Search

In a study of 62 plant species of the Cretan flora for their phytotoxic activity, plants were extracted successively with CH2Cl2, MeOH and H2O. Phytotoxicity evaluation of the 240 extracts was performed against Lactuca sativa L. and Agrostis stolonifera L.. The MeOH extract of Bellis longifolia was the most phytotoxic. Bioassay-guided fractionation revealed that a fraction consisting mainly of saponins was the most effective. Separation of the saponins was performed using initially a step-gradient Centrifugal Partition Chromatography (CPC). Investigation of the active fraction led to the isolation and structure elucidation of three previously undescribed triterpene saponins, 3-O-β-D-fucopyranosyl polygalacic acid, 28-O-α-L-rhamnopyranosyl-(1 → 2)-β-D-fucopyranosyl polygalacic acid and 3-O-β-D-fucopyranosyl-2α,3β,23-trihydroxyolean-12-en-28-oic acid, which were present as the main phytotoxic compounds of the methanol extract. Two triterpenes, polygalacic acid and bellisonic acid and four kaempferol glucosides, as well as chlorogenic acid were also isolated. 3-O-β-D-fucopyranosyl polygalacic acid and 3-O-β-D-fucopyranosyl-2α,3β,23-trihydroxyolean-12-en-28-oic acid had phytotoxic activity similar to some commercial herbicides (IC50 values of ca. 25 μM) against duckweed (Lemna paucicostata).

Camphorweed, Heterotheca subaxillaris (Lam.) Britt. & Rusby, has a camphor-like odor, and its leaf surfaces contain glandular trichomes of the type shown to contain high levels of isoprenoids in other species. Borneol (1), the phytotoxic calamenene-type sesquiterpenes (2-5, 9-11), and methylated flavones (12-15) were isolated from the dichloromethane rinsate of camphorweed aerial tissues. The strongest plant growth inhibitor against Agrostis stolonifera and Lactuca sativa seedlings, as well as duckweed (Lemna pausicostata), was 2-methoxy-calamenene-14-carboxylic acid (2). Esterification of calamenene carboxylic acids decreased their biological activity.

The essential oil from the leaves of Callicarpa japonica was analyzed by GC-MS, and 84 compounds were identified. The main constituents of the essential oil were spathulenol (18.1%), germacrene B (13.0%), bicyclogermacrene (11.0%), globulol (3.3%), viridiflorol (2.6%), alpha-guaiene (2.3%), and gamma-elemene (2.0%). The essential oil constituents of C. japonica were significantly different from those found in our previous work on Callicarpa americana. The oil of C. japonica was selectively phytotoxic to bentgrass compared to lettuce seeds, with 80-100% growth reduction observed at 0.3 mg/ml.

The relative abundance of steam-distillable isoprenoids and other volatile compounds in leaves of glanded and glandless Artemisia annua L. was investigated. Steam distillation of leaves bearing glandular trichomes yielded 0.24% essential oil on a fresh weight basis. Monoterpenes were predominant in this essential oil, of which α -pinene (26.7%), pinocarvone (15.8%), and artemisia ketone (11.0%) were the major constituents. The essential oil distilled from glandless leaves amounted to 0.06% of fresh weight, and consisted mostly of sesquiterpenes, of which germacrene- D (49.8%) and β -caryophyllene (25.1%) were the major components. Only one monoterpene, β -ocimene, present in trace amounts, was detected in the oil distilled from the glandless tissue. The sesquiterpene artemisinin, found only in the glanded biotype, is heat-labile and cannot be extracted by steam distillation, during which it decomposes into non-distillable products.

Dehydrozaluzanin C, a natural sesquiterpene lactone, is a weak plant growth inhibitor with an I50 of about 0.5 mM for lettuce root growth. It also causes rapid plasma membrane leakage in cucumber cotyledon discs. Dehydrozaluzanin C is more active at 50 μM than the same concentration of the herbicide acifluorfen. Symptoms include plasmolysis and the disruption of membrane integrity is not light dependent. Reversal of its effects on root growth was obtained with treatment by various amino acids, with histidine and glycine providing ca. 40% reversion. The strong reversal effect obtained with reduced glutathione is due to cross-reactivity with DHZ and the formation of mono- and di-adducts. Photosynthetic, respiratory and mitotic processes, as well as NADH oxidase activity appear to be unaffected by this compound. Our results indicate that dehydrozaluzanin C exerts its effects on plants through two different mechanisms, only one of which is related to the disruption of plasma membrane function.

