Kevin C. Vaughn

Bio: Kevin C. Vaughn is an academic researcher from Agricultural Research Service. The author has contributed to research in topics: Microtubule & Chloroplast. The author has an hindex of 40, co-authored 134 publications receiving 4884 citations. Previous affiliations of Kevin C. Vaughn include United States Department of Agriculture & University of Arkansas.

Function of polyphenol oxidase in higher plants

Abstract: Recent evidence has supported the folllowing views: 1 Polyphenol oxidase (PPO) is a plastidic enzyme that is unclear-coded, but is inactive until incorporated into the plastid 2 In healthy green tissues PPO exists in a latent form on the thylakoid membrane and is not involved in synthesis of phenolic compounds In leucoplasts, proplastids, or amyloplasts PPO is often present in a latent form in rudimentary thylakoids 3 PPO normally functions as a phenol oxidase in vivo only in sencent or damaged cells 4 In the functional chloroplast, PPO may be involved in some aspect of oxygen chemistry – pherhaps mediation of pseudocyclic photophosphorylation

Role of peroxidase in the development of water-impermeable seed coats in Sida spinosa L

Abstract: The seed coats of S. spinosa (prickly sida, Malvaceae) become impermeable to water during seed development on the mother plant. After the seeds have dehydrated during the final maturation stages, piercing of seed coats is necessary to induce imbibition of water and germination. Onset of impermeability occurs during seed coat browning, well in advance of seed dehydration. I. Marbach and A.M. Mayer (1975, Plant Physiol. 56, 93-96) implicated polyphenol oxidase (PO; EC 1.10.3.1) as catechol oxidase in the formation of insoluble polymers during development of coat impermeability in a wild strain of pea (Pisum elatius) seeds. We found, however, that peroxidase (EC 1.11.1.7), not PO, is instrumental in the development of water-impermeable seed coats in prickly sida. We isolated coats and embryos from seeds harvested at several stages of development. Highest peroxidase activity of coat extracts correlated well with the developmental stages of maximum conversion of soluble phenolics to insoluble lignin polymers. Although seed extracts oxidized dihydroxyphenylalanine, this activity was eliminated by catalase, indicating that the oxidation of phenolics in the coat is catalyzed by peroxidase rather than PO. Histochemical localization of peroxidase was strongest in the palisade layer; both the level and time of appearance of activity was proportional to the spectrophotometric assays of seed-coat extracts. The presence of peroxidase and the absence of PO in the seed coat were also confirmed with immunocytochemistry. Our results support the view that peroxidase is involved in the polymerization of soluble phenolics to insoluble lignin polymers during development of prickly sida seed coats, causing the formation of a water-impermeable barrier prior to seed dehydration. As dehydration proceeds, the chalazal area finally becomes impermeable resulting in the hard mature seeds of prickly sida.

Polyphenol oxidase: The chloroplast oxidase with no established function

Abstract: Vaughn, K. C, Lax, A. R. and Duke, S. O. 1988. Polyphenol oxidase: The chloroplast oxidase with no established function. - Physiol. Plant. 72: 659–665. Polyphenol oxidase (PPO) is an enzyme localized on the thylakoids of chloroplasts and in vesicles or other bodies in non-green plastid types. Although virtually all plastids contain PPO, little or no detectable activity is associated with guard cell and bundle sheath cell chloroplasts. Despite this nearly ubiquitous occurrence, no function for this enzyme has been established. The enzyme is nuclear-encoded and, unlike most chloroplast proteins is not encoded as a larger M, precursor molecule. This lack of a transit peptide sequence may be related to a unique mechanism of uptake, apparently involving inner envelope-derived vesicles. The M, range of most of the PPO forms is 36–45 kDa. PPO is apparently not involved in phenolic biosynthesis but is probably involved with the production of o-quinones during pathogen invasion. A role for PPO as an “oxygen buffer” is postulated, but little concrete data have been collected on any other functional role for this enzyme.

