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II. Production and Use of Biological Weed Control Agents

II. Production and Use of Biological Weed Control Agents

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Table I

Chemical Names of Herbicides and Other Modifiers Mentioned by Common Name in the Text



Common name

Acifluorfen

Alachlor

Amitrole

Atrazine

Benomyl

Bentazon

Benzadox

Bialophos

Butylate

Chloramben

Chlorofenprop-methyl

Chlorpropham

Chlorsulfuron

Cinmethylin

Dalapon

Dietholate

Diphenamid

Diuron



EPTC



Fluazifop

Fluorodifen

Flurazole

Glufosinate

Glyphosate

Imazaquin



MCPA



Mecoprop

Methoxyphenone

Metolachlor

Metribuzin

Molinate

Monuron

Paraquat

Phenmedipham

Picloram

Propanil

Pyrazon

Quinclorac

Sulfometuron

Terbutryn

Tridiphane

Trifluralin

2,4-D

2,4,5-T



Chemical name

5-[2-Chloro~-(trifluoromethyl)phenoxy]-2-nitrobenzoic

acid

2-Chloro-N-(2,6-diethylphenyl)-N-(methox~ethyl)acetamide

1H-l,2,4-triazol-3-amine

'-(l-methylethyl)-l,3,5-triazine-2,4-diamine

6-Chloro-N-ethyl-N

Methyl l-(butylcarbamoyl)-2-benzimidazole-carbamate

3-(l-Methylethyl)-( lH)-2,1,3-benzothiadiazin4(3H)-one-2,2-dioxide

(Benzamidooxy)acetic acid



~-2-Amino-4-[~ydroxy)-(methyl)phosphinoyl]-butyryl-~-alanyl-~-



alanine

S-ethyl bis(2-methylpropyl)carbamothioate

3-Amino-2,5-dichlorobenzoicacid

Methyl-2-chloro-3-(4-chlorophenyl)propionate

1-Methylethyl-3-chlorophenylcarbamate

2-Chloro-N-[

[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]

benzenesulfonamide

exo-1-Methyl-4-(l-methylethyl)-2-[(2-methylphenyl)methoxy]-7oxabicyclo[2.2.llheptane

2,2-Dichloropropanoic acid

0,O-diethyl-0-phenylphosphorothioate

N,Ndimethyl-a-phenylbenzeneacetamide

N '-(3,4-dichlorophenyl)-N,N-dimethylurea

S-ethyl dipropyl carbamothioate

( f )-2-[4-[[5-(Trifluoromethyl)-2-pyridinyl]oxy]phenoxy]

propanoic

acid

p-Nitrophenyl-a,a,c~,-trifluoro-2-nitro-p-tolyl

ether

Phenylmethyl 2-chloro4-(trifluoromethyl)-S-thiazole-carboxylate

~.~-2-Amino-4-~ydroxy)-(methyl)phosphinoyl-butyric

acid

N-(phosphonomethy1)glycine

2-[4,5-Dihydro-4-methyl-4-(

1-rnethylethyl)-5-oxo-lH-imidazol-2-yl-3quinolinecarboxylic acid

(4-Chloro-2-methy1phenoxy)aceticacid

( + )-2-(4-Chloro-2-methylphenoxy)propanoicacid

(4-Methoxy-3-methylphenyl)-(3-methylphenyl)methanone

2-Chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-l

methylethyl)

acetamide

4-Amino-6-(1,l -dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one

S-ethyl hexahydro-1H-azepine-l-carbothioate

N '-(-4-chlorophenyl)-N,N-dimethylurea

1,l '-Dimethyl4,4'-bipyridiniumion

3-[(Methoxycarbonyl)amino]phenyl(3-methylphenyl)carbamate

4-Amino-3,5,6-trichloro-2-pyridinecarboxylic

acid

N-(3,4-dichlorophenyl)propanamide

5-Amino-4chloro-2-phenyl-3(2H)-pyridazinone

3,7-Dichloro-8-quinolinecarboxylicacid

2[[[[(4,6-Dimethyl-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]

benzoic acid

N-(1,1-dimethylethyl)-N'-ethyl-6-(methylthio)-1,3,5-triazine-2,4diamine

2-(3,5-Dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane

2,6-Dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine

(2,4-Dichlorophenoxy)acetic acid

(2,4,5-Trichlorophenoxy)acetic acid



328



KRITON K. HATZIOS



research over the past two decades has resulted in the establishment of two

major strategies for the biological control of weeds: the classical and the

bioherbicide approach (Charudattan and Walker, 1982; Templeton et al.,

1986).



