoglycan provides structural strength, protects cells from osmotic pressure and contributes to shape. All bacterial glycans are repeating disaccharides of N-acetylglucosamine (GlcNAc) β-(1–4)-linked to N-acetylmuramic acid (MurNAc). Borrelia burgdorferi, the tick-borne Lyme disease pathogen, produces glycan chains in which MurNAc is occasionally replaced with an unknown sugar. Nuclear magnetic resonance, liquid chromatography–mass spectroscopy and genetic analyses show that B. burgdorferi produces glycans that contain GlcNAc–GlcNAc. This unusual disaccharide is chitobiose, a component of its chitinous tick vector. Mutant bacteria that are auxotrophic for chitobiose have altered morphology, reduced motility and cell envelope defects that probably result from producing peptidoglycan that is stiffer than that in wild-type bacteria. We propose that the peptidoglycan of B. burgdorferi probably evolved by adaptation to obligate parasitization of a tick vector, resulting in a biophysical cell-wall alteration to withstand the atypical torque associated with twisting motility.
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    An Updated Perspective on Sinorhizobium meliloti Chemotaxis to Alfalfa Flavonoids
    Compton, K. Karl; Hildreth, Sherry B.; Helm, Richard F.; Scharf, Birgit E. (Frontiers, 2020-10-23)
    The symbiotic interaction between leguminous plants and their cognate rhizobia allows for the fixation of gaseous dinitrogen into bioavailable ammonia. The perception of host-derived flavonoids is a key initial step for the signaling events that must occur preceding the formation of the nitrogen-fixing organ. Past work investigating chemotaxis – the directed movement of bacteria through chemical gradients – of Bradyrhizobium japonicum, Rhizobium leguminosarum, and Rhizobium meliloti discovered chemotaxis to various organic compounds, but focused on chemotaxis to flavonoids because of their relevance to the symbiosis biochemistry. The current work sought to replicate and further examine Sinorhizobium (Ensifer) meliloti chemotaxis to the flavonoids previously thought to act as the principal attractant molecules prior to the initial signaling stage. Exudate from germinating alfalfa seedlings was analyzed for composition and quantities of different flavonoid compounds using mass spectrometry. The abundance of four prevalent flavonoids in germinating alfalfa seed exudates (SEs) was at a ratio of 200:5:5:1 for hyperoside, luteolin, luteolin-7-glucoside, and chrysoeriol. Using quantitative chemotaxis capillary assays, we did not detect chemotaxis of motile S. meliloti cells to these, and two other flavonoids identified in seed exudates. In support of these findings, the flavonoid fraction of seed exudates was found to be an insignificant attractant relative to the more hydrophilic fraction. Additionally, we observed that cosolvents commonly used to dissolve flavonoids confound the results. We propose that the role flavonoids play in S. meliloti chemotaxis is insignificant relative to other components released by alfalfa seeds.