Natural metabolites for parasitic weed management

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ReviewReceived: 5 August 2008 Revised: 29 October 2008 Accepted: 29 October 2008 Published online in Wiley Interscience: 05 March 2009

(www.interscience.wiley.com) DOI 10.1002/ps.1742

Natural metabolites for parasitic weedmanagementMaurizio Vurro,a∗ Angela Boari,a Antonio Evidente,b Anna Andolfib andNadjia Zermanec

Abstract

Compounds of natural origin, such as phytotoxins produced by fungi or natural amino acids, could be used in parasitic weedmanagement strategies by interfering with the early growth stages of the parasites. These metabolites could inhibit seedgermination or germ tube elongation, so preventing attachment to the host plant, or, conversely, stimulate seed germinationin the absence of the host, contributing to a reduction in the parasite seed bank. Some of the fungal metabolites assayed werevery active even at very low concentrations, such as some macrocyclic trichothecenes, which at 0.1 µM strongly suppressed thegermination of Orobanche ramosa L. seeds. Interesting results were also obtained with some novel toxins, such as phyllostictineA, highly active in reducing germ tube elongation and seed germination both of O. ramosa and of Cuscuta campestris Yuncker.Among the amino acids tested, methionine and arginine were particularly interesting, as they were able to suppress seedgermination at concentrations lower than 1 mM. Some of the fungal metabolites tested were also able to stimulate thegermination of O. ramosa seeds. The major findings in this research field are described and discussed.c© 2009 Society of Chemical Industry

Keywords: fungal metabolites; toxins; amino acids; parasitic weeds; biological weed control

1 INTRODUCTIONDifficulties in controlling parasitic weeds are due to theirphysiological traits and life cycle.1,2 Preventing early growthstages, such as seed germination, host attachment and tubercledevelopment, could be a strategy to interfere successfully withthe parasite, resulting in its management. Natural compoundsthat inhibit or stimulate seed germination could be attractive andenvironmentally friendly tools to reach that objective.

Microbial organisms are an enormous source of partially ex-plored bioactive metabolites. Toxins produced by plant pathogensare of utmost interest because they are produced duringhost–pathogen interactions. Phytotoxins from fungal pathogensof crops have received considerable attention in helping to un-derstand the development of diseases and consequently to setup strategies for disease control. Toxic metabolites producedby fungal pathogens can belong to different chemical families,have different ecological and environmental roles, have differentbehaviours with respect to the host and act with different mecha-nisms. Several studies have also considered the possibility of usingsuch metabolites as tools in biological and integrated weed man-agement, e.g. as novel and environmentally friendly herbicides, asbiomarkers for the selection of more efficacious biocontrol agents,as templates for novel compounds or as sources of unknownmechanisms of action.3 – 6

Many compounds can cause macroscopic effects such aschlorosis, necrosis, reduction of rootlet development, wilting andinhibitory effects on seed germination, but only a few studies havesuggested the possibility of using these metabolites to preventgermination of seeds of parasitic weeds.

Certain amino acids applied in millimolar amounts can inhibitplants. This inhibition can be attributed to malregulation of

enzymes in a biosynthetic pathway. This type of malregulation canoften be reversed by addition of the other amino acid end-productsof the biosynthetic pathway. For example, lysine, threonine andmethionine belong to the same biosynthetic pathway, and theabundance of lysine and threonine shuts down the entire pathwayat the top, leading to methionine starvation.7 However, notall plants and not all tissues within a plant regulate a givenbiosynthetic pathway with the same end-product. Certain aminoacids will inhibit one plant species and actually stimulate another.No studies have been carried out on the effect of amino acids onparasitic weeds.

This paper takes stock of the situation, describing previousand more recent results achieved in this field of research, mainlyregarding the target Orobanche ramosa L., and to a lesser extentStriga hermonthica (Del.) Benth. and Cuscuta campestris Yuncker.Attention was focused on: (a) fungal toxins that inhibit seedgermination; (b) amino acids inhibiting seed germination andtubercle development; (c) fungal metabolites stimulating ‘suicidal’germination.