In the classical approach, a biocontrol agent is simply introduced or

released into a weed population to establish itself and control the weed

population, requiring no further manipulation (Templeton et al. 1979). Insects, fungi, nematodes, fish, and other biological systems have all been

used with varying degrees of success as biocontrol agents in the classical approach of biological weed control (Charudattan and Walker, 1982).

The bioherbicide approach employs the massive, usually annual, release

of a biocontrol agent into specific weed-infested fields to infect and kill

susceptible weeds (Templeton, et al., 1986). In modern literature, the term

“bioherbicide” refers to microbial plant pathogens which are applied as

sprays that uniformly kill or suppress the growth of weeds (Templeton and

Smith, 1977). Thus, in the bioherbicide approach, microbial plant

pathogens are applied to target weeds in a manner similar to chemical herbicides. This strategy has received considerable attention and the subject

has been reviewed extensively (Charudattan and Walker, 1982; Quimby and

Walker, 1982; Scheepens and van Zon, 1982; TeBeest and Templeton, 1985;

Templeton and Smith, 1977; Templeton et al., 1979, 1986). Fungi, bacteria,

and viruses offer great promise for use as bioherbicides in modern weed

management. At present, however, only the potential use of fungal

pathogens as mycoherbicides has been studied in depth (Scheepens and van

Zon, 1982; Templeton et al., 1986). Phytopathological agents developed as

bioherbicides may not be very effective in nature but are made effective for

biological control by applying an abnormally high inoculum pressure at an

appropriate time (Scheepens and van Zon, 1982). Thus, in the bioherbicide

approach, the climax of an epidemic is reached artificially early in the

season. Fungal pathogens that are virulent (capable of causing injury), host

specific, and genetically stable but constrained naturally by low inoculum

production and poor dissemination are good candidates for development as

mycoherbicides (Templeton et al., 1986). The exploitation of plant

pathogenic bacteria as bioherbicides seems feasible because they can be host

specific and can easily be cultivated in vitro (Scheepens and van Zon, 1982).

In contrast, plant pathogenic viruses are more problematic since they are

often not host specific and a vector is usually necessary for their transmission from one plant to another. Other factors influencing the potential use

of plant disease-causing organisms as bioherbicides include: low virulence,

stringent temperature and moisture requirements, wounding requirements,

and specific physiological interactions with the host plant (TeBeest and

Templeton, 1985).

The utilization of biological weed control agents in weed management



BIOTECHNOLOGYAPPLICATIONS IN WEED MANAGEMENT



329



offers many advantages including (1) a high degree of specificity for the

target weed; (2) no effects on nontarget and beneficial plants or man; (3)

absence of weed resistance development; (4) absence of residue buildup in

the environment; and ( 5 ) potential impact from biotechnological research

and development (Khachatourians, 1986; Templeton et al., 1986). Some

drawbacks associated with the production and use of bioherbicides include

1. Bioherbicides have to be registered with the Environmental Protection

Agency (E.P.A.) and the registration process may be lengthy.

2. Suppression or killing of weeds by bioherbicides may be a slow process.

3. Stability of bioherbicides under field conditions is highly dependent on

environmental conditions.

4. Production of a bioherbicide for large-scale application may be an expensive process.

5 . Numerous fungi need to be discovered and developed as bioherbicides

because of the high degree of specificity of these agents (Khachatourians,

1986; Templeton et al., 1986).

As a consequence of these advantages and disadvantages, present work

with bioherbicides is concentrated on their use for the control of species that

escape standard chemical control. These hard-to-control weeds, spread

across a wide area, present an economic opportunity that could fit into an

established chemical control program. Thus, at present, bioherbicides are

not seen as alternatives to chemical herbicides but as complementary adjuncts to current weed management systems (Templeton et d., 1986).



B.



COMMERCIAL PROSPECTS AND CONSIDERATIONS



The concept of using fungi, bacteria, and even viruses as bioherbicides is

biologically feasible with several host-pathogen combinations. Today,

however, only fungal pathogens have been exploited commercially as

mycoherbicides, marketed for practical applications. Detailed overviews of

several aspects pertaining to the commercialization of bioherbicides such as

large-scale production, formulation, application, storage, and market

potential have been reported by Quimby (1986) and Templeton et al. (1986).