Artemisinin, a Constituent of Annual Wormwood (Artemisia annua), is a Selective Phytotoxin

Abstract: Artemisinin (qinghaosu), a sesquiterpenoid lactone peroxide constituent of annual wormwood (Artemisia annua L. # ARTAN) that is used as an antimalarial drug, was tested for phytotoxic properties. It inhibited germination of lettuce (Lactuca sativa L.) and annual wormwood, and growth of roots and shoots of lettuce, redroot pigweed (Amaranthus retroflexus L. # AMARE), pitted morningglory (Ipomoea lacunosa L. # IPOLA), annual wormwood, and common purslane (Portulaca oleracea L. # POROL) was inhibited at 33 μM. No effects of 33 μM artemisinin were detected on growth of velvetleaf (Abutilon theophrasti Medik. # ABUTH) and grain sorghum [Sorghum bicolor (L.) Moench.]. Chlorophyll content was not affected in lettuce, and chlorosis was not observed in any species tested. The probable biosynthetic precursors of artemisinin, arteannuin B and qinghao acid, had no effect on growth or chlorophyll content of lettuce; however, they inhibited lettuce seed germination. Artemisinin and cinmethylin {exo-1-methyl-4-(1-methylethyl)-2-[(2-methylphenyl)methoxy]-7-oxabicyclo [2.2.1] heptane} were equally effective in reducing growth of lettuce; however, cinmethylin had no effect on germination. Respiration of lettuce roots or cotyledons was not inhibited by artemisinin. Artemisinin only marginally increased the mitotic index of lettuce root tips at 33 μM. At the ultrastructural level, however, chromosomes were less condensed during mitosis in artemisinin-treated than control meristematic cells. The growth-inhibiting ability of artemisinin could not be reduced by feeding the plants with hydrolyzed protein or treatment with putrescine. Artemisinin is a selective phytotoxin that reduces growth by a mechanism other than mitotic disruption or inhibition of protein synthesis.

Mitotic Disrupter Herbicides

Abstract: Approximately one-quarter of all herbicides that have been marketed affect mitosis as a primary mechanism of action. All of these herbicides appear to interact directly or indirectly with the microtubule. Dinitroaniline and phosphoric amide herbicides inhibit microtubule polymerization from free tubulin subunits. Because of the loss of spindle and kinetochore microtubules, chromosomes cannot move to the poles during mitosis, resulting in cells exhibiting an arrested prometaphase configuration. Nuclear membranes re-form around the chromosomal masses to form lobed nuclei. Cortical microtubules, which influence cell shape, are also absent, and, as a result, the cell expands isodiametrically. In root tips and other structures that are normally elongated, these herbicides induce a characteristic club-shaped swelling. Pronamide and MON 7200 induce similar effects, except that tufts of microtubules remain at the kinetochore region of the chromosomes. The carbamate herbicides barban, propham, and chlorpropham alter the organization of the spindle microtubules so that multiple spindles are formed. Chromosomes move to many poles and multiple nuclei result. Abnormal branched cell walls partly separate the nuclei. Terbutol induces “star anaphase” chromosome configurations in which the chromosomes are drawn into an area at the poles in a star-like aggregation. DCPA's most dramatic effect is on phragmoplast microtubule arrays. Multiple, branched, and curved phragmoplasts are found after herbicide treatment. These disrupters should prove to be useful tools in investigations of the proteins and structures required for a successful cell division.

Microtubules as a target for anticancer drugs.

Abstract: Highly dynamic mitotic-spindle microtubules are among the most successful targets for anticancer therapy. Microtubule-targeted drugs, including paclitaxel and Vinca alkaloids, were previously considered to work primarily by increasing or decreasing the cellular microtubule mass. Although these effects might have a role in their chemotherapeutic actions, we now know that at lower concentrations, microtubule-targeted drugs can suppress microtubule dynamics without changing microtubule mass; this action leads to mitotic block and apoptosis. In addition to the expanding array of chemically diverse antimitotic agents, some microtubule-targeted drugs can act as vascular-targeting agents, rapidly depolymerizing microtubules of newly formed vasculature to shut down the blood supply to tumours.

Fungal endophytes: diversity and functional roles.

Abstract: Summary 1 Summary All plants in natural ecosystems appear to be symbiotic with fungal endophytes. This highly diverse group of fungi can have profound impacts on plant communities through increasing fitness by conferring abiotic and biotic stress tolerance, increasing biomass and decreasing water consumption, or decreasing fitness by altering resource allocation. Despite more than 100 yr of research resulting in thousands of journal articles, the ecological significance of these fungi remains poorly characterized. Historically, two endophytic groups (clavicipitaceous (C) and nonclavicipitaceous (NC)) have been discriminated based on phylogeny and life history traits. Here, we show that NC-endophytes represent three distinct functional groups based on host colonization and transmission, in planta biodiversity and fitness benefits conferred to hosts. Using this framework, we contrast the life histories, interactions with hosts and potential roles in plant ecophysiology of C- and NC-endophytes, and highlight several key questions for future work in endophyte biology.