∗ Correspondence to: Maurizio Vurro, Istituto di Scienze delle ProduzioniAlimentari, CNR, via Amendola 122/O, 70125 Bari, Italy.E-mail: [email protected]

a Istituto di Scienze delle Produzioni Alimentari, CNR, via Amendola 122/O, 70125Bari, Italy

b Dipartimento di Scienze del Suolo, della Pianta, dell’Ambiente e delle ProduzioniAnimali, Universita di Napoli Federico II, Via Universita 100, 80055 Portici, Italy

c Departement de Botanique, Institut National Agronomique (INA), El-Harrach16200 Alger, Algeria

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Natural metabolites for parasitic weed management www.soci.org

2 FUNGAL TOXINS INHIBITING SEED GERMI-NATIONIn an early study, 14 fungal toxins were assayed on S. hermonthicaseeds.8 Two of the toxins tested, T2 toxin and deoxynivalenol,completely inhibited seed germination when applied at 0.1 mM.Other toxins, e.g. cytochalasin E, tenuazonic acid or enniatin, weresomewhat active at that concentration, inhibiting the germinationof about 50% of the seeds, whereas a third group of compounds,among which were cytochalasin B and fusapyrone, had negligibleor no effects. The effect of these metabolites on parasitic plantseeds is unrelated to their previously reported toxicities. In fact,some of them (e.g. tenuazonic acid) that are not consideredharmful mycotoxins had the same effect as other toxins (e.g.nivalenol) considered highly toxic to mammals. This supportedthe hope of finding safe and highly active fungal metabolites, andof using them to suppress seed germination of parasitic weeds.Several metabolites were later assayed to O. ramosa, chosen usingdifferent approaches: (1) already known microbial compoundsproduced by Fusarium species and available commercially orfor laboratories; (2) compounds obtained from the cultures ofpotential biocontrol agents of O. ramosa; (3) novel phytotoxinsrecently isolated and chemically characterised in the authors’

laboratories from the cultures of fungal pathogens of weeds otherthan parasitic ones.

Eighteen already known toxins produced by different Fusariumspp. were used, twelve of which were commercially available, andthe others available from laboratories.9 A total of 53 pathogenicfungal isolates from diseased O.ramosa plants taken in field surveyscarried out in Southern Italy10 yielded one strain of Myrotheciumverrucaria (Alb. & Schwein) Ditmar and one strain of Fusariumcompactum (Wollenw.) able to produce metabolites activelyinhibiting O. ramosa germination.11 Eight metabolites wereisolated from M. verrucaria culture extracts: the main metabolitewas identified as verrucarin E, a disubstituted pyrrole, whereasthe other seven were macrocyclic trichothecenes: verrucarins A, B,M and L acetate, roridin A, isotrichoverrin B and trichoverrol B.12

The main metabolite produced by F. compactum was identified asneosolaniol monoacetate, another well-known trichothecene.12

Liquid cultures of weed pathogens yielded more toxins.Phyllostictine A (Fig. 1) was purified and chemically characterisedas a new oxazatricycloalkenone from Phyllosticta cirsii (Desm.),a potential mycoherbicide for the biological control of Cirsiumarvense (L.) Scop.13 This toxin, isolated together with three otherrelated metabolites (named phyllostictines B to D), was highlyphytotoxic on leaves at 6 mM, had no antifungal activity and had

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Figure 1. Chemical structures of fusicoccin, ophiobolin A and phyllostictine A.

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antibiotic and zootoxic activity only when tested at concentrationshigher than 1 mM. Drechslera gigantea (Heald & FA Wolf), apotential mycoherbicide of grass weeds, produced four phytotoxicmetabolites in liquid culture. The main metabolite was identifiedas ophiobolin A, a phytotoxic sesterterpene (Fig. 1).14 The resultsobtained for all these metabolites in the different series ofexperiments are summarised in Table 1.

Seven out of the 18 already known toxins assayed caused 100%inhibition of seed germination when tested at 100 µM (Table 1). Allthe others were toxic at different levels when assayed at 100 µM,ranging from 63% germination (moniliformin) to almost inactive(92% germination in the case of enniatin). At 10 µM the seven mostactive metabolites were still highly active, causing the completeinhibition of germination, whereas the other compounds hadnegligible or no effects (Table 1). The strongest toxins, T-2, HT-2, neosolaniol and diacetoxyscirpenol, caused almost completeinhibition of seed germination at 1 µM (data not shown). Some ofthe latter were still very active at 0.1 µM. The active compounds

