Host plant chemistry

In the family Chrysomelidae (leaf beetles), the presence of defensive glands located on the elytra and on the pronotum has been reported for adults of 4 of the 19 subfamilies. As these beetles are phytophagous, it is not surprising that their host plant chemistry frequently plays a prominent role in their defensive... 

Host plant-specific specialists occur within bacteria, fungi, and herbivores. The interaction of the former two groups is a central topic for plant pathologists. They often find that susceptible and nonsusceptible microbe strains exist. In most cases, it is not known how these microbial specialists achieved a relationship with the host plant chemistry, for example, whether they degrade secondary metabolites or whether they simply toler-... 

Fogleman, J.C., Abril, R. (1990). Ecological and evolutionary importance of host plant chemistry. In Barker, J.S.F., MacIntyre, R.J., Starmer, W.T. (eds.). Ecological and Evolutionary Genetics of Drosophila, 121-143. New York, NY Plenum Press. 

The suitability of arthropods as prey or as hosts of insect predators and parasitoids has been repeatedly shown to be affected by host plant chemistry. The mechanisms by which plant chemical factors mediate suitability of herbivores as hosts (prey) for their natural enemies fall into two broadly overlapping categories. The first involves host plant compounds ingested, and in some cases sequestered, by the herbivores that are toxic or distasteful to parasitoids or predators (4-6). Such compounds may be active as repellents or as acute or chronic toxins ( 5, 7-10). The second category involves the nutritional quality of the host plant mediating herbivore utilization by parasitoids and predators (11-14). Changes in herbivore size due to host plant effects, for example, have been associated with differences in size and sex ratio of parasitoids (13, 12), and differences in functional responses of parasitoids and predators (16). 

One additional factor that comes into play in the overall chemistry of the communication system relates to chemical signals from host plants that can override the photoperiodic control of phermone production. With the com earworm, it was found that a volatile chemical signal from com silk, probably ethylene, was required by wild insects for stimulation of pheromone production (33). This signal probably interacts with controls on the photoperiodic release of PBAN. 

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... 

An insect host s exposure to parasites and predators may be increased by variable plant defenses in three ways. First, by restricting feeding activity to certain tissue types or portions of the host plant, the position of insect hosts becomes more predictable. Parasites (24,46,42) or predators (48) able to recognize physical plant traits such as tissue color or form, or those capable of employing the unique chemistry of the preferred tissues as cues (47,49) would be able to locate their hosts more readily by focusing their search on these traits. 

Each step in the process of host-plant selection may be mediated by plant components. Both the secondary chemistry and nutritional value play a major role in the suitability of the host. 

Chapters in this volume consider how plants use chemicals to defend themselves from insect herbivores the complexity of floral odors that mediate insect pollination tritrophic interactions of plants, herbivores, and parasitoids, and the chemical cues that parasitoids use to find their herbivore hosts the semiochemically mediated behaviors of mites pheromone communication in spiders and cockroaches the ecological dependence of tiger moths on the chemistry of their host plants and the selective forces that shape the pheromone communication channel of moths. 

All of the alkaloids shown in Figure 23-1 are substances with very pronounced physiological action. Indeed, alkaloids in general have been used and abused for centuries as medicinals, drugs, and poisons. However, only in this century have their structures become known, and we are still a long way from understanding the chemistry that leads to their pronounced physiological effects. It is not even understood what function, if any, these compounds have in the host plant. 

Ehrlich, P.R. and Murphy, D.D., Plant chemistry and host range in insect herbivores, Ecology, 69, 908, 1988. 

Chemical factors are also involved in the resistance of plants to disease and in the competitive ability of a plant to survive within a community of plants. Plant stress may also generate a chemical response giving rise to compounds known as the phytoalexins, the nature of which will depend on the chemistry of the host plant (18, 19). Such response to injury or infection is of great Interest because it has stimulated investigations of the nature of the bloregulatory processes involved. 

Most insects are herbivores, and adaptation to host plants and their chemistry is often very close and complex 1,4,10,14,15,28-33, 494-496,503). Whereas insects rely on plants for food, many plants need insects for pollination and seed dispersal. In the latter context we often find that plants attract insects by chemical means (colors, fragrances, sugars, amino acids). At the same time, other secondary metabolites are employed to discourage the feeding on flowers and seeds. 

Insect herbivores can be divided into two large groups whose strategies with respect to the plant s defense chemistry differ substantially (75). The polyphagous species can exploit a wide range of host plants, whereas the mono-/oligophagous insects are often specialized on one or a small number of (often systematically related) hosts. 

In contrast, mono- and oligophagous species often select their host plants with respect to the composition of the nutrients and secondary metabolites present. For these specialists the originally noxious defense compounds are often attractive feeding and oviposition stimulants. These insects either tolerate the natural products or, more often, actively sequester and exploit them for their own defense against predators or for other purposes 1,4,10-12,14-17,28,31,33,494-496). These observations seem to contradict the first statement, that secondary metabolites are primarily defense compounds, and a number of renowned authors have fallen into this logical pit, such as Mothes 35) and Robinson 505). However, these specialized insects are exceptions to the general rule. For these specialists, the defense chemistry of the host plant is usually not toxic, but they are susceptible to the toxicity of natural toxins from non-host plants 32). As compared to the enormous number of potential herbivores, the number of adapted monophagous species is usually very small for a particular plant species. 

Plants that defend themselves effectively constitute an ecological niche almost devoid of herbivores and pathogens. It is not surprising that during evolution a number of organisms evolved which have specialized on a particular host plant species and found ways to tolerate, or even to exploit, the defense chemistry of their hosts 4,10-22). As compared to the huge number of potential enemies, the number of adapted specialists is usually small, and in general a status quo or equilibrium can be observed between the specialists (or parasites) and their hosts. A specialist is not well advised to kill its host, since this would destroy its own resources a mutualism is more productive for survival. 

On the other hand, microorganisms and herbivores rely on plants as a food source. Since both have survived, there must be mechanisms of adaptations toward the defensive chemistry of plants. Many herbivores have evolved strategies to avoid the extremely toxic plants and prefer the less toxic ones. In addition, many herbivores have potent mechanisms to detoxify xenobiotics, which allows the exploitation of at least the less toxic plants. In insects, many specialists evolved that are adapted to the defense chemicals of their host plant, in that they accumulate these compounds and exploit them for their own defense. Alkaloids obviously function as defense molecules against insect predators in the examples studied, and this is further support for the hypothesis that the same compound also serves for chemical defense in the host plant. 

Insects can also manipulate plant chemistry for purposes other than nutrition. For example, Pontania spp. sawflies increase the tannin content preferentially at the outer gall regions for defensive purposes,and the gall wasp Anistrophus rufis modifies the pattern of volatile terpenes emitted hy Asteraceae host plants to generate sex pheromone-like substances that serve as localization cues for male wasps. ... 

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