Biomedical subjects

Stephen O Duke

Publications and source records attributed to Stephen O Duke.

At least 19 recordsLinked to original sources

PubMed2006

Functional characterization of desaturases involved in the formation of the terminal double bond of an unusual 16:3Delta(9,12,150) fatty acid isolated from Sorghum bicolor root hairs.

Sorgoleone, produced in root hair cells of sorghum (Sorghum bicolor), is likely responsible for much of the allelopathic properties of sorghum root exudates against broadleaf and grass weeds. Previous studies suggest that the biosynthetic pathway of this compound initiates with the synthesis of an unusual 16:3 fatty acid possessing a terminal double bond. The corresponding fatty acyl-CoA serves as a starter unit for polyketide synthases, resulting in the formation of 5-pentadecatrienyl resorcinol. This resorcinolic intermediate is then methylated by an S-adenosylmethionine-dependent O-methyltransferase and subsequently dihydroxylated, yielding the reduced (hydroquinone) form of sorgoleone. To characterize the corresponding enzymes responsible for the biosynthesis of the 16:3 fatty acyl-CoA precursor, we identified and cloned three putative fatty acid desaturases, designated SbDES1, SbDES2, and SbDES3, from an expressed sequence tag (EST) data base prepared from isolated root hairs. Quantitative real-time RT-PCR analyses revealed that these three genes were preferentially expressed in sorghum root hairs where the 16:2 and 16:3 fatty acids were exclusively localized. Heterologous expression of the cDNAs in Saccharomyces cerevisiae revealed that recombinant SbDES2 converted palmitoleic acid (16:1Delta(9)) to hexadecadienoic acid (16:2Delta(9,12)), and that recombinant SbDES3 was capable of converting hexadecadienoic acid into hexadecatrienoic acid (16:3Delta(9,12,15)). Unlike other desaturases reported to date, the double bond introduced by SbDES3 occurred between carbons 15 and 16 resulting in a terminal double bond aliphatic chain. Collectively, the present results strongly suggest that these fatty acid desaturases represent key enzymes involved in the biosynthesis of the allelochemical sorgoleone.

Amino Acid Sequence↗

PubMed2006

The current status and environmental impacts of glyphosate-resistant crops: a review.

Glyphosate [N-(phosphonomethyl) glycine]-resistant crops (GRCs), canola (Brassica napus L.), cotton (Gossypium hirsutum L.), maize (Zea mays L.), and soybean [Glycine max (L.) Merr.] have been commercialized and grown extensively in the Western Hemisphere and, to a lesser extent, elsewhere. Glyphosate-resistant cotton and soybean have become dominant in those countries where their planting is permitted. Effects of glyphosate on contamination of soil, water, and air are minimal, compared to some of the herbicides that they replace. No risks have been found with food or feed safety or nutritional value in products from currently available GRCs. Glyphosate-resistant crops have promoted the adoption of reduced- or no-tillage agriculture in the USA and Argentina, providing a substantial environmental benefit. Weed species in GRC fields have shifted to those that can more successfully withstand glyphosate and to those that avoid the time of its application. Three weed species have evolved resistance to glyphosate in GRCs. Glyphosate-resistant crops have greater potential to become problems as volunteer crops than do conventional crops. Glyphosate resistance transgenes have been found in fields of canola that are supposed to be non-transgenic. Under some circumstances, the largest risk of GRCs may be transgene flow (introgression) from GRCs to related species that might become problems in natural ecosystems. Glyphosate resistance transgenes themselves are highly unlikely to be a risk in wild plant populations, but when linked to transgenes that may impart fitness benefits outside of agriculture (e.g., insect resistance), natural ecosystems could be affected. The development and use of failsafe introgression barriers in crops with such linked genes is needed.

Argentina↗

PubMed2006

Zebra mussel antifouling activity of the marine natural product aaptamine and analogs.