The potential commercialization of a microbial phytopathogenic agent is

dependent greatly on whether this microbe possesses properties that allow it

to be handled like a chemical herbicide or not. Ideally, a commercialized

bioherbicide should be long-lived and insensitive to manipulations in the industrialization process (Kenney, 1986). Thus, determination of the life

cycles of endemic pathogens of major weeds and development of methods

for mass production of stable reproductive units (spores) of these pathogens



330



KRITON K. HATZIOS



are of paramount importance for the commercialization of microbial

pathogens as bioherbicides. In addition, the small size of current markets of

bioherbicides, their marketability in situations where a selective host range

is required, and stabilization of the product under field conditions are factors or obstacles that need to be addressed during the commercialization

process (Khachatourians, 1986; Templeton et ul., 1986).

Utilization of innovative approaches for large-scale production and

stabilization under field conditions has resulted in the commercialization

and registration of two fungi as mycoherbicides. A formulation of the soilborne fungus Phytophthorupulmivora (butler) Butler was registered in 198 1

as the first selective mycoherbicide for the control of a strangler (milkweed)

vine (Morreniu odorutu Lindl.) in Florida citrus groves under the trade

name DeVineO (Abbott Laboratories, North Chicago, Illinois) (Kenney,

1986; Ridings, 1986). Phytophthoru pulmivoru is a facultative parasite that

produces a lethal root rot of its host plant (strangler vine) and persists

saprophytically in the soil for extended periods of time (Ridings, 1986).

Host-specific strains of this fungus are formulated as liquid suspensions

consisting largely of chlamydospores of the fungus (Kenney, 1986). These

suspensions are normally applied with boom and nozzle sprayer systems to

the soil surface under tree canopies. Rapid loss in spore viability required

Abbott Laboratories to provide strict quality control checks on viability and

to market DeVineO on a “made-to-order” basis (Kenney, 1986). Thus,

DeVineO must be ordered prior to the season in which it will be applied,

and it is distributed and stored under refrigeration. DeVineO proved to be

an extremely effective mycoherbicide. Groves treated with DeVineO in

1978-1980 are still seeing 95-100% control of strangler vine from a single

treatment, in spite of a continuous infestation of new seedlings originating

from wind-blown seeds (Kenney, 1986).

The second commercially developed mycoherbicide is marketed under

the trade name CollegoO by the TUCO division of the Upjohn Company,

Kalamazoo, Michigan (Bowers, 1986; Smith, 1986). CollegoO is a formulation of Colletotrichum gloesporoides (Penz.) Sacc. f. sp. aeschynomene, an

endemic anthracose fungus, registered in 1982 for the selective control of

northern jointvetch [Aeschynomene virginica (L.) B.S.P.] in rice (Oryza

sutivu L.) and soybean [Glycine mux (L.) Merr.] fields of Arkansas,

Louisiana, and Mississippi (Smith, 1986; Templeton et al., 1986). In 1983, a

total of 619 acres were treated with CollegoO in rice and soybean fields of

the aforementioned southern states (TeBeest, 1986). Colletotrichum

gloesporoides is a facultative sporophyte that causes a lethal stem and

foliage blight of its host weed when inoculated with spores (Smith, 1986).

CollegoO is a dry powder containing 15% spores (conidia) of C .

gloesporoides as active ingredient and 85 9’0 inert ingredients. The formulation is rehydrated and resuspended in a sugar medium before being mixed



BIOTECHNOLOGY APPLICATIONS IN WEED MANAGEMENT



33 1



with water in the spray tank of an application system (TeBeest and

Templeton, 1985). CollegoO is normally applied aerially to rice and soybeans at dusk, but tractor-mounted spray equipment could be also used for

applications in soybean fields (TeBeest and Templeton, 1985).

Necessary precautions that must be taken when using DeVineO and

College@ in the field include the prevention of exposure of these formulations to wetting agents, fertilizers, and chemical pesticides that are detrimental to the viability of their spores (TeBeest and Templeton, 1985). Recent field

and laboratory tests demonstrated that Collego@ can be integrated with

several chemical herbicides used in soybeans, such as acifluorfen and

bentazon, in tank-mixed application (TeBeest and Templeton, 1985).