Table 1. Effect of fungal toxins on the germination of Orobancheramosa seedsa

Inhibition of seedgerminationb

Toxin Source 10 µM 100 µM

Beauvericin Fusarium proliferatum −7 20

Dehydrofusaric acid Fusarium nygamai −3 27

Dehydrofusaric acidmethyl ester

F. nygamai 0 18

Deoxyfusapyrone Fusarium semitectum 5 –

Deoxynivalenol Fusarium sp. 100 100

Diacetoxyscirpenol Fusarium sambucinum 100 100

Enniatin Fusarium sp. 4 8

Fusapyrone F. semitectum 4 –

Fusarenon X Fusarium nivale 100 100

Fusaric acid F. nygamai −1 27

Fusaric acid methyl ester F. nygamai −3 31

HT-2 toxin Fusarium sp. 100 100

Isotrichoverrin B Myrothecium verrucaria 66 100

Moniliformin F. proliferatum 5 37

Neosolaniol Fusarium sp. 100 100

Neosolaniolmonoacetate

Fusarium compactum 100 100

Nivalenol F. nivale 100 100

Ophiobolin Ac Drechslera gigantea 8 100

Phyllostictine Ac Phyllosticta cirsii 5 28

Roridin A Myrothecium verrucaria 100 100

T-2 toxin Fusarium sp. 100 100

Trichoverrol B M. verrucaria 19 100

Verrucarin A M. verrucaria 100 100

Verrucarin B M. verrucaria 100 100

Verrucarin E M. verrucaria – 3

Verrucarin L acetate M. verrucaria 100 100

Verrucarin M M. verrucaria 100 100

Zearalenol Fusarium sp. −3 12

Zearalenone Fusarium sp. 2 19

a Summarises data previously given in Refs 9 and 12.b The in vitro germination assay is described in Ref. 6. Data showreduction in germination, expressed as percentage of control.c Previously unreported data.

caused complete failure of germination, suggesting that they aretrue inhibitors. This was further supported by the observation thattoxin removal by seed washing did not result in germination (datanot shown).

Of the compounds isolated from pathogens of parasitic weeds(Table 1), all the metabolites caused the total inhibition ofstimulated germination at 100 µM, except verrucarin E, whichproved to be inactive. The trichothecenes, except isotrichoverrinB, were still highly active at 10 µM, causing total inhibition of seedgermination. Many of the metabolites were still active at 1 µM,except verrucarin M, which proved to be almost inactive at thatconcentration. Roridin A and neosolaniol monoacetate were bothable to cause 100% inhibition of seed germination at 1 µM (datanot shown).

Ophiobolin A was very efficacious, causing 100% inhibitionwhen tested at 100 µM (Table 1). Its efficacy was much lower at10 µM (around 10%), and nil at even lower concentrations (datanot shown). Phyllostictine A caused around 28% inhibition ofgermination at 100 µM (Table 1), whereas at lower concentrationsits effect was almost negligible or nil. This is the first report ofthe toxic effect of these two compounds on the germination of O.ramosa seeds, and confirms the possibility of finding interestingand active compounds with herbicidal properties by studyingpathogens of other weeds.

Some of the most active toxins belong to the trichothecenegroup, a well-known group of mammalian mycotoxins. Thus, thecommon chemical structure of these compounds seems to playan important role in blocking the germination of stimulated seeds.The trichothecenes are a family of tetracyclic sesquiterpenoidsproduced by several species of Fusarium. They cause a wide varietyof biological effects owing to the diversity of chemical structureswithin the group. They are all potent inhibitors of protein synthesisin eucaryotic cells.

Verrucarins A, B, M and L acetate belong to a subgroup ofmacrocyclic trichothecenes having a differently functionalisedlactone ring located between C-4 and C-15. This macrocycle wassubstantially different in roridin A and isotrichoverrin B, and alsoopen in trichoverrol B, compounds that belong to two othersubgroups of the macrocyclic trichothecene family.

The results in Table 1 are in agreement with a previous studyusing 14 fungal toxins on germination of seeds of S. hermonthica.8

In fact, T-2 and deoxynivalenol were also the most active at very lowconcentrations against Striga. Enniatin and beuvericin were slightlymore active against Striga, whereas nivalenol was more activeagainst Orobanche. The activities of fusaric and dehydrofusaricacid, as well as of their respective methyl esters, were muchstronger against Striga than against Orobanche (data not shown).