Several aaptamine derivatives were selected as potential zebra mussel (Dreissena polymorpha) antifoulants because of the noteworthy absence of fouling observed on Aaptos sponges. Sponges of the genus Aaptos collected in Manado, Indonesia consistently produce aaptamine-type alkaloids. To date, aaptamine and its derivatives have not been carefully evaluated for their antifoulant properties. Structure-activity relationship studies were conducted using several aaptamine derivatives in a zebra mussel antifouling assay. From these data, three analogs have shown significant antifouling activity against zebra mussel attachment. Aaptamine, isoaaptamine, and the demethylated aaptamine compounds used in the zebra mussel assay produced EC(50) values of 24.2, 11.6, and 18.6 microM, respectively. In addition, neither aaptamine nor isoaaptamine produced a phytotoxic response (as high as 300 microM) toward a nontarget organism, Lemna pausicostata, in a 7-day exposure. The use of these aaptamine derivatives from Aaptos sp. as potential environmentally benign antifouling alternatives to metal-based paints and preservatives is significant, not only as a possible control of fouling organisms, but also to highlight the ecological importance of these and similar biochemical defenses.

Animals↗

PubMed2006

Antifungal activity of thiophenes from Echinops ritro.

Extracts from 30 plants of the Greek flora were evaluated for their antifungal activity using direct bioautography assays with three Colletotrichum species. Among the bioactive extracts, the dichloromethane extract of the radix of Echinops ritro (Asteraceae) was the most potent. Bioassay-guided fractionation of this extract led to the isolation of eight thiophenes. Antifungal activities of isolated compounds together with a previously isolated thiophene from Echinops transiliensis were first evaluated by bioautography and subsequently evaluated in greater detail using a broth microdilution assay against plant pathogens Colletotrichum acutatum, Colletotrichum fragariae, Colletotrichum gloeosporioides, Botrytis cinerea, Fusarium oxysporum, Phomopsis viticola, and Phomopsis obscurans. 5'-(3-Buten-1-ynyl)-2,2'-bithiophen (1), alpha-terthienyl (2), and 2-[pent-1,3-diynyl]-5-[4-hydroxybut-1-ynyl]thiophene (5) at 3 and 30 microM were active against all three Colletotrichum species, F. oxysporum, P. viticola, and P. obscurans.

Ascomycota↗

PubMed2005

Algicidal and antifungal compounds from the roots of Ruta graveolens and synthesis of their analogs.

Bioassay-guided fractionation of the ethyl acetate extract of Ruta graveolens roots yielded rutacridone epoxide with potent selective algicidal activity towards the 2-methyl-isoborneol (MIB)-producing blue-green alga Oscillatoria perornata, with relatively little effect on the green alga Selenastrum capricornutum. The diol-analog of rutacridone epoxide, gravacridondiol, which was also present in the same extract, had significantly less activity towards O. perornata. Rutacridone epoxide also showed significantly higher activity than commercial fungicides captan and benomyl in our micro-bioassay against the agriculturally important pathogenic fungi Colletotrichum fragariae, C. gloeosporioides, C. acutatum, and Botrytis cineara and Fusarium oxysporium. Rutacridone epoxide is reported as a direct-acting mutagen, precluding its use as an agrochemical. In order to understand the structure-activity relationships and to develop new potential biocides without toxicity and mutagenicity, some analogs containing the (2-methyloxiranyl)-dihydrobenzofuran moiety with an epoxide were synthesized and tested. None of the synthetic analogs showed comparable activities to rutacridone epoxide. The absolute stereochemistry of rutacridone was determined to be 2'(R) and that of rutacridone epoxide to be 2'(R), 3'(R) by CD and NMR analysis.

Antifungal Agents↗

PubMed2005

Detoxification and transcriptome response in Arabidopsis seedlings exposed to the allelochemical benzoxazolin-2(3H)-one.

