Defense compounds, plants

Perhaps the most interesting arthropodan defensive compounds from the point of view of structural diversity are the alkaloids. While alkaloids had long been believed to arise only as a consequence of plant secondary metabolism, it has become apparent over the last few decades that arthropods are both prolific and innovative alkaloid chemists. The millipede Polyzonium rosalbum, once thought to secrete camphor (20), in fact gives off a camphoraceous/earthy aroma produced by the spirocyclic isoprenoid imine polyzonimine (21). 

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. 

As far as we know, one species, or a few closely related ones, often have a monopoly on a particular compound. The tobacco plant (Nicotiana tabacum), for example, synthesizes nicotine to defend itself from attack by herbivores (animals that feed on plants) and as a convenient way of storing nitrogen temporarily. Neither roses, elephants, nor starfish contain nicotine. In fact, apart from a few of tobacco s close relatives, we know of no other living organism that makes nicotine. In the same way, other plants have devised their own defensive compounds for protection from herbivores. 

Many aposematic lepidopteran insects are associated with poisonous plants and sequester the toxins from their host instead of, or in some cases in addition to, biosynthesizing their own defensive compounds. 

In birds, the trigeminal nerve appears to play a role in food selection. Starlings more easily accepted commercial feed treated with otherwise avoided coniferyl benzoate after bilateral section of the ophthalmic branch of the trigeminal nerve. Therefore, the trigeminal nerve may help to protect the animal by detecting plant defense compounds. Many of these compounds are astringent or irritating (Jakubas and Mason, 1991). 

How do herbivores avoid or deal with ill effects of plant defense compounds. ... 

To the bitter end distribution of defense compounds within plants... 

How much a mammal eats of a given plant often depends on the levels of different classes of chemical constituent, notably nutrients and plant secondary metabolites. As in birds, it is not the plant defense compounds alone, but rather complex balances between nitrogen and carbohydrate contents, levels of defense compounds, and fiber that determine palatability. 

Some animals process plants to reduce defense compounds even before eating. The meadow vole M. pennsylvanicus cuts winter branches of white spruce, Norway spruce, white pine, and Norway pine and leaves them on the snow for 2-3 days before eating them. This reduces the levels of condensed tannins and other phenolics by one half, to their summer levels. A high level of protein 12%) and reduced phenolics (1.5% of dry matter) now render the food acceptable. It is not clear how the phenolics are being lost, possibly by polymerization or oxidation (Roy and Bergeron, 1990b). 

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