However, tank-mixtures of CollegoO with the herbicide propanil, an important week killer in rice, were incompatible (TeBeest and Templeton, 1985).

Although weeds can become resistant to mycoherbicides following

repeated applications of these biocontrol organisms in an intensive weed

management system, so far, this has not been the case with CollegoO and

DeVine@. Development of biotypes of northern jointvetch with resistance

to Colletotrichum gloesporoides has not been observed during the 14-year

period of field tests conducted with this mycoherbicide (Templeton, 1986).

Similarly, there is presently no indication of any changes in virulence or

specificity of the fungus C. gloesporoides during this time.

Apart from DeVineO and CollegoO , a number of other endemic fungi

are in various stages of research and development as mycoherbicides, including the following:

1. Colletotrichum malvarum (A. Braun and Casp.) Southworth, an

anthracnose-inducingfungus, for the control of prickly sida (Sidaspinosa L.)

in cotton (Gossypium hirsutum L.) and soybean (Kirkpatrick et al., 1982).

2. Alternaria cassiae Jurair and Khan, a blight-inducing fungus, for the

control of sicklepod (Cassia obtusifolia L.) in soybean and cotton (Walker

and Riley, 1982).

3. Alternaria macrospora Zimm.for the control of spurred anoda [Anoda

cristata (L.)Schlecht.] in cotton (Walker, 1981; Walker and Sciumbato, 1979).

4. Cercospora rodmanii Conway, a leaf spot-inducing fungus, for the

growth suppression of water hyacinth [Eichhorniacrassipes (Mart .) Solms]

in waterways (Conway et al., 1978)

5. Colletotrichum coccodes (Wallr.) Hughes for the control of velvetleaf

(Abutilon theophrasti Medic.) (Gotlief et al., 1984).



c.



BIOTECHNOLOGY

AND BIOHERBICIDE

PRODUCTION



Many aspects of the biology of phytopathogenic agents that affect their

bioherbicide potential could be improved greatly with the application of



332



KRITON K. HATZIOS



innovative technological procedures. Biotechnology, in particular, is expected to have a far-reaching impact on the production and use of

phytopathogenic agents as bioherbicides in the future. The efficacy, industrial production, application, and cost-effectiveness of biological weed

control agents could all be enhanced by applying biotechnology.

Advances of fermentation and formulation technology of plant

pathogens used as bioherbicides are needed to improve the costeffectiveness and industrial production of these biological weed control

agents. Experience obtained with the two commercially developed mycoherbicides indicates that opportunities exist for improvements in the capacity

of a fungus to yield spores in submerged fermentation or to tolerate adverse

drying procedures (Templeton ei al., 1986).

Techniques such as genetic engineering may allow us to bring about changes

in the genome of fungal or bacterial plant pathogens that could result in an increase of their favorable properties when used as bioherbicides. Isolation of the

genetic determinants of virulence, specificity, sporulation capacity, toxin production, and tolerance to climatic stresses as well as their transfer from

pathogen to pathogen appear now quite promising. Virulence of a host-specific

pathogen could be increased by strain selection or genetic improvement of this

pathogen. Early work by Sands and Rovira (1972) demonstrated that genetic

changes in plant pathogenic agents could be induced, making them less virulent

to crop species and more virulent to weeds. Fungi that produce a sexual stage

may be improved by crossing compatible strains. Advances in protoplast fusion

technology with fungi make it possible now to use parasexual crossing with

strains that do not produce a sexual stage (Templeton et al., 1986). In a way,

this biotechnological approach will enable us to produce biological analogs of

successful bioherbicides.

Pathogen strain improvement may be also achieved by mutagenesis induced either by irradiation or by chemical treatment. This approach seems

particularly promising for the selection of strains of bioherbicides that are

tolerant to chemical pesticides. In a recent report. TeBeest (1984) used

chemical mutagenesis to select several benomyl-tolerant strains of the

mycoherbicide College@ . These strains contained all the desirable properties

of the naturally occuring strains of C. gloesporoides. This successfulexample

illustrates that development of strains of phytopathogenic agents resistant to

selected chemicals will allow the wider use of bioherbicides in integrated pest

management programs of selected crops (Templeton et al., 1986).