AAL-toxins and fumonisin B 1 are phytotoxic to susceptible plants by inhibiting the enzyme ceramide synthase. Australifungin, which is structurally unrelated to these toxins, inhibits the same enzyme in the sphingolipid biosynthesis pathway of animals. In duckweed ( Lemna pausicostata ) cultures, 5 μM australifungin caused accumulation of the sphingolipid precursors, phytospingosine and sphinganine, although less so than AAL-toxin T A or FB 1 at 1 μM. Phytosphingosine and sphinganine began to accumulate after 12 h, followed by increased electrolyte leakage at 24 h. Electrolyte leakage with 5 μM australifungin was somewhat less than 1 μM fumonisin B 1 or AAL-toxin T A . Morphological effects were not identical; fumonisin B 1 and AAL-toxin T A mainly caused bleaching, while australifungin caused clumping of duckweed fronds. Our study shows that australifungin inhibits sphingolipid synthesis in plants, but is about eight times less potent than AAL-toxin T A or fumonisin B 1 .

Fumonisins A1, A2, B2, B2, and B3 are a series of secondary metabolite analogues produced by Fusarium moniliforme. The A series fumonisins differ from B series by possession of a N terminal acetyl group. Hydrolytic removal of the propanetricarboxylic acid moieties from fumonisins B1, and B2 yields the aminoalcohols HB1, and HB2, respectively. AAL-toxin is a chemically related phytotoxin (only one propanetricarboxylic acid) produced by Alternaria alternata. In a duckweed (Lemna pausicostata) bioassay, AAL-toxin and the B series fumonisins at 1,μM caused pronounced cellular leakage of electrolytes and photodegradation of chlorophylls. These four compounds also caused the most marked reductions in duckweed growth. HB1, at 1 μM was a moderate growth inhibitor and caused a low level of cellular leakage. The other compounds were inactive at this concentration. Significant effects on cellular leakage were measured at 0.04 μM for both AAL-toxin and fumonisin B1, the two most active analogues. The propanetricarboxylic acid groups of fumonisins and AAL-toxin are necessary for significant herbicidal activity of this series of compounds, whereas acetylation of the terminal amine group greatly reduces their activity. The structurally related sphingolipids, phytosphingosine and sphingosine, were about two orders of magnitude less phytotoxic than fumonisin B1; however, the phytotoxicity symptoms were similar.

Sorgoleone, a major component of the hydrophobic root exudate of sorghum [Sorghum bicolor (L.) Moench], is one of the most studied allelochemicals. The exudate also contains an equivalent amount of a lipid resorcinol analog as well as a number of minor sorgoleone congeners. Synthesis of sorgoleone is constitutive and compartmentalized within root hairs, which can accumulate up to 20 microg of exudate/mg root dry weight. The biosynthesis pathway involves unique fatty acid desaturases which produce an atypical 16:3 fatty acyl-CoA starter unit for an alkylresorcinol synthase that catalyzes the formation of a pentadecatrienylresorcinol intermediate. This intermediate is then methylated by SAM-dependent O-methyltransferases and dihydroxylated by cytochrome P450 monooxygenases. An EST data set derived from a S. bicolor root hair-specific cDNA library contained all the candidate sequences potentially encoding enzymes involved in the sorgoleone biosynthetic pathway. Sorgoleone interferes with several molecular target sites, including inhibition of photosynthesis in germinating seedlings. Sorgoleone is not translocated acropetally in older plants, but can be absorbed through the hypocotyl and cotyledonary tissues. Therefore, the mode of action of sorgoleone may be the result of inhibition of photosynthesis in young seedlings in concert with inhibition of its other molecular target sites in older plants. Due to its hydrophobic nature, sorgoleone is strongly sorbed in soil which increases its persistence, but experiments show that it is mineralized by microorganisms over time.