Benzoxazolin-2(3H)-one (BOA) is an allelochemical most commonly associated with monocot species, formed from the O-glucoside of 2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one by a two-step degradation process. The capacity of Arabidopsis to detoxify exogenously supplied BOA was analyzed by quantification of the major known metabolites BOA-6-OH, BOA-6-O-glucoside, and glucoside carbamate, revealing that detoxification occurs predominantly through O-glucosylation of the intermediate BOA-6-OH, most likely requiring the sequential action of as-yet-unidentified cytochrome P450 and UDP-glucosyltransferase activities. Transcriptional profiling experiments were also performed with Arabidopsis seedlings exposed to BOA concentrations, representing I(50) and I(80) levels based on root elongation inhibition assays. One of the largest functional categories observed for BOA-responsive genes corresponded to protein families known to participate in cell rescue and defense, with the majority of these genes potentially associated with chemical detoxification pathways. Further experiments using a subset of these genes revealed that many are also transcriptionally induced by a variety of structurally diverse xenobiotic compounds, suggesting they comprise components of a coordinately regulated, broad specificity xenobiotic defense response. The data significantly expand upon previous studies examining plant transcriptional responses to allelochemicals and other environmental toxins and provide novel insights into xenobiotic detoxification mechanisms in plants.

Amino Acid Motifs↗

PubMed2005

Taking stock of herbicide-resistant crops ten years after introduction.

Since transgenic, bromoxynil-resistant cotton and glufosinate-resistant canola were introduced in 1995, planting of transgenic herbicide-resistant crops has grown substantially, revolutionizing weed management where they have been available. Before 1995, several commercial herbicide-resistant crops were produced by biotechnology through selection for resistance in tissue culture. However, non-transgenic herbicide-resistant crops have had less commercial impact. Since the introduction of glyphosate-resistant soybean in 1996, and the subsequent introduction of other glyphosate-resistant crops, where available, they have taken a commanding share of the herbicide-resistant crop market, especially in soybean, cotton and canola. The high level of adoption of glyphosate-resistant crops by North American farmers has helped to significantly reduce the value of the remaining herbicide market. This has resulted in reduced investment in herbicide discovery, which may be problematic for addressing future weed-management problems. Introduction of herbicide-resistant crops that can be used with selective herbicides has apparently been hindered by the great success of glyphosate-resistant crops. Evolution of glyphosate-resistant weeds and movement of naturally resistant weed species into glyphosate-resistant crop fields will require increases in the use of other herbicides, but the speed with which these processes compromise the use of glyphosate alone is uncertain. The future of herbicide-resistant crops will be influenced by many factors, including alternative technologies, public opinion and weed resistance. Considering the relatively few recent approvals for field testing new herbicide-resistant crops and recent decisions not to grow glyphosate-resistant sugarbeet and wheat, the introduction and adoption of herbicide-resistant crops during the next 10 years is not likely to be as dramatic as in the past 10 years.

Agriculture↗

PubMed2005

Phytotoxic and antifungal compounds from two Apiaceae species, Lomatium californicum and Ligusticum hultenii, rich sources of Z-ligustilide and apiol, respectively.

The seeds of two Apiaceae species, Ligusticum hultenii and Lomatium californicum, were investigated. Preliminary bioassays indicated that methylene chloride extracts of seeds of both species contained selective phytotoxic activity against monocots and antifungal activity against Colletotrichum fragariae. Active constituents were isolated by bioassay-guided fractionation, and the structures were elucidated by NMR and GC-MS as apiol and Z-ligustilide, isolated from L. hultenii and L. californicum, respectively. Apiol and Z-ligustilide had I50 values of about 80 and 600 microM, respectively, for inhibition of the growth of Lemna paucicostata. The methylene chloride (CH2Cl2) extracts of the seeds and the isolated and purified compounds were tested against the 2-methylisobomeol-producing cyanobacterium (blue-green alga) Oscillatoria perornata, and the green alga Selenastrum capricornutum. The CH2Cl2 extracts of both Apiaceae species and apiol were weakly toxic to both species of phytoplankton, while Z-ligustilide was toxic to both with a lowest complete inhibitory concentration (LCIC) of 53 microM. Seeds of L. californicum and L. hultenii were found to be rich sources of Z-ligustilide (97 mg/g of dry seed) and apiol (40 mg/g of dry seed), respectively.

