Host plant allelochemics

It has been shown that in insects cytochrome P450 monooxygenases are involved in metabolism of host plant allelochemicals by Lepidoptera larvae. Expression of the CYP6B1 alleles in lepidopteran cell lines has demonstrated that both P450 isoforms metabolize substantial amounts of linear furanocoumarins, such as xanthotoxin and bergapten, but not angular furanocoumarins, such as angelicin (4) and sphondin [249]. 

Kennedy, G. G., R. R. Farrar, Jr, and M. R. Riskallah. 1987. Induced tolerance in Heliothis zea neonates to host plant allelochemicals and carbaryl following incubation of eggs on foliage of Lycopersicon hirsutum f. glabratum. Oecologia 73 615-620,... 

Induction of MFO activity in insects to synthetic insecticides was discovered nearly two decades ago. This phenomenon as mediated by natural plant products has received increased attention in the last few years. Induction refers to the temporarily accelerated production of enzyme protein. A large number of natural plant chemicals are known to induce the MFO system (Brattsten et al., 1977) and host plant allelochemic induction of MFO activity may mediate resistance to synthetic insecticides as well (Yu etal., 1979). There can also be genetic bases to cross-resistance between pesticides and plant defenses (Gould et al., 1982), thus posing another significant consideration in breeding host plant resistance. 

The plant cell is a basic unit for germination, growth and development of plants. The allelochemicals first come in contact with the cell and then allelopathic interactions take place. Some allelochemicals have broad-spectrum activity that extends to the tissues of host plants, where their effects may be either beneficial or deleterious to plant germination, growth, development or yield. Hence, to understand the mechanisms of such intercations interactions the study of the cell and its various processes is very necessary. Therefore, this book has been been prepared (a) to make available all methods for such studies and (b) scientists can understand the scope of allelopathic research in relation to the cell. Hence, we have explained and discussed various techniques to study cell processes etc. 

Yu SJ (1983) Induction of detoxifying enzymes by allelochemicals and host plants in the fall armyworm. Pestic Biochem Physiol 19 330-336 Yu SJ (1984) Interactions of allelochemicals with detoxification enzymes of insecticide-susceptible and resistant fall armyworms. Pestic Biochem Physiol 22 60-68 Yu SJ (1986) Consequences of induced foreign compound-metabolizing enzymes in insects. In Brattsten LB, Ahmad S (eds) Molecular aspects of insect-plant associations. Plenum, New York, pp 153-174... 

Most parasites have to survive prolonged periods of time apart from the host plant. Consequently, the formation of resting propagules, such as sclerotia, constitutes a critical part of the parasite s life cycle. Several observations indicate that formation of sclerotia may be stimulated by allelochemicals (, 9). Brandt and Reese (10) concluded that Verticillium dahliae produces a diffusible morphogenetic factor that stimulates production of microsclerotia. When low concentrations of the diffusible factor were added to cultures of the pathogen, the hyphae swelled and became constricted, septation was increased, and cell walls became thickened. 

Nutrient-Allelochemical Interactions in Host Plant Resistance... 

Vinson, S. B., Elzen, G. W. and Williams, H. J. (1987). The influence of volatile plant allelochemicals on the third trophic level (parasitoids) and their herbivorous hosts. In Insect-Plants, eds. V. Labeyerie, G. Fabres and D. Lachaise, pp. 109-114. 

Abstract. Allelochemals induced in mycorrhizal plants play an important role in disease resistance. Mycorrhizal associations are the most important symbiosis systems in terrestrial ecosystems and offer many benefits to die host plant. Arbuscular mycorrhizal associations can reduce damage caused by soil and root - borne pathogens. 

Yu S. J. (1983) Induction of detoxifying enzymes by allelochemicals and host plants in the fall armyworm. Pest. Biochem. Physiol. 19, 330-336. 

Ingested alkaloids can also be utilized for defense even if these compounds are still present in the intestine. For example, grasshoppers and the larvae of many species of butterflies and moths regurgitate when tactually disturbed. The enteric fluid, fortified with alkaloids such as PAs, can function as a highly effective topical deterrent against small invertebrate predators.33 Thus the plant has adapted its host plant s allelochemicals to function as a secretion that can be accurately applied in aggressive confrontations. 

In animals, we can observe the analogous situation in that many insects and other invertebrates (especially those which are sessile and unprotected by armor), but also some vertebrates, store secondary metabolites for their defense which are often similar in structure to plant allelochemicals 1,4,12,16,17,28-30,494-496,503). In many instances, the animals have obtained the toxins from their host plants 4,12,15,17,27-33). Hardly any zoologist or ecologist doubts that the principal function of these secondary metabolites (which are often termed toxins in this context) in animals is that of defense against predators or microorganisms 1,17,28,494-496). 

The quantity and quality of food in the diet on which insects are reared may affect their size and survival capacity. Variegated cutworm larvae fed peppermint leaves were more tolerant of the insecticides carbaryl, acephate, methomyl, and malathion than larvae fed snap bean leaves. Increased tolerance for carbaryl and methomyl was also observed in larvae of the alfalfa looper and cabbage looper when they were fed peppermint plants instead of their favored host plants alfalfa and broccoli. Furthermore, fall armyworm larvae fed corn leaves became less susceptible to the insecticides methomyl, acephate, methamido-phos, diazinon, trichlorfon, monocrotophos, permethrin, and cypermethrin than those fed soybean leaves. Although nutrition may play some role in these cases, it was found that enhanced insecticide tolerance caused by these host plants was mainly due to plant allelochemicals, which induced detoxification enzymes in the insects (Yu, 1986). 

Yu, S.J. and Hsu, E.L., Induction of hydrolases by allelochemicals and host plants in fall army worms (Lepidoptera Noctuidae) larvae, Environ. Entomoi, 14, 512,1985. 

Allelochemicals are most notable in examples of host-plant and nonhost-plant selection, a process that is vital for phytophagous pest insects to locate food plants and avoid unsuitable or toxic hosts. Aphids have taxonomi-cally unrelated summer and winter hosts. The bird-cherry-oat aphid, Rhopalosiphum yadi. is able to detect species-specific odors from each of its hosts, the bird-cherry tree. Prunus padus, in winter and summer cereal crops. One such allelochemical from the winter host is methyl... 

In contrast to the leaf surfaces much more is known about the chemistry of the leaf interior, which is probably considerably more complex. These compounds representing nutrients, toxic allelochemicals and allelochemicals with a host plant sign character, influence feeding behavior (see Scriber, Chapter 7). As can be seen from Table 1.1, contact chemoreceptors for all types of compounds have been identified in a variety of insect species. With the exception of the two beetle larvae and the locust, all of the examples are lepidopteran larvae mainly because the two pairs of sensilla styloconica on the galea (maxilla) of these larvae can be investigated easily (Schoonhoven and Dethier, 1966). The relatively small number of contact chemoreceptors (four sensilla with four neurons) and their importance for food plant discrimination (Hanson and Dethier, 1973) make them ideal for the study of contact chemoreception in relation to host plant selection. 

The adaptive value of deterrent receptor cells in preventing toxication by allelochemics in non-host plants seem to be evident. Other examples which, in terms of our present knowledge, are more difficult to explain on the basis of selective value, are deterrent cells sensitive to allelochemics that do not occur in the insects normal habitat (Schoonhoven, 1981). Possible explanations could be (i) a redundancy from earlier phylogeny and, (ii) a still unknown chemical similarity of the used stimulating deterrent with compounds present in the insect s natural environment. 

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