Plant defensive compounds and their molecular targets

Higher plants are sessile and are consumed by motile organisms, namely other eukaryotes and prokaryotes. Plants defend themselves by physical barriers including cell walls at the cellular level, by the waxy cuticle of leaves and by bark and thorns at the macroscopic level. Plants also defend themselves from fungal and bacterial pathogens and animal herbivores by elaborating a variety of bioactive secondary metabolites and defensive proteins. There may be as many as 100,000 different kinds of plant defensive compounds of which about 30,000 have been isolated and structurally characterized. Biochemical targets have been determined in vitro or in vivo for some thousands of the defensive compounds isolated to date. 

The word target is being used rather broadly and loosely here to encompass the molecular sites of interaction demonstrated for such compounds. However, the demonstrated binding of a plant compound to a protein in vitro or in vivo does not necessarily mean that this particular interaction is actually the critical site of action of the defensive compound. Further, a particular defensive compound may have multiple molecular sites of action and may well have synergistic effects with other such compounds. This book is concerned with the biochemical targets of plant defensive compounds. 

Plants provide a bulk supply of carbohydrate (typically as seed or tuber starch) to support the global human population that now totals 6 billion as compared to an estimated 1 million 

Herbal medicinal traditions can be traced back to our primate forebears. Thus, parasite-infected chimpanzees make recourse to particular plants, which they evidently associate with symptomatic relief. Human cultures in general have accumulated medicinal protocols based on use of plants, major traditions including Chinese medicine and Indian Ayurvedic herbal medicine. As detailed in this book, in some instances, specific bioactive substances from medicinal plants (or derivatives of such compounds) have found application in conventional medicine. Thus, the cardiotonic cardiac glycoside sodium pump (Na+, K+-ATPase) inhibitors derived from the initial use for cardiac insufficiency of digitalis (dried leaves of the foxglove, Digitalis purpuremri). 

Determining the molecular sites of action of bioactive medicinal plant constituents is clearly important for establishing the chemical and physiological basis for herbal medicinal efficacy, for quality control of commercial herbal preparations and for the discovery of lead compounds for synthetic (or semi-synthetic) pharmaceutical development. Of course, it must be recognized that medicinal plant efficacy may derive from complex synergistic effects or even from quasi-placebo effects connected with the taste, mild effects and appearance of the preparation. While recognizing these possible holistic complications, in order to find out how such preparations work, it is clearly important to initially isolate, structurally characterize and define the biochemical targets of plant bioactive substances. 

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Plant defensive compounds and their molecular targets... 

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