Defensive molecules

The defensive molecules of plants, often called secondary substances, are probably derived evolu-tionarily from materials with a basic metabolic function. Some continue to serve more than one function, e.g. defense, providing structure and, perhaps, controlling water loss (some plant resins). Unlike molecules that are necessary to basic cellular metabolism, secondary substances are strikingly different 

Based on this type of reasoning, investigators have divided defensive substances into (1) the acute toxins (qualitative defenses) that are present in very low concentrations in plant tissues and which exert their effects on herbivores by interfering with some basic metabolic process such as transmission of nervous impulses, and (2) digestibility-reducing substances (quantitative defenses) that are present in higher concentrations in plant tissues, that act in the gut of the animal to reduce its ability to utilize its food, particularly proteins, and whose effectiveness increases directly with their concentration (Cates and Rhoades, 1977 Feeny, 1970, 1976 McKey, 1974 Rhoades and Cates, 1976). Qualitative defenses 

Animals also defend themselves chemically against predators but this is less widespread than might be expected, probably because large predators on most animals are closely related to their prey, and hence are biochemically much more similar than animals are to plants. Therefore, effective defenses are likely to be autotoxic (Orians and Janzen, 1974). Plants, however, are able to evolve chemicals that attack nerves, muscles, and hormonal systems. 

Many plants release chemicals that inhibit the germination and growth of other plants. Included among them are some of the terpenes responsible for the fragrance of many plants. They may travel from the leaves of plants to the soil as the leaves decompose. Many allelopathic substances are released from the roots of plants and have important effects on soil bacteria and fungi. The role that they play in organizing plant communities in nature is still controversial. 

The structural molecules of the skeletons and shells of invertebrates, which function as physical defenses against predation, are important in marine environments, where they produce carbonate and silicate rocks. Deposition in anaerobic environments has also been the basis for the formation of the extensive deposits of gas and oil that now fuel modern industrial societies. Removal of carbon from the biosphere by organisms to produce carbonate rocks, coal, oil, and hydrocarbon gases has been responsible for the presence of oxygen in the atmosphere of the Earth. Reversal of this process by human consumption of fossil fuels has already produced a detectable increase in atmospheric carbon dioxide. 

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Mollusca Gastropods, Bivalves Lectins, PO, AMPs, molluscan defense molecules, fibrinogen related proteins, cytokines... 

Bloc, A. et al., An invertebrate defense molecule activates membrane conductance in mammalian cells by means of its lectin-like domain, Dev. Comp. Immunol., 26, 35, 2002. 

Host defense peptide hydrophobicity (H) is defined as the proportion of hydrophobic amino acids within a peptide. Typically, these peptides are comprised of >30% hydrophobic residues and this governs the ability of a host defense peptide to partition into the lipid bilayer, an essential requirement for antimicrobial peptide-membrane interactions. Typically, the hydrophobic and hydrophilic amino acids of natural peptides are segregated to create specific regions or domains that allow for optimal interaction with microbial membranes. This likely represents evolutionary optimization to maximize the selectivity of these defense molecules. It has been established that increasing antimicrobial peptide hydrophobicity above a specific threshold correlates... 

These plant defense molecules are thus of interest for a number of reasons, including their potential applications in the discovery of new pharmacological substances, their adaptation to nonproducing species, for example, for protection of crop plants from insect pests delivered either topically or via incorporation into transgenic plants, and as new structural scaffolds for protein engineering approaches. 

As is clear from the preceding discussion, plants express a vast array of toxic peptides in their defense that are interesting as structural models, active compounds in crop protection, and active compounds in pharmaceutical applications. In the plant kingdom the defense mechanisms involved have produced peptides for many kinds of microorganisms and predators, and in future it is possible that for any new problem in bacterial, viral, or fungal infection, a solution can be found on the basis of leads from plant defense molecules. 

It has been suggested that the synthesis of flavones, flavanones, and flavonols could have evolved primarily as chemical messengers, such as defense molecules. Indeed, flavonoids have recently been described as a novel class of hormones. Interestingly, in this regard, the... 

Iwanaga, S. and Kawabata, S. (1998) Evolution and phylogeny of defense molecules associated with innate immunity in horseshoe crab. Front. Biosci. 3, D973-D984. 

Hancock, R., McPhee, J. Salmonella s sensor for host defense molecules. Cell 122 (2005) 320-322. 

Fig. 9.3 The amoebocytes of horseshoe crab. Some small antimicrobial molecules like antimicrobial peptides that have been identified are in small granules, while Factor C and other defense molecules are localized in the large granules. Adapted from (Iwanaga and Lee, 2005), with permission from Editor-in-Chief of the Journal of Biochemistry and Molecular Biology...

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. 

A network of antimicrobial salivary defense includes numerous salivary proteins. Although some defense molecules are present in a rather low concentration in whole sahva, it should be considered that local concentrations of these proteins nearby the mucosal surfaces (mucosal transudate), periodontal sulcus (gingival crevicular fluid), and oral wounds and ulcers (transudate) may be much greater (2). Furthermore, the effects are addable, synergistic, and in many cases reinforced by immune and/or inflammatory reactions (2, 16, 17). 

Moss B, Shisler JL, Xiang Y et al. Immune-defense molecules of molluscum contagiosum virus, a human poxvirus. Trends Microbiol 2000 8 473-477. 

In reaction to this infection, the plant produces defense molecules, which are either constitutive (phytoanticipins) or de novo induced in reaction to the attack (phytoalexins). Nonetheless, pathogenic fungus pathogens have developed detoxification processes of these defense secondary metabolites, which are mediated by their chemical transformation into less toxic compounds. 

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