Antifungal Agents↗

PubMed2004

Aminomethylphosphonic acid, a metabolite of glyphosate, causes injury in glyphosate-treated, glyphosate-resistant soybean.

Glyphosate-resistant (GR) soybean [Glycine max (L.) Merr.] was developed by stable integration of a foreign gene that codes insensitive enzyme 5-enolpyruvylshikimate-3-phosphate synthase, an enzyme in the shikimate pathway, the target pathway of glyphosate. Application of glyphosate to GR soybean results in injury under certain conditions. It was hypothesized that if GR soybean is completely resistant to the glyphosate, injury could be caused by a metabolite of glyphosate, aminomethylphosphonic acid (AMPA), a known phytotoxin. Glyphosate and AMPA effects on one- to two-trifoliolate leaf stage (16-18-days old) GR and non-GR soybean were examined in the greenhouse. In GR soybean, a single application of glyphosate-isopropylammonium (1.12-13.44 kg/ha) with 0.5% Tween 20 did not significantly reduce the chlorophyll content of the second trifoliolate leaf at 7 days after treatment (DAT) or the shoot dry weight at 14 DAT compared with Tween 20 alone. A single application of AMPA (0.12-8.0 kg/ha) with 0.5% Tween 20 reduced the chlorophyll content of the second trifoliolate leaf by 0-52% at 4 DAT and reduced shoot fresh weight by 0-42% at 14 DAT in both GR and non-GR soybeans compared with Tween 20 alone. AMPA at 0.12 and 0.50 kg/ha produced injury in GR and non-GR soybean, respectively, similar to that caused by glyphosate-isopropylammonium at 13.44 kg/ha in GR soybean. AMPA levels found in AMPA-treated soybean of both types and in glyphosate-treated GR soybean correlated similarly with phytotoxicity. These results suggest that soybean injury to GR soybean from glyphosate is due to AMPA formed from glyphosate degradation.

Dose-Response Relationship, Drug↗

PubMed2004

Phytotoxins from the leaves of Ruta graveolens.

Bioassay-guided fractionation of the ethyl acetate extract of Ruta graveolens (common rue) leaves led to the isolation of the furanocoumarins 5-methoxypsoralen (5-MOP), 8-methoxypsoralen (8-MOP), and the quinolone alkaloid graveoline as phytotoxic constituents. Graveoline and 8-MOP substantially inhibited growth of Lactuca sativa (lettuce) seedlings and reduced chlorophyll content at 100 microM; this effect was not due to a direct effect on chlorophyll synthesis. Radical growth of L. sativa was inhibited by 10 microM 8-MOP. Graveoline inhibited growth of Lemna paucicostata (duckweed) at 100 microM. This is the first report of the phytotoxic activity of graveoline. Growth of Agrostis stolonifera (bentgrass) was inhibited by 5-MOP at 30 microM. All three compounds substantially reduced cell division in Allium cepa (onion) at or below 100 microM. None of the compounds caused significant cellular leakage of Cucumis sativus (cucumber) cotyledon disks at 100 microM. All three compounds inhibit plant growth, at least partially through inhibition of cell division.

5-Methoxypsoralen↗

PubMed2004

Molluscicidal activity of vulgarone B against ram's horn snail (Planorbella trivolvis).

The ram's horn snail (Planorbella trivolvis (Say)) is an intermediate host for a digenetic trematode (Bolbophorus confusus (Krause) Dubois) that has recently been discovered to be a significant problem in commercial channel catfish (Ictalurus punctatus Raf) production ponds in the Mississippi Delta region in the USA. In these catfish ponds, the digenetic life cycle of this parasitic trematode involves two intermediate hosts, the ram's horn snail and the channel catfish, and the final host, the American white pelican (Pelecanus erythrorhynchos Gmelin). One approach to eradicate this problem is to disrupt the life cycle of the parasitic trematodes by eliminating the snails. During our search for natural-product-based molluscicides to control the snails in the catfish ponds, vulgarone B, isolated from the steam distillate of the aerial parts of the plant Artemisia douglasiana Besser (Asteraceae), was found to be active towards the snails with a LC50 of ca 24 microM. Channel catfish toxicity studies indicated a LC50 of ca 207 microM. Vulgarone B may be an environmentally acceptable alternative for snail control in aquaculture.