A variety of terrestrial and aquatic plant species were tested for the presence of polyphenol oxidase (PPO), using three different assay protocols. All aquatic green algae (Chlorella, Stigeoclonium, Microspora, Ulva and Spirogyra) tested have no detectable PPO except for a Trebouxioid alga (lichen symbiont), and Coleochaete. Of the Charalean algae Chara and Nitella, only the latter had measurable activity. Some hepatics (Conocephalum and Marchantia) possess the enzyme, but two species of Riccia tested do not. The anthocerotes Phaeoceros, Anthoceros and Notothylas have low, but easily detectable PPO. The mosses, Dicranum, Sphagnum and Thuidium, have no detectable activity. All of the ferns and fern allies tested have PPO activity. Among seed plants, PPO is undetectable in Ginkgo or any of the conifers tested. All but two species of the angiosperms surveyed possessed the enzyme. The Mr as determined by semi-native electrophoretic mobility, ranged widely from 30 000 to 200 000, depending upon species. All species with PPO activity measured by spectrophotometric or gel assay had cytochemically-detected activity localized to the thylakoid membranes, with cytochemical product accumulating in the lumen of the thylakoid.

p-Hydroxyphenylpyruvate dioxygenase (HPPD) is a key enzyme in tyrosine catabolism and is the molecular target site of beta-triketone pharmacophores used to treat hypertyrosinemia in humans. In plants, HPPD is involved in the biosynthesis of prenyl quinones and tocopherols, and is the target site of beta-triketone herbicides. The beta-triketone-rich essential oil of manuka (Leptospermum scoparium), and its components leptospermone, grandiflorone and flavesone were tested for their activity in whole-plant bioassays and for their potency against HPPD. The achlorophyllous phenotype of developing plants exposed to manuka oil or its purified beta-triketone components was similar to that of plants exposed to the synthetic HPPD inhibitor sulcotrione. The triketone-rich fraction and leptospermone were approximatively 10 times more active than that of the crude manuka oil, with I50 values of 1.45, 0.96 and 11.5 microg mL(-1), respectively. The effect of these samples on carotenoid levels was similar. Unlike their synthetic counterpart, steady-state O2 consumption experiments revealed that the natural triketones were competitive reversible inhibitors of HPPD. Dose-response curves against the enzyme activity of HPPD provided apparent I50 values 15.0, 4.02, 3.14, 0.22 microg mL(-1) for manuka oil, triketone-rich fraction, leptospermone and grandiflorone, respectively. Flavesone was not active. Structure-activity relationships indicate that the size and lipophilicity of the side-chain affected the potency of the compounds. Computational analysis of the catalytic domain of HPPD indicates that a lipophilic domain proximate from the Fe2+ favors the binding of ligands with lipophilic moieties.

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.

The inhibitory activity of 34 natural products of various structural classes on hydroxyphenylpyruvate dioxygenase (HPPD), the target site for triketone herbicides, and the mode of interaction of selected natural products were investigated. Recombinant HPPD from arabidopsis is sensitive to several classes of natural compounds including, in decreasing order of sensitivity, triketones, benzoquinones, naphthoquinones and anthraquinones. The triketone natural products acted as competitive tight-binding inhibitors, whereas the benzoquinones and naphthoquinones did not appear to bind tightly to HPPD. While these natural products may not have optimal structural features required for in vivo herbicidal activity, the differences in their kinetic behavior suggest that novel classes of HPPD inhibitors may be developed based on their structural backbones.

The effects of sublethal treatments of the herbicide glyphosate [ N -(phosphonomethyl)glycine] on the accumulation of shikimate, hydroxybenzoic acids, and hydroxycinnamic acids in Abutilon theophrasti (velvetleaf) were measured. In newly developing tissues (both leaf and flower), the concentrations of shikimate and protocatechuate were greatly increased by the herbicide, whereas hydroxybenzaldehyde and p -coumarate levels were decreased. During recovery from glyphosate treatment there was a rapid loss of protocatechuate and the levels of other phenolic acids rapidly returned to normal. These indicate that phenolic acid pools in higher plants are regulated and that simple phenolic acids do not accumulate as end products.

Phytochrome was spectrophotometrically determined to be differentially concentrated among separated tissues of dark-grown, norflurazon-treated barley l