Ten phytotoxins were tested against C. campestris (dodder)at concentrations below 1 mM: phyllostictine A, ophiobolin A,fusicoccin (FC) and seven of its derivatives (Table 2). In general,the phytotoxins tested had no inhibiting effect on dodder seedgermination except for dideacetyl-FC (DAF), which significantlyreduced the percentage of seed germination compared withthe control (around 28% reduction). Four toxins, ophiobolin A,fusicoccin and two of its derivatives, greatly inhibited growth ofthe parasite seedlings by 94, 82, 77 and 33% respectively (Table 2).Assayed at tenfold lower concentration, Ophiobolin A was stillvery active, causing almost total inhibition of the seedling growth,whereas at 100-fold dilution it had a lesser effect (data not shown).At lower concentrations, fusicoccin and its two derivatives didnot show any toxicity (data not shown). Besides their effect inreducing the seedling length of the parasite, the three toxins

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Table 2. Effect of some phytotoxins on Cuscuta campestris seed germination and seedling growtha

Treatments Concentration (µM) Seed germination (%) Seedling lengthb (mm) Healthy seedlings (%)

Control (water) – 72.3a 57.4b 100 a

Fusicoccin (FC) 820 71.3a 10.2d 1.0c

Dideacetyl-FC 840 51.7b 12.9d 0.0c

Isomer of 16-O-demethyl-de-tert-pentenylhexaacetyl-FC

460 69.3ab 63.9a 100a

Isomer of 16-O-demethyl-de-tert-pentenylpentaacetyl-FC

290 59.0ab 65.0a 100a

8,9-Isopropylidene FC-aglycone 270 77.3a 65.6a 100a

Triacetyl-8-oxo-FC 940 65.3ab 61.8a 100a

D2,6-1,22,23-Trihydro-16-O-demethyl-FC 260 66.0ab 37.6c 98.3 a

19-Trityl-12-oxo-8,9-isopropylidene FC-aglycone 310 67.7ab 64.8a 100a

Ophiobolin A 1525 62.3ab 3.5e 7.7b

Phyllostictine A 800 62.3ab 56.9b 100a

a Values within each column followed by different letters are significantly different from the non-inoculated control according to the Duncan test atP ≤ 0.05. Values are the mean of one experiment with three replicates.b Mean of ten seedlings per replicate for three replicates.

ophiobolin A, fusicoccin and DAF caused necrosis and browningof almost all the treated seedlings, leading to their death. To theauthors’ best knowledge, this is the first report on the toxicityof fungal compounds on dodder seedlings. Conversely, otherputative phytotoxins slightly stimulated the growth of dodderseedlings (Table 2).

From a practical point of view, inhibition of the elongation of thegerm tube can be as important as inhibition of the germination.Once the seed has started the germination process, if the germtube is not able to elongate sufficiently, then the attachment andthe successive damaging phases can be prevented, and the cropprotected.

3 AMINO ACIDS AS SAFE HERBICIDESThis approach was based on the mechanism of action ofthe so-called ‘frenching disease’, a physiological disorder oftobacco caused by saprophytic bacteria growing on the rootsand overproducing isoleucine, which is described and discussedseparately.15 Briefly, the idea was to supply high or unbalancedamounts of essential amino acids to cause disorders in themetabolic processes occurring during the germination of seeds,then inhibiting the germination or the germ tube elongation.These are the same final results as with some chemical herbicidesthat inhibit single enzymes in plants (such as ALS), making treatedplants incapable of producing metabolites essential for plantgrowth.

In early preliminary experiments,16 the L forms of 13 aminoacids were assayed at 2 mM to evaluate their ability to inhibitthe germination of seeds of parasitic plants. Orobanche ramosaseeds were particularly sensitive to methionine, arginine, prolineand histidine, whereas the seeds of S. hermonthica were moresensitive to leucine, threonine and tyrosine. More specifically,proline, arginine and histidine completely inhibited O. ramosaseed germination. Glycine, methionine, alanine and lysine werealso very active, with an inhibitory activity ranging between 70and 90%.16 The other amino acids (i.e. valine, serine, glutamine,leucine, cysteine and threonine) had low or no effects.

Attention was focused on two amino acids as models:methionine and arginine. Methionine was chosen because, besides

its high efficacy, at lower doses it caused an interesting shorteningof the germination tubes and their enlargement and deformation.Arginine was chosen because it was one of the most activecompounds, and because it is one of the commonest, cheapestand least hazardous amino acids.