Ill. NATURALLY OCCURRING HERBICIDES

As a consequence of worldwide growing concern about the environmental aspects of selected pesticides, the need for the use of easily degradable



BIOTECHNOLOGY APPLICATIONS IN WEED MANAGEMENT



333



pesticides with good selectivity is greater than ever. Recent advances in

microbial and plant biochemistry have stimulated scientific interest into the

possible role of secondary plant products and microbial toxins as natural

pesticides (Cutler, 1984; Glick et al., 1984; Duke, 1986a,b; Kurz and

Constabel, 1985; McLaren, 1986; Misato and Yamaguchi, 1984; Rice, 1984;

Sandermann, 1985; Seikizawa and Takematsu, 1983). Furthermore, advances in plant cell culture, fermentation technology, molecular genetics,

and genetic engineering make it now possible to exploit biotechnologically

plants and microorganisms as potential sources of naturally occurring

chemical compounds that could be developed as herbicides.

At present, two major areas of research appear attractive because of their

potential commercial applications. They include (1) isolation and characterization of microbialtoxins or secondaryplant metabolitesthat could be used effectively as herbicides, and (2) evaluation of plant secondary metabolites and

microbial toxins with novel chemistries which could be used as leads for the

chemical synthesis of new herbicides. Selected examples illustrating the potential of these approaches are discussed briefly in the following sections.

A. UTILIZATION OF PLANT AND MICROBIAL

PHYTOTOXINS AS HERBICIDES

The use of biologically derived chemicals as herbicides has already

achieved respectable levels. In principle, this approach is based on fermenting bacteria and fungi, testing fermentation broths for activity, and

isolating active compounds from these broths (Adler et al., 1985; Egorov

and Landau, 1983). Plant cell cultures could be used in a similar fashion

(McLaren, 1986; Rhodes and Kirshop, 1982). Although both of these procedures are tedius and long-term operations, they do possess practical and

economical merit. Japan is presently considered to be the most advanced

country in the development of microbial pesticides and pharmaceuticals

(Dibner, 1985). Pesticides of microbial origin, commonly referred to as

“agricultural antibiotics,” are highly specific for target organisms and supposed to be inherently biodegradable because they are synthesized

biologically (Misato and Yamaguchi, 1984). In addition, according to Duke

(1986a), purified natural compounds appear to have many practical advantages over bioherbicides as weed control agents such as longer self life, a

wider range of storage conditions, a broader environmental window for application, lower storage space requirements, and greater ease of application.

Extensive research during the last two decades has demonstrated that

several plant secondary metabolites (allelochemicals) as well as fungal and

microbial toxins possess good herbicidal activity. A brief list of selected

naturally occurring compounds that exhibit promising herbicidal activity is



334



KRITON K. HATZIOS



presented in Table 11. Detailed reviews of this topic are available (Balandrin

et al., 1985; Dodge and Knox, 1986; Duke, 1986a, b; Fellows, 1986; Fischer

and Bellus, 1983; Misato and Yamaguchi, 1984; Putnam, 1985). Of the

naturally occurring compounds listed in Table 11, only bialophos is currently marketed under the trade name HerbiaceaeO as a commercially

Table 11

Examples of Microbial or Plant Phytotoxins with Promising Herbicidal Activity

Natural

phytotoxin



Plant or microbial source



Reference



Anisomycin

Bialophos



Streptomyces sp.



Yamada et a/. (1972)



Streptomyces hygroscopicus

Streptomyces viridochromogenes



Cytochalasins



Phomopsis sp.



Mase (1984)

Tachibana (1987)

Cole et al. (1981)



Cercosporin



Phloridzin

Phosalacine

Psoralen

Rhizobitoxine



Cercospora sp.

Pseudocercosporella capsella

Coffee plants

Scytonema hofmanni

Sorghum plants

Spurge plants

Streptomyces saganonensis

Black walnut trees

Aspergillus terreus

Fusarium moniliforme

Penicillium sp.

Xylaria sp.

