Tens of thousands of secondary products of plants have been identified and there are estimates that hundreds of thousands of these compounds exist. There is growing evidence that most of these compounds are involved in the interaction of plants with other species-primarily the defense of the plant from plant pests. Thus, these secondary compounds represent a large reservoir of chemical structures with biological activity. This resource is largely untapped for use as pesticides. This review will provide an overview of those compounds from plants that have been utilized for pest control, examples of some compounds with pesticidal potential, and a discussion of considerations in development of natural plant compounds for pesticidal use.
A few highly phytotoxic plant-produced compounds have been discovered. However, none have been developed as herbicides. The sesquiterpenoid lactone, artemisinin from Artemisia annua L., was found to inhibit plant growth as well as the commercial herbicide cinmethylin. Other compounds, such as 2,4-dihydroxy-1,4-benzoxazin-3-one are as active as plant growth inhibitors as many herbicides. Plants produce many photodynamic compounds, such as hypericin (Fig. 1), that are strongly phytotoxic, provided they can be introduced into the plant cell. These compounds are unlikely to be developed as pesticides because, in the presence of light, they are toxic to all living organisms. However, any plant can be caused to generate phytotoxic levels of photodynamic porphyrin compounds by treating the plant with both d-aminolevulinic acid (Fig. 1), a natural porphyrin precursor, and 2,2'-dipyridyl, a synthetic compound. This relatively safe combination of compounds is being developed as the "laser" herbicide. Several classes of commercial herbicides have recently been shown to act by causing target plant species to accumulate phytotoxic levels of protoporhyrin IX a photodynamic chlorophyll and heme precursor. Thus, a natural product, not the synthetic herbicides is the acutely toxic compound in these cases. Application of protoporhyrin IX alone to plant issues, however, is not effective, apparently because it does not reach the proper cellular compartment in sufficient quantity.
A problem with plant-produced phytotoxins as potential herbicides is that in the native state, they are generally only weakly active compared to commercial herbicides. Most known allelochemicals would have to be applied at rates of more than 10 kg/ha to achieve significant weed control whereas, most recently marketed herbicides would achieve the same level of control at levels three orders of magnitude smaller. This is not unexpected, because production of highly phytotoxic compounds would lead to strong autotoxicity unless the producing plant develops metabolic or physical mechanisms to cope with its own phytotoxins. Some of the more potent allelochemicals are toxic to the producing species and this autotoxicity has been implicated in vegetation shifts. Microbial conversion of relatively non-phytotoxic compounds in the soil to highly phytotoxic derivatives has been documented.
Plants have been much more successfully exploited as sources of pesticides for pests other than weeds. This is probably due to several factors. The selection pressure caused by pathogens and herbivores has probably been more acute and intense than that caused by plant competitors. A plant species can effectively compete with plant foes in many ways other than by poisoning them and having to cope with autotoxicity. Pathogens and herbivores have many potential physiological and biochemical sites of action for pesticides that the plant does not share. Biosynthesis of a compound to affect one of these sites reduces the chance of autotoxicity. Thus, the chemical option is generally a more attractive option in responding to a herbivore or pathogen that can rapidly devour or invade the plant than it is in responding to a plant competitor.
Another plant terpenoid, camphene (Fig. 2), was a very successful herbicide in its polyhalogenated form. Sold as Toxaphrenereg., this product was the leading insecticide in the United States before it was removed from the market. Although this product was a mixture of over two hundred chlorinated forms of camphene, certain specific compounds in the mixture were found to be much more active than the mixture on a unit weight basis. Many other terpenoids have been demonstrated to have insecticidal or other insect-inhibiting activities. For instance, azadirachtin and other terpenoids of the limonoid group from the families Meliaceae and Rutaceae are potent growth inhibitors of several insect species.
Nicotine (Fig. 2) and nornicotine, components of several members of the genus Nicotiana, have been used commercially as insecticides. N. rustica is the chief commercial source. Other natural analogues of nicotine have been shown to have significant insecticidal properties and one, anabasine or neonicotine (Fig. 2), has been produced as an insecticide from the shrub, Anabasis aphylla, in the Soviet Union. Synthetic variations of nicotine such as 5'-methylnornicotine have been demonstrated to be effective insecticides. Ryanodine, an alkaloid from the tropical shrub, Ryania speciosa, has been used as a commercial insecticide against European corn borer. Physostigmine, an alkaloid from Physostigma venenosum was the compound upon which carbamate insecticides were designed. Furo-quinoline and beta-carboline alkaloids such as dictamine and harmaline, respectively, are potent photosensitizing compounds that are highly toxic to insect larvae in sun light. The relative high cost toxicity to mammals, and limited efficacy have limited the use of natural alkaloid insecticides.