A good dose–response relationship was achieved with arginine,with a regression curve y = 19.9 − 23.6 × ln(x), P < 0.01, andwith an ED50 (dose inhibiting 50% of seed germination) of 0.55 mM

(Boari A and Vurro M, unpublished).Seed germination occurs 3 days after the application of the

stimulant. Thus, several attempts were made to understand theeffect of the timing of the application of methionine with respectto the timing of the application of the germination stimulant. Therewas no germination in any of the treatments, regardless of whethermethionine was applied before, simultaneously or after stimulantsupplement. A lower inhibition (19% germination) was observableonly when the amino acid was applied 3 days after stimulation,when the germ tube elongation had already started. A higherpercentage of germination (52%) was observed when methioninewas applied 3 days after the stimulant but then washed out afterone further day.16 In both cases, clear effects on size and shapeof germ tube were evident. If methionine was rinsed away beforestimulant application, it was unable to interfere with germinationor elongation when supplied 1 or 2 days before the beginning ofthese processes.

The possibility of increasing the efficacy of amino acids or, on thecontrary, suppressing their inhibitory effect, was also investigatedin preliminary tests using combinations of two different aminoacids, i.e. methionine with threonine, two amino acids belongingto the same biosynthetic pathway, and lysine with proline, fromdifferent biosynthetic pathways. When applied jointly, both theapplication of methionine with threonine and in particular theapplication of lysine with proline were more active in inhibitingseed germination compared with the effects caused by the singleapplication of each amino acid.16

Methionine was also tested in vivo against O. ramosa bygrowing tomato seedlings in the presence of O. ramosa seeds,the germination and tubercle development of which were thenstimulated by the host roots, and adding methionine solutions (2or 10 mM) at 2 week intervals up to 3 times. All the treatments


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(1–3 applications) with the methionine solution at the highestconcentration (10 mM) strongly inhibited tubercle development.The application of solution at a lower concentration (2 mM) causeda significant effect when the solution was applied 2 or 3 times,16

whereas it was not detectable if the solution was applied only once.The inhibitory effect of certain amino acids can be attributed to

malregulation of enzymes in the biosynthetic pathway.7 In plants,several such amino acid biosynthetic pathways with multipleproducts are regulated by single end-products. Interestingly, notall plants and not all tissues within a plant regulate a givenbiosynthetic pathway with the same end-product. Certain aminoacids will inhibit one plant species and actually stimulate another

4 FUNGAL METABOLITES FOR ‘SUICIDAL’GERMINATIONFusicoccin (FC) (Fig. 1), the major toxin of Fusicoccum amygdali(Delacr.),17 the causative fungal agent of peach and almond canker,is able, together with the structurally related compound cotylenol,to induce seed germination of S. hermonthica and Orobanche mi-nor Smith.18 A structure–activity study was carried out, testing 25compounds at 10 and 100 µM: fusicoccin, its aglycone, several fu-sicoccin derivatives and natural analogues and cotylenol.19 Somenatural fusicoccin analogues and derivatives were more activethan fusicoccin, and the stimulatory properties were modulatedby chemical modifications, essentially in the functionalities and/orthe conformation of the carbotricyclic diterpenoid ring. Among theglucosides, the most active compound was dideacetyl-fusicoccin,which could be of practical interest because it can be easilyprepared and in high yields by fusicoccin. The importance ofthe presence of a free primary hydroxy group at C-19 was evi-dent. Some fusicoccin glucosides, with acetylation of all hydroxygroups and other significant modifications of functionalities andconformation of the carbotricyclic ring, had decreased stimulantactivity. The same structure–activity relationships were observedtesting the fusicoccin aglycones. Among these, the most active(58%) proved to be the isopropylidene derivative, followed by thefusicoccin aglycone (16%). In fact, both compounds have a freehydroxy group at C-19. The most important feature imparting stim-ulation of the germination O. ramosa seeds in fusicoccin-relatedcompounds seems to be the presence of the hydroxy group onC-19, both in the fusicoccin glucosides and in the fusicoccin agly-cones. The alteration of the functionalities and conformation ofthe carbotricyclic ring induces a further diminution in activity.