Macrophomina phaseolina

Apple roots

Kitasatosporia phosalacinea

Psoralea plants

Rhizobium japonicum



Stemphyloxin 1

Tabtoxin

Tentoxin

Toyocamycin

Trimethylxanthine

Viridiol

Ziniol



Stemphylium botryosum

Pseudomonas tabaci

Alternaria alternata

Strephtomyces toyocanensis

Coffee plants

Gliocladium virens

Alternaria carthami



Caffeine

Cyanobacterin

Dhurrin

Gallic acid

Herbicidins

Juglone

Mevinolin

Moniliformin

Patulin

Phaseolinone



Durbin (1981)

Putnam (1985); Duke (1986a)

Gleason er al. (1987)

Putman (1985)

Putnam (1985); Duke (1986a)

Takiguchi et al. (1979)

Reitveld (1983)

Bach and Lichtenhaler (1983)

Cole et al. (1973)

Putnam (1985)

Karl et a/. (1986)

Dhar et a/. (1982)

Putnam (1985)

Omura et a/. (1984)

Putnam (1985); Duke (1986a)

Giovanelli et a/. (1971);

Lieberman (1979)

Barash et al. (1982)

Langston-Unkefer et al. (1984)

Duke (1986a)

Yamada et al. (1972)

Rizvi et a/. (1980)

Howell and Stipanovich (1984)

Robeson and Strobel (1984)



BIOTECHNOLOGY APPLICATIONS IN WEED MANAGEMENT



335



developed herbicide in Japan (Mase, 1984;Tachibana, 1987). Bialophos is a

microbial product isolated from the fermentation broth of Streptomyces

hygroscopicus and S. viridochromogenes and exhibits strong herbicidal activity against a wide spectrum of grass and broadleaf weeds following application to their foliage (Mase, 1984; Misato and Yamaguchi, 1984).

Bialophos exhibited very little phytotoxicity when applied to the soil, and

this is believed to be due to its remarkable biodegradability by soil

microorganisms (Misato and Yamaguchi, 1984). A detailed description of

the biosynthetic pathway used by Streptomyces hygroscopicus to synthesize

bialophos was recently reported by Imai et al. (1986).Utilization of blocked

mutants and of biosynthetic intermediates was instrumental in the elucidation of this pathway. Several of the biosynthetic genes of bialophos were

recently cloned (Imai et al., 1986). In sensitive plants, bialophos is

metabolized to phosphinothricin { L-2-amino-4-[(hydroxy)-(methyl)

phosphinoyll-butyric acid}, a phytotoxic metabolite which inhibits the enzyme glutamine synthetase in such plants (Lea et d., 1984). Glutamine synthetase is important in ammonia assimilation and the genes coding for this

enzyme in alfalfa (Medicago sativa L.) plants have been recently cloned

(Tischer et al., 1986). Treatment of plants with phosphinothricin causes ammonia to accumulate at levels exceeding those known to uncouple

photophosphorylation and as a result CO, assimiliation is greately reduced

(Kocher, 1983; Lea et al., 1984). Phosalacine is another microbial compound containing phosphinothricin and its herbicidal behavior is similar to

that of dialophos (Omura et al., 1984).

Of the other natural compounds listed in Table 11, tentoxin has been

studied the most. Tentoxin is a cyclic tetrapeptide produced by Alternaria

alternata and causes marked chlorosis to many grass and broadleaved weed

species. Several crops such as corn (Zea mays L.) and soybeans are tolerant

to this toxin (Duke et al., 1980; Durbin and Uchytil, 1977). However, in

spite of its clear-cut crop selectivity and excellent activity, tentoxin has not

been developed commercially as a herbicide. The role of several hostspecific phytotoxins produced by Alternaria sp. in host-parasite interactions has been reviewed by Ueno (1987).

With respect to most allelochemicals from higher plants, a major problem that hinders their development as herbicides is their limited selectivity

and lack of stability (Duke, 1986a). Recent advances in genetic engineering

appear promising in overcoming these problems. Some of the plant enzymes

mediating the biosynthesis of secondary plant products have already been

isolated. The induction of such enzymes has been studied at the levels of

transcription and translation, and cDNAs for some biosynthetic enzymes

have been cloned as a prerequisite for the isolation of the respective genes

(Hahlbrook et al., 1985). However, although attempts to introuce genes



336



KRITON K. HATZIOS



coding for the biosynthesis of natural pesticides into crop plants are underway in several laboratories, this would be a long-term process because

several genes may have to be simultaneously transferred to achieve this goal

(Sandermann, 1985).



B.