Animals↗

PubMed2004

Structural activity relationship studies of zebra mussel antifouling and antimicrobial agents from verongid sponges.

Several dibromotyramine derivatives including moloka'iamine were selected as potential zebra mussel (Dreissena polymorpha) antifoulants due to the noteworthy absence of fouling observed on sponges of the order Verongida. Sponges of the order Verongida consistently produce these types of bromotyrosine-derived secondary metabolites. Previously reported antifouling data for the barnacle Balanus amphitrite(EC50 = 12.2 microM) support the results reported here that the compound moloka'iamine may be a potential zebra mussel antifoulant compound (EC50 = 10.4 microM). The absence of phytotoxic activity of the compound moloka'iamine toward Lemna pausicostata and, most importantly, the compound's significant selectivity against macrofouling organisms such as zebra mussels suggest the potential utility of this compound as a naturally derived antifoulant lead.

Animals↗

PubMed2004

Somatic mutation-mediated evolution of herbicide resistance in the nonindigenous invasive plant hydrilla (Hydrilla verticillata).

Hydrilla (Hydrilla verticillata L.f. Royle) was introduced to the surface water of Florida in the 1950s and is today one of the most serious aquatic weed problems in the USA. As a result of concerns associated with the applications of pesticides to aquatic systems, fluridone is the only USEPA-approved chemical that provides systemic control of hydrilla. After a decrease in fluridone's efficacy at controlling hydrilla, 200 Florida water bodies were sampled to determine the extent of the problem and the biological basis for the reduced efficacy. Our studies revealed that hydrilla phenotypes with two- to six-fold higher fluridone resistance were present in 20 water bodies. Since fluridone is an inhibitor of the enzyme phytoene desaturase (PDS), the gene for PDS (pds) was cloned from herbicide-susceptible and -resistant hydrilla plants. We report for the first time in higher plants three independent herbicide-resistant hydrilla biotypes arising from the selection of somatic mutations at the arginine 304 codon of pds. The three PDS variants had specific activities similar to the wild-type enzyme but were two to five times less sensitive to fluridone. In vitro activity levels of the enzymes correlated with in vivo resistance of the corresponding biotypes. As hydrilla spread rapidly to lakes across the southern United States in the past, the expansion of resistant biotypes is likely to pose significant environmental challenges in the future.

Base Sequence↗

PubMed2004

Dose-response relationships between herbicides with different modes of action and growth of Lemna paucicostata: an improved ecotoxicological method.

Lemna species are widely used in ecotoxicological research to assess the phytotoxicity of environmental compounds. The purpose of the work described in this paper was to further refine the Lemna spp. bioassay, making it more robust and useful in comparing results between laboratories. A nondestructive image analysis method was used to measure growth as affected by herbicides during a time course. We tested 26 herbicides, with as many as 19 different modes of action, on leaf area growth of Lemna paucicostata to establish complete dose-response relationships. By using 3,5-dichlorophenol as a reference compound, different EC50 (concentration that inhibited growth by 50%) values of the herbicides can be compared. The EC50 values ranged from 0.003 microM for sulcotrione to 407 microM for asulam. Complete dose-response parameters were obtained that will better allow comparison of these results to those of other laboratories. These results should give useful information to anyone who works with Lemna species to address questions on herbicide residues in environmental compartments, screening of new herbicidal compounds, or the assessment of phytotoxic side effects of any compound.

Araceae↗

PubMed2003

Ecophysiological aspects of allelopathy.

Allelochemicals play an important role in explaining plant growth inhibition in interspecies interactions and in structuring the plant community. Five aspects of allelochemicals are discussed from an ecophysiological perspective: (i) biosynthesis, (ii) mode of release, (iii) mode of action, (iv) detoxification and prevention of autotoxicity, and (v) joint action of allelochemicals. A discussion on identifying a compound as an allelochemical is also presented.