Copper Active Sites in Biology

Abstract: Based on its generally accessible I/II redox couple and bioavailability, copper plays a wide variety of roles in nature that mostly involve electron transfer (ET), O2 binding, activation and reduction, NO2− and N2O reduction and substrate activation. Copper sites that perform ET are the mononuclear blue Cu site that has a highly covalent CuII-S(Cys) bond and the binuclear CuA site that has a Cu2S(Cys)2 core with a Cu-Cu bond that keeps the site delocalized (Cu(1.5)2) in its oxidized state. In contrast to inorganic Cu complexes, these metalloprotein sites transfer electrons rapidly often over long distances, as has been previously reviewed.1–4 Blue Cu and CuA sites will only be considered here in their relation to intramolecular ET in multi-center enzymes. The focus of this review is on the Cu enzymes (Figure 1). Many are involved in O2 activation and reduction, which has mostly been thought to involve at least two electrons to overcome spin forbiddenness and the low potential of the one electron reduction to superoxide (Figure 2).5,6 Since the Cu(III) redox state has not been observed in biology, this requires either more than one Cu center or one copper and an additional redox active organic cofactor. The latter is formed in a biogenesis reaction of a residue (Tyr) that is also Cu catalyzed in the first turnover of the protein. Recently, however, there have been a number of enzymes suggested to utilize one Cu to activate O2 by 1e− reduction to form a Cu(II)-O2•− intermediate (an innersphere redox process) and it is important to understand the active site requirements to drive this reaction. The oxidases that catalyze the 4e−reduction of O2 to H2O are unique in that they effectively perform this reaction in one step indicating that the free energy barrier for the second two-electron reduction of the peroxide product of the first two-electron step is very low. In nature this requires either a trinuclear Cu cluster (in the multicopper oxidases) or a Cu/Tyr/Heme Fe cluster (in the cytochrome oxidases). The former accomplishes this with almost no overpotential maximizing its ability to oxidize substrates and its utility in biofuel cells, while the latter class of enzymes uses the excess energy to pump protons for ATP synthesis. In bacterial denitrification, a mononuclear Cu center catalyzes the 1e- reduction of nitrite to NO while a unique µ4S2−Cu4 cluster catalyzes the reduction of N2O to N2 and H2O, a 2e− process yet requiring 4Cu’s. Finally there are now several classes of enzymes that utilize an oxidized Cu(II) center to activate a covalently bound substrate to react with O2. Figure 1 Copper active sites in biology. Figure 2 Latimer Diagram for Oxygen Reduction at pH = 7.0 Adapted from References 5 and 6. This review presents in depth discussions of all these classes of Cu enzymes and the correlations within and among these classes. For each class we review our present understanding of the enzymology, kinetics, geometric structures, electronic structures and the reaction mechanisms these have elucidated. While the emphasis here is on the enzymology, model studies have significantly contributed to our understanding of O2 activation by a number of Cu enzymes and are included in appropriate subsections of this review. In general we will consider how the covalency of a Cu(II)–substrate bond can activate the substrate for its spin forbidden reaction with O2, how in binuclear Cu enzymes the exchange coupling between Cu’s overcomes the spin forbiddenness of O2 binding and controls electron transfer to O2 to direct catalysis either to perform two e− electrophilic aromatic substitution or 1e− H-atom abstraction, the type of oxygen intermediate that is required for H-atom abstraction from the strong C-H bond of methane (104 kcal/mol) and how the trinuclear Cu cluster and the Cu/Tyr/Heme Fe cluster achieve their very low barriers for the reductive cleavage of the O-O bond. Much of the insight available into these mechanisms in Cu biochemistry has come from the application of a wide range of spectroscopies and the correlation of spectroscopic results to electronic structure calculations. Thus we start with a tutorial on the different spectroscopic methods utilized to study mononuclear and multinuclear Cu enzymes and their correlations to different levels of electronic structure calculations.