Chemical defence compounds

Nicotiana alkaloids, which serve as chemical defence compounds, are synthesized in the roots and are transported to other plant organs, such as aerial parts, via the xylem. These alkaloids accumulate in vacuoles. PMT and A622 oxidoreductase are strongly expressed in the endodermis and outer cortex cells of tobacco root tips and to a lesser degree in other parts of the cortex and parenchyma cells surrounding the xylem (Shoji et ah, 2002). The localization of nicotine biosynthesis in the parenchyma cells surrounding the xylem may aid the loading of the xylem with nicotine. 

Sparteine is a drug with antiarrhythmic properties. It has been deduced from pharmacological and electrophysiological studies that sparteine acts via a reduction of the Na inward current, e.g. during the upstroke of cardiac action potentials. This process was elucidated by the determination of sodium currents, in isolated muscle fibbers, by loose patch clamp measurements [236]. The IC50 value for half-maximal blocking of the sodium current was 168.8 )iM, which is in accordance witli the antiarrhythmic activity of sparteine. The importance of sparteine on Na channels inhibition was further analysed because of its potencial strong interference in neuronal transmission, particularly in herbivores. This emphasizes the role of sparteine as a chemical defence compound for the plants that produce it. 

Alkaloids as Chemical Defence Compounds Against Herbivores and Predators... 

Typical of other sesquiterpenoid and diterpenoid-based isonitriles are 136 and 137 from Halchondria spp and 138 and 139 from Adocia spp . Presumably there are chemical defence compounds. 

The first volume ends with a chapter by G. Pohnert on chemical defence in the marine environment. Defense compounds, which can be regarded as allomones, are often, but not always, more complex than other semiochemicals and may have unique modes of action. The biological mechanisms are not always easy to unravel, which is shown by some of examples. The reader may be tempted to compare the chemical complexity with that of terrestrial insect defence, which can be found in the second volume chapter by D. Daloze and J.-C. Braekman. Insects thus do not only produce interesting pheromones, but also complex allelochemicals for their own protection. 

Defensive Compounds. Many data on chemical defences of rove beetles have been compiled by Dettner [118]. Recent taxonomic compilations indicate that this beetle family with its omaliine, oxyteline, tachyporine and staphylinine subgroups consists of about 60,000 species, worldwide [ 119]. Within all four groups, chemical defensive systems evolved independently, because free living rove bee-... 

In some species, however, e.g. ash, Fraxinus excelsior, cells of the traumatic axial parenchyma of the compartmentalization wall 4 may show no evidence of cell wall alterations, yet appear to act normally as a functional barrier to decay (Pearce, R.B., unpublished data). It is to be presumed that the spread of decay fungi is arrested either by chemical defences or by environmental constraints (cf. 26-28) in such species. Clearly, a contribution may be made by these defences in suberizing species also phytoalexin-like antifungal compounds have been detected in association with a suberized wall 4 barrier in Acer saccharinum (42). More work will be required to elucidate the long-term effectiveness of the various mechanisms maintaining the function of these barrier walls. 

J.R. Hancock, J.M. McAndless and R.P. Hicken, A solid adsorbent based system for the sampling and analysis of organic compounds in air an application to compounds of chemical defence interest, J. Chromatogr. Sci., 29, 40-45 (1991). 

J.R. Hancock and G.R. Peters, Retention index monitoring of compounds of chemical defence interest using thermal desorption gas chromatography, J. Chromatogr., 538, 249-257 (1991). 

This group of alkaloids is found in a wide range of families, centred around the Asteraceae, Boraginaceae and Fabaceae (Hartmann and Witte, 1995 Hartmann, 2007). PAs are metabolically activated in the liver of herbivores and can then alkylate DNA and proteins, leading to mutations and even cancer. The occurrence of PAs in many Senecio species accounts for the high toxicity of these plants. PAs function as defence compounds against many herbivores however, a number of specialized insects are known which store and utilize the dietary defence chemicals (for a review, see Wink, 1993 Hartmann and Witte, 1995). 

Plate 5 Costs of chemical defence and signal compounds. Abbreviations ATP, adenosine triphosphate NADPH2, nicotinamide adenine dinucleotide phosphate (reduced form). (Fig. 1.6, p. 14)... 

Active secondary metabolites are often deterrent to more than one predator. Algal secondary metabolites have been shown to deter both sea urchins and fish [67]. Compounds from a Bahamian sponge deter feeding among a natural assemblage of fish [68]. Ascidlans have also been shown to contain a chemical defence against reef fish in field assays [61]. 

Moreover, the observation of an interesting antibiotic activity for some Lactarius extracts (3) stimulated the search of new biologically active compounds among those isolated from these mushrooms. In fact, simple bioassays (4) led to the identification of new products with antimicrobial, cytotoxic, antifeedant and other interesting activities. No less important was the observation that some species seem to withstand attack from parasites such as snails and insects better than others. In fact, it has been shown that resistant species are armed with a chemical defence system which protects the mushrooms from predators and invaders. 

Harborne, J. B. Role of secondary metabolites in chemical defence mechanisms in plants. In Bioactive Compounds from Plants (Chadwick, D. J., Marsh, J., Eds). John Wiley Sons Chichester,... 

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