Ecology↗

PubMed2003

Natural fungicides from Ruta graveolens L. leaves, including a new quinolone alkaloid.

Bioassay-directed isolation of antifungal compounds from an ethyl acetate extract of Ruta graveolens leaves yielded two furanocoumarins, one quinoline alkaloid, and four quinolone alkaloids, including a novel compound, 1-methyl-2-[6'-(3' ',4' '-methylenedioxyphenyl)hexyl]-4-quinolone. The (1)H and (13)C NMR assignments of the new compound are reported. Antifungal activities of the isolated compounds, together with 7-hydroxycoumarin, 4-hydroxycoumarin, and 7-methoxycoumarin, which are known to occur in Rutaceae species, were evaluated by bioautography and microbioassay. Four of the alkaloids had moderate activity against Colletotrichum species, including a benomyl-resistant C. acutatum. These compounds and the furanocoumarins 5- and 8-methoxypsoralen had moderate activity against Fusarium oxysporum. The novel quinolone alkaloid was highly active against Botrytis cinerea. Phomopsis species were much more sensitive to most of the compounds, with P. viticola being highly sensitive to all of the compounds.

Alkaloids↗

PubMed2003

Isoflavone, glyphosate, and aminomethylphosphonic acid levels in seeds of glyphosate-treated, glyphosate-resistant soybean.

The estrogenic isoflavones of soybeans and their glycosides are products of the shikimate pathway, the target pathway of glyphosate. This study tested the hypothesis that nonphytotoxic levels of glyphosate and other herbicides known to affect phenolic compound biosynthesis might influence levels of these nutraceutical compounds in glyphosate-resistant soybeans. The effects of glyphosate and other herbicides were determined on estrogenic isoflavones and shikimate in glyphosate-resistant soybeans from identical experiments conducted on different cultivars in Mississippi and Missouri. Four commonly used herbicide treatments were compared to a hand-weeded control. The herbicide treatments were (1) glyphosate at 1260 g/ha at 3 weeks after planting (WAP), followed by glyphosate at 840 g/ha at 6 WAP; (2) sulfentrazone at 168 g/ha plus chlorimuron at 34 g/ha applied preemergence (PRE), followed by glyphosate at 1260 g/ha at 6 WAP; (3) sulfentrazone at 168 g/ha plus chlorimuron at 34 g/ha applied PRE, followed by glyphosate at 1260 g/ha at full bloom; and (4) sulfentrazone at 168 g/ha plus chlorimuron at 34 g/ha applied PRE, followed by acifluorfen at 280 g/ha plus bentazon at 560 g/ha plus clethodim at 140 g/ha at 6 WAP. Soybeans were harvested at maturity, and seeds were analyzed for daidzein, daidzin, genistein, genistin, glycitin, glycitein, shikimate, glyphosate, and the glyphosate degradation product, aminomethylphosphonic acid (AMPA). There were no remarkable effects of any treatment on the contents of any of the biosynthetic compounds in soybean seed from either test site, indicating that early and later season applications of glyphosate have no effects on phytoestrogen levels in glyphosate-resistant soybeans. Glyphosate and AMPA residues were higher in seeds from treatment 3 than from the other two treatments in which glyphosate was used earlier. Intermediate levels were found in treatments 1 and 2. Low levels of glyphosate and AMPA were found in treatment 4 and a hand-weeded control, apparently due to herbicide drift.

Drug Resistance↗

PubMed2003

Weeding with transgenes.

Transgenes promise to reduce insecticide and fungicide use but relatively little has been done to significantly reduce herbicide use through genetic engineering. Recently, three strategies for transgene utilization have been developed that have the potential to change this. These are the improvement of weed-specific biocontrol agents, enhancement of crop competition or allelopathic traits, and production of cover crops that will self-destruct near the time of planting. Failsafe risk mitigation technologies are needed for most of these strategies.

Agriculture↗