Preparations of roots from the genera Derris, Lonchocarpus, and Tephrosia, containing rotenone (Fig. 2), were commercial insecticides in the 1930s. Rotenone is a flavonoid derivative that strongly inhibits mitochondrial respiration. No other phenolic compound has been used commercially as an insecticide, although the content of certain phenolic compounds in plant tissues have been correlated with host plant resistance to insects and many have been demonstrated to be strong insect growth inhibitors and antifeedants.
As in plants, delta-aminolevulinic acid (ALA), in combination with 2,2'-dipyridyl, can cause accumulation of toxic levels of photodynamic porphyrin compounds. Larvae of several insect species, when fed these compounds and exposed to light were rapidly killed. Protoporphyrin IX the same compound caused to accumulate in plants by certain photobleaching herbicides, is the prophyrin responsible for the toxicity of these compounds to insects. Other photodynamic compounds from plants such as polyacetylenes are acutely toxic to insects, however, their general toxicity would probably preclude them from commercial use.
Control of insects can be achieved by means other than causing rapid death. Plants produce many compounds that are insect repellents or act to alter insect feeding behavior, growth and development ecdysis (molting), and behavior during mating and oviposition. Most insect repellents are volatile terpenoids such as terpenen-4-ol. Other terpenoids can act as attractants. In some cases, the same terpenoid can repel certain undesirable insects while attracting more beneficial insects. For instance, geraniol will repel houseflies while attracting honey bees. Compounds from many different chemical classes have been reported to act as insect antifeedants. Thus, polygodial a sesquiterpenoid from Polygonum hydropiper, is a potent inhibitor of aphid feeding. Several plant-derived steroids that are close analogues of the insect molting hormone, ecdysterone, prevent insect molting. Other chemically unrelated terpenoids inhibit molting by unknown mechanisms. Plant terpenoids that act as locomotor excitants, biting or piercing suppressants, ovipositioning deterrents, or mating behavior disruptants have been described. More than a dozen plant-produced terpenoid juvenile hormone mimics have been found to effectively sterilize insects. Plants contain a myriad of compounds with potential for commercial development in controlling insects.
Several plant-derived compounds have been demonstrated to be strong elicitors of phytoalexins. For instance, certain oligosaccharide components of cell walls from stressed or dying higher plant cells will act as elicitors. Further knowledge of plant-derived phytoalexin elicitors could lead to their use as fungicides. Several isoflavonoid compounds, such as glyceollin, phaseolin, and pisatin (Fig. 3) in soybean, garden bean, and pea, respectively have been implicated in protection of these crops from pathogens. Many other confirmed or suspected phytoalexins have been identified. Some of these compounds have demonstrated utility against fungi under field conditions. Foliar application of the phenolic lactone juglone (Fig. 3), a product of several walnut species, provides better protection of bean seedlings from rust than some commercial fungicides. Terpenoid phytoalexins) and fungicides are known and some have been tested for commercial efficacy. Wyerone, an acetylenic acid derivative produced by legumes as a phytoalexin has a wide fungicidal spectrum against plant pathogens and has been successfully tested against fungal infection of crop plants. Despite a repertoire of many antifungal and antibacterial compounds, plant products have not been used to any significant extent in the development of antimicrobial pesticides.
The plant-derived saponins are generally highly toxic to snails. Cyanogenic glucosides are responsible for resistance of certain legumes to snails and slugs. No plant-derived natural products are commercial products are available for control of snails and slugs.
Considering the probability of plant secondary products being involved in plant-pest interactions, the strategy of randomly isolating, identifying, and bioassaying these compounds may also be an effective method of pesticide discovery. Biologically active compounds from plants will often have activity against organisms with which the producing plant does not have to cope. Many secondary compounds described in the natural product, pharmacological and chemical ecology literature have not been screened for pesticidal activity. This is due, in part, to the very small amounts of these compounds that have been available for screening.
The discovery process for natural pesticides is more complicated than that for synthetic pesticides (Fig. 4). Traditionally, new pesticides have been discovered by synthesis, bioassay, and evaluation If the compound is sufficiently promising, quantitative structure-activity relationship-based synthesis of analogues is used to optimize desirable pesticidal properties. The discovery process with natural compounds is complicated by several factors.
First, the amount of purification initially conducted is a variable for which there is no general rule. Furthermore, secondary compounds are generally isolated in relatively small amounts compared to the amounts of synthesized chemicals available for screening for pesticide activity. Therefore, bioassays requiring very small amounts of material will be helpful in screening natural products from plants. A number of published methods for assaying small amounts of compounds for pesticidal and biological activities are available in the allelochemical and natural product literature. At some point in the discovery process, structural identification is a requirement. This step can be quite difficult for some natural products. Finally, synthesis of the compound and analogues must be considered. This is generally much more difficult than identification. Despite these difficulties, modern instrumental analysis and improved methods are reducing the difficulty, cost and time involved in each of the above steps.
