Host-plant resistance

Most biological and cultural pest controls return greater profits than pesticides. For example, biological pest controls are reported to return from 30 to 300 per dollar invested in control (16). Various cultural controls like host plant resistance, crop rotations, and tillage, also return 30 to 300 per dollar Invested in pest control (16). 

Extracts of plants have been used as insecticides by humans since before the time of the Romans. Some of these extracts have yielded compounds useful as sources (e.g., pyrethrins, rotenoids, alkaloids), others as models (e.g., pyrethrins, physostigmine) of commercial insecticides. Recent technological advances which facilitate the isolation and identification of the bioactive constituents of plants should ensure the continued usefulness of plant compounds in commercial insect control, both as sources and models of new insect control agents and also as components in host plant resistance mechanisms. The focus in this paper will be on several classes of compounds, including limonoids, chromenes, ellagitannins, and methyl ketones, which were found to be components of the natural defenses of both wild and cultivated plants and which may be useful in commercial insect control. 

In summary, natural plant compounds have been exploited commercially as sources (e.g., pyrethrins, rotenoids, alkaloids) and models (e.g., pyrethrins, physostlgmine) of insecticides. Other plant compounds are currently being evaluated for similar uses (e.g., chromenes, limonoids). Still others are being evaluated for use in host plant resistance (e.g., long-chain methyl ketones). Such novel chemicals with potent and often unique biological activities will continue to be discovered and exploited through bioassay and... 

Plant furanocoumarins occur widely in nature and provide formidable obstacles to grazing by herbivorous animals. Some insect species have nevertheless adapted to circumvent this powerful host-plant-resistance mechanism. It has been proposed that the leaf-rolling habit of some insect species may be an evolutionary adaptation to avoid light and thus avoid the toxic effects of furanocoumarins (21). Also, evidence has recently been obtained that the capacity of at least one leaf-mining Insect species to detoxify furanocoumarins allows the utilization of furanocoumarln-contalnlng plants as hosts (29). 

Figure 3. Cuticular Diterpenoids—Duvanes and Labdanes important in host-plant resistance to insects.

Two dlterpenoid acids, (-)-cis- and (-)-trans-ozlc acids, may also contribute to host plant resistance to several insect species in Hellanthus occidentalls (54). 

The cited observations suggest that it is possible to identify potato cultivars with low or high phenolic acid content for human use and to select processing conditions that minimize losses of phenolic compounds. In summary, the methods we developed and used to determine the content and distribution of phenolic compounds in potato plant flowers, leaves, and tubers, in the peel and flesh parts of potato tubers, and in freeze-dried and processed commercial potatoes merit application in numerous studies designed to assess the role of potato phenolic compounds in host-plant resistance, plant breeding, plant molecular biology, food chemistry, nutrition, and medicine. The described wide distribution of phenolic compounds in different commercial... 

Incorporate into plant varieties, perhaps through genetic engineering, specific antifungal genes expressed in the specific plant tissues, e. g. seed tissues contaminated by aflatoxigenic strains (a host-plant resistance approach), and... 

Examples of natural products that may have potential in augmenting host plant resistance against A. flavus infection are certain plant derived volatile compounds (78, 85) as described earlier. 

Volatile compounds originating from the aflatoxin susceptible crop, cotton, and other plant-derived compounds that inhibit aflatoxin production have been identified. The various compounds are being tested for their potential to enhance host plant resistance by inhibition of fungal growth/aflatoxin production. 

Widstrom, N. W. "Breeding strategies to control aflatoxin contamination of maize through host plant resistance" In Aflatoxin in maize A proceedings of the workshop-, Zuber, M. S., et al., Eds. CIMMYT Mexico City, Mexico, 1987, pp. 212-220. 

The recent advances in identifying and utilizing allelochemlcs Involved in host-plant resistance has drawn the attention of the pesticide Industry. A potential problem that may not be recognized, is the effect on insects if analogs of plant protective chemicals are sprayed on agricultural crops. Insects treated with such analogs, could rapidly become tolerant not only to the analog, but also to the natural allelochemlc. 

B. in "Host Plant Resistance to Pests", ACS Syii5>oslum Series, No. 62 Hedin, P. A., Ed. American Chemical Society Washington D. C., 1977, p. 179. 

De Novo Synthesis of Phytoalexins. Phytoalexins have been studied in great depth by plant pathologists. Excellent review papers are available in Hedin s ACS symposium volume. Host Plant Resistance to Plants (42), and more recently in the book edited by Horsfall and Cowling, Plant Disease (43). The antiherbivory effect of phytoalexins, however, is only now beginning to be fully appreciated. It is apparent that pathogen induced phytoalexins do have a definite effect on insect herbivores. There is mounting evidence that herbivore-inflicted injury may also result in the induction of phytoalexin production and accumulation. 

Nutrient-Allelochemical Interactions in Host Plant Resistance... 

Research supported by University of Delaware Research Foundation grant. Plant Defense to Insect Attack and Hatch Project 215, Host Plant Resistance to Black Cutworms in Corn. 

In the coevolutionary interactions of plants and animals, lipids play a major role. They function as ecomones (pheromones, allomones and kairomones) and have been classified by their function. Host plant resistance is partially dependent on these chemical constituents. Lipids may be subdivided into two types. Volatile lipids are generally involved in long distance interactions whereas non-volatile lipids are generally involved after the insect has contacted the host plant. Several examples of each are reviewed. Utilization of these compounds to promote increased host plant resistance could be accomplished by selection of plants rich in allomones, lacking kairomones for a particular pest or those with inducible systems of defense. Another approach is to isolate the defensive compounds of one plant and apply them to crop plants. Trap crops could also be used to lure insects away from other crops. 

In "Host Plant Resistance to Pests" (P. A. Hedin, ed.) 185-196. Symposium Series No. 62, Amer. Chem. Soc., Washington, 1977. 

In summary, then, whether acting alone or in conjunction with other chemicals, the unique and potent physiological, biochemical, and morphological effects Induced by the phytoecdysones confers an integral role for them in host plant resistance. 

Kubo, I. Nakanlshl, K. Host Plant Resistance to Pests, ACS Symposium Series 62, American Chemical Society, Washington, D.C., 1977, 165. 

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