Plant toxins Secondary metabolites

While poisonous plants on grazing lands have a significant impact on livestock production throughout the world, the natural toxins (secondary metabolites) in the plant may have multiple and diverse functions, not only for the plant world but also for the benefit of mankind. Many current pharmaceuticals have been chemically optimized from natural toxins of plant origin. New plant compounds and familiar compounds with renewed interest, e.g., nutraceuticals, herbal preparations, nutritional supplements, etc, are increasingly finding their value in human nutrition and health. 

In order for allelochemicals to enter the body of a herbivore, absorption must occur across the gut lining. Curtailing the initial absorption of dietary allelochemicals may be a herbivore s first line of defense against plant toxins. Studies have citied the lack of absorption or metabolism of lipophilic plant secondary metabolites (i.e., terpenes), conducive to phase I or II detoxification, in the gut of terrestrial herbivores rather these compounds are excreted unchanged in the feces (Marsh et al. 2006b). While physical barriers or surfactants have been used to explain this limited adsorption in both marine and terrestrial herbivores (Lehane 1997 Barbehenn and Martin 1998 Barbehenn 2001 for review of marine herbivores, see Targett and Arnold 2001), active efflux of plant allelochemicals out of enterocytes into the gut lumen has received limited attention until now. 

Since a detailed review of antibiotics is beyond the scope of this chapter, we focus on the recent works on microbial secondary metabolites toxic to animals. In addition, there are many reviews on herbicides produced by microbes3-5 and toxins of plant pathogenic fungi.6 7... 

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). 

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 produce a large diversity of natural products, the so-called secondary metabolites. These are of great importance for the plant for its interaction with the environment due to their roles as pollinator attractants, for symbiosis and for defence against attacks by microorganisms, other plants or animals. Moreover, they are economically important to man as a source of pharmaceuticals, flavours, fragrances, insecticides, dyes, food additives, toxins, etc. Structures of an estimated 200,000 natural products have been elucidated [1] and each year approximately 4,000-5,000 novel compounds are characterised. 

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