The toxicological and environmental properties of the compound must be considered. Simply because a compound is a natural product does not insure that it is safe. The most toxic mammalian poisons known are natural products and many of these are plant products. Introduction of levels of toxic natural compounds into the environment that would never be found in nature could cause adverse effects. However, evidence is strong that natural products generally have a much shorter half-life in the environment than synthetic pesticides. In fact, the relatively short environmental persistence of natural products may be a problem, because most pesticides must have some residual activity in order to be effective. As with pyrethroids, chemical modification can increase persistence.
After promising biological activity is discovered, extraction of larger amounts of the compound for more extensive bioassays can be considered. Also, analogues of the compound should be made by chemical alteration of the compound and/or chemical synthesis. Structural manipulation could lead to improvement of activity, toxicological properties, altered environmental effects, or discovery of an active compound that can be economically synthesized. This has been the case with several natural compounds that have been used as a template for commercial pesticides (e.g., pyrethroids).
Before a decision is made to produce a natural pesticide for commercial use, the most cost-effective means of production must be found. Although this is a crucial question in considering the development of any pesticide, it is even more complex and critical with natural products. Historically, preparations of crude natural product mixtures have been used as pesticides. However, the potential problems in clearing a complex mixture of many biologically active compounds for use by the public may be prohibitive in today's regulatory climate. Thus, the question that will most probably be considered is whether the pure compound will be produced by biosynthesis and purification or by traditional chemical synthesis.
Before considering any other factors, there are two advantages to the pesticide industry to industrial synthesis. They have invested heavily in personnel and facilities for this approach. Changing this approach may be difficult for personnel trained in disciplines geared to use it. Secondly, in addition to the patent for use, patents for chemical synthesis often further protect the investment that a company makes in development of a pesticide.
However, many natural products are so complex that the cost of chemical synthesis would be prohibitive. Even so, more economically synthesized analogues with adequate or even superior biological activity may tip the balance toward industrial synthesis. If not biosynthesis must be considered. There are a growing number of biosynthetic options.
The simplest method is to extract the compound from field-grown plants. To optimize production, the species and the variety of that species that produce the highest levels of the compound must be selected and grown under conditions that will optimize their biosynthetic capacity to produce the compound. Genetically manipulating the producing plants by classical or biotechnological methods could also increase production of some secondary products. For instance, low doses of diphenyl ether herbicides can cause massive increases in phytoalexins in a variety of crop species.
Another alternative is to produce the compound in tissue or cell culture. With these methods, cell lines that produce higher levels of the compound can be rapidly selected. However, genetic stability of such traits has been a problem in cell culture production of secondary products. Cells that produce and accumulate massive amounts of possibly autotoxic secondary compounds are obviously at a metabolic disadvantage and are thus selected against under many cell or tissue culture conditions. A technique, such as an immobilized cell column that continuously removes secondary products can increase production by decreasing feedback inhibition of synthesis, reducing autotoxicity, and possibly increasing generic stability. Other culture methods that optimize production can also be utilized. For instance, supplying inexpensively synthesized metabolic precursors can greatly enhance biosynthesis of many secondary products. Also, plant growth regulators, elicitors, and metabolic blockers can be used to increase production.
Genetic engineering and biotechnology may allow for the production of plant-derived secondary products by gene transfer to microorganisms and production by fermentation. This concept is attractive because of the existing fermentation technology for production of secondary products. However, it may be prohibitively difficult for complex secondary products in which several genes control the conversion of several complex intermediates to the desired product.
Genetic engineering might also be used to insert the genetic information for production of plant-produced pesticides from one plant species to another species to be protected from pests. However, such transgenic manipulation of the complex metabolism of a higher plant might be extremely difficult. A simpler alternative might be to infect plant-colonizing microbes with the desired genetic machinery to produce the natural pesticide, as has been done with bacterial-produced insecticides.
Fig. 1. Some plant-produced compounds and derivatives with herbicidal activity. I-1,8-cineole, II-cinmethylin, III-hypericin, IV-delta-aminolevulinic acid.
Fig. 2. Some plant-produced compounds with insecticidal activity. I-camphene, II-nicotine, III-anabasine, IV-rotenone.
Fig. 3. Some plant-produced compounds with fungicidal nematicidal and rodenticidal activity. I-pisatin II-juglone, III-alpha-terthienyl, IV-strychnine.
Fig. 4. Pesticide discovery strategies for synthetic versus natural products.