SYNTHETIC DERIVATIVES

OF NATURALLY

OCCURRING

COMPOUNDS AS HERBICIDES



The problems of high phytotoxicity, limited crop selectivity, and instability under field conditions associated with several naturally occurring

compounds hinder their commercial development as herbicides. These

problems can be overcome by a biorational synthesis of more selective and

stable analogs of these chemicals. Thus, microbial toxins and

allelochemicals provide us with novel chemistries that could be manipulated

in order to produce commercial herbicides. Selected examples of commercially developed herbicides which are chemically similar to naturally occurring phytotoxic compounds are listed in Table 111. In most of the presented

cases, it is unclear whether these herbicides were developed by means of a

biorational design based on natural phytotoxin chemistry or through random screening.

The development of the herbicide methoxyphenone as a synthetic analog

of the microbial toxin anisomycin represents one of the few successful efforts employing the biorational chemical synthesis of a herbicide based on a

Table I11

Commercially Developed Herbicides Based on Natural Chemistry

Natural

product



Plant or

microbial source



Herbicide



Manufacturer/

country



Anisomycin

Cineole



Streptomyces sp.

Widespread in plants



Methoxyphenone NihodJapan

Cinmethylin

She1MJ.S.A.



Benzoxazinones

(hydroxamic acids)

Iprexil



Graminae plants



Benzazin



BASF/Federal Republic

of Germany



Benzadox



Fusaric acid

Moniliformin



Iprex pachyon

Fusarium sp.

Fusarium moniliforme



Gulf/U S .A,

Dow/U.S.A.



Quinolinic acid



Nicotiana tabacum



Phosphinothricin



Streptomyces

viridochromogenes



Picloram

3,44ibutoxymoniliformin

Quinclorac

Glufosinate



.



CIBA-GEIGY/

Switzerland

BASF/Federal Republic

of Germany

HoechstIFederal Republic

of Germany



BIOTECHNOLOGY APPLICATIONS IN WEED MANAGEMENT



337



natural chemistry (Munakata et al., 1973). Methoxyphenone is marketed in

Japan as a selective herbicide for the control of barnyardgrass [Echinochloa

crusgalli (L.) Beauv.] in rice and is easily degraded in soil (Duke, 1986a;

Munakata et al., 1973). The ammonium salt of glufosinate, a synthetic

racemic mixture based on the chemistry of phosphinothricin, has been

recently introduced by Hoechst Aktiegesellschaft Co. (West Germany) as a

nonselective herbicide marketed under the trade name Basta@ (Kocher,

1983). As discussed earlier, phosphinothricin is the active ingredient of the

microbial herbicide bialophos (Herbiaceae@).

Iprexil and its synthetic analog benzadox appear to act as proherbicides.

Following their application to plants they are activated by being converted

to amino-oxyacetic acid, a potent inhibitor of pyridoxyl phosphate-requiring enzymes (Duke, 1986a; Fischer and Bellus, 1983).

Fusaric acid is a marasmin produced by many species of Fusarium fungi

and has been detected in infected tomato (Lycopersicon esculentum Mill.)

plants and wilted cotton (Owens, 1969). Picloram, a chlorinated analog of

fusaric acid, has been marketed as a herbicide for many years. Peterson et

al. (1974) reported that picloram caused desiccation and wilting in red

maple (Acer rubrum L.) plants resembling the symptoms caused by fusaric

acid. Similarities in physiological actions of picloram and several other synthetic or natural derivatives of picolinic acid have been also reported by

Chang and Foy (1982).

These examples demonstrate that the structures of naturally occurring

phytotoxins can serve as leads for the synthesis of new successful herbicides.

Undoubtedly, the use of biorational design for the discovery of new herbicide chemistries will become increasingly important in the future following the isolation and characterization of additional microbial toxins and

allelochemicals from higher plants.



IV. GENETIC IMPROVEMENT OF CROP

TOLERANCE TO HERBICIDES

OF PLANT TOLERANCE

TO HERBICIDES

A. MECHANISMS



As mentioned earlier, one of the most important characteristics of

chemical herbicides is their ability to control selectively a wide spectrum of

weeds in a variety of crops. Such selectivity implies that some crop or weed

species are able to survive and grow at agriculturally recommended rates of

a herbicide but not at rates which are several times higher. When this happens we talk about crop or weed tolerance to this particular herbicide. The

continuous application of herbicides for weed control in modern crop production systems, however, creates a new selective pressure that could lead to



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II. Production and Use of Biological Weed Control Agents

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