5 DISCUSSIONThe possibility of using fungal toxins as natural herbicides to inhibitgermination of parasitic plant seeds seems not to be so remote, assome toxins are effective at very low concentrations. Many toxinsare not selective, as they are able to cause the same toxic effectsboth on host and on non-host plants. For this reason, the toxicityto crop plants has to be ascertained. In any case, their applicationat a very low concentration when the host plants are already fullydeveloped and their quick degradation after inhibition of seedgermination should avoid any toxic effect.

Most of the crop plants parasitised by broomrape are irrigated,and toxins or amino acids could be introduced by drip irrigationsystems at very low amounts near the host roots. This could preventthe germination of the seeds only where needed, preventingattachment of haustoria to the host roots. This should minimise

environmental risks, reducing the amount of toxins to be appliedand avoiding toxin dispersal.

Several approaches are possible for applying amino acids,besides their possible direct application to the soil. Beingprimary metabolites, they could easily be bioinactivated by soilmicroorganisms soon after their application. A further approachcould be to coat or pelletise crop seeds or drench croptransplant roots with sufficient amino acids to inhibit Orobanchegermination and infection. Even in these cases, they probablycould be degraded by rhizosphere organisms. Various aminoacids negatively affect both size and shape of the germ tube.This shortening effect could further protect the host plant fromthe parasite, because the germinating seeds would be lessable to reach the root of the host. Another approach wouldbe to select or engineer crop plants that overproduce aminoacids in the root zone. Selecting rhizosphere microflora thatoverproduce and excrete the amino acid might be a morefeasible approach. These amino-acid-overproducing bacteria orfungi would be coated onto the seed or the transplant priorto planting. Another possible approach would be to inoculatecrop seeds with an amino acid or toxin-excreting pathogen ofOrobanche. Several strains of Fusarium spp. were isolated andproposed as biological control agents against broomrapes.20 Ifthese pathogens saprophytically grew along the root systemof crop plants, then they would attack and eventually kill anybroomrape seedlings attempting to attach and penetrate thecrop plant root. Selection of an amino-acid/toxin-overproducingand -excreting strain of Fusarium should make the pathogenmore effective than the wild strains at controlling broomrapeinfection, and even more attractive commercially.15 Some fungalmetabolites were effective in stimulating the germination ofparasitic weed seeds, so supporting the idea of their use toinduce the ‘suicidal’ germination of parasitic plant seeds, in theabsence of the host, in order to reduce the seed bank and allowlong-term management.

There are many other aspects that would need to be clarifiedin order to make the use of natural metabolites a more practicaland exploitable approach. For example, although some of thetoxins tested have been known for a long time, their mechanismsof action for preventing germination and germ tube elongationof parasitic weeds are still completely unknown. These aspectswould be particularly interesting in the case of novel toxins,such as phyllostictine A, the biological properties of which havestill only been partially investigated. Another open question istheir fate after their release into the environment, and into thesoil in particular. From a practical point of view, an extremelyfast degradation would render the metabolites more attractiveas environmentally friendly compounds, but also would makethe application timing much more influential as regards thepossibility of control. The modality of application would be anotherimportant aspect to consider. The use of drip irrigation or seedcoating, for example, would be of utmost importance both indelivering the compounds to the right site and in reducingthe costs of application and the amount of product necessaryfor the treatments. This aspect would be strictly related to thedose–response relationships, which can be determined only withtreatments in real field conditions. One of the limiting factorsin using fungal metabolites at the field level or carrying outmore extensive studies of their potential is that very often thesecompounds are produced by microorganisms in low quantities,or are not easily purified. Scaled-up fermentation and purificationprocesses could allow higher production of toxins and reduce

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the costs of their production, making their use more feasible.Moreover, study of structure–activity relationships of the noveltoxins would allow identification of the active sites of the moleculesto modify their structures, changing their chemical or physicalproperties.

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13 Evidente A, Cimmino A, Andolfi A, Vurro M, Zonno MC andMotta A, Phyllostictines A–D, oxazatricycloalkenones produced byPhyllosticta cirsii, a potential mycoherbicide for Cirsium arvensebiocontrol. Tetrahedron 64:1612–1617 (2008).

14 Evidente A, Andolfi A, Cimmino A, Vurro M, Fracchiolla M andCharudattan R, Herbicidal potential of Ophiobolins produced byDrechslera gigantea. J Agric Food Chem 54:1779–1783 (2006).

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Natural metabolites for parasitic weed management