Chemically induced defenses

Keywords Marine chemical defense Activated defense Induced defense Plankton Chemical ecology... 

UV-C technology is widely used as an alternative to chemical sterilization and microorganism reduction in food products (Lamikanra 2002 Fan and others 2008). Ultraviolet light also induces biological stress in plants and defense mechanisms in plant tissues with the consequent production of phytochemical compounds (Lee and Kader 2000). Phytoalexin accumulation could be accompanied by other inducible defenses such as cell-wall modifications, defense enzymes, and antioxidant activity, which have been reported with health benefits (Gonzalez-Aguilar and others 2007). It is well documented that UV-C irradiation has an effect in secondary metabolism. 

Graeve M, Dauby P, Scailteur Y (2001) Combined lipid, fatty acid and digestive tract content analyses a penetrating approach to estimate feeding modes of Antarctic amphipods. Polar Biol 24 853-862 Harvell CD (1990) The ecology and evolution of inducible defenses. Q Rev Biol 65 323-340 Hay ME (1996) Marine chemical ecology what s known and what s next J Exp Mar Biol Ecol 200 103-134... 

Hansson, LA, Gustafsson S, Rengefors K, Bomark L (2007) Cyanobacterial chemical warfare affects zooplankton community composition. Freshwater Biol 52 1290-1301 Harvell CD (1986) The ecology and evolution of inducible defenses in a marine bryozoan cues, costs, and consequences. Am Nat 128 810-823... 

In some cases, browsing does not appear to lead to induced defenses. Two species of northern European birch (B. pendula and B. pubescens) do not seem to respond to moose browsing with increased chemical defenses (Danell et ah, 1985). After experimental defoliation, the Emory oak of southwestern North America, Quercus emoryi, produces in its regrown leaves 2.5 times as much hydrolyzable tannins than in mature leaves, but lower amounts of condensed tannins. Refoliated branches suffered more, not less herbivory. New leaves have more protein, more water, and are tender. These properties may override any effects of induced chemical defenses, at least for the current growing season (Eaeth, 1992), but the induced compounds may still protect a plant later on, especially in winter. 

This type of correlative approach is widespread, as only a few marine studies involving inducible defenses (and none with mobile invertebrates) have directly demonstrated that the induction results in a decrease in the susceptibility of the organism to predation.71,72 Statistically significant differences in shell thickness or concentrations of defensive chemicals may or may not meaningfully affect predator preferences in ecologically relevant field situations. For chemical defenses, compound dose-response relationships may be nonlinear, and threshold levels of defense could be sufficient to deter predators so that further induction has little additional benefit. Thus, future studies should focus on directly demonstrating whether an induced response reduces predation on prey organisms. 

Environmental conditions may also affect chemical defense concentrations indirectly by changing the intensity of grazing, which can alter defense concentrations in algae with inducible defenses. Evidence for the direct effects of environmental factors on secondary metabolite concentrations are discussed below. 

Many intertidal and shallow subtidal habitats are highly variable over space or time. Consequently, the concentrations of chemical defenses that are appropriate at one place or time may be inappropriate at another. A strategy for dealing with this variability is the detection of an environmental cue that can be correlated with an appropriate level of defense for that environment at that time. One of the best examples of environmental cues causing changes in chemical defense concentrations are herbivore-induced defenses in which chemical levels are increased in response to an attack by a consumer.136 137 Induced defenses require that herbivory be variable and unpredictable, that the inducing cues produce effective increases in defenses, and that the induction is cost-effective and rapid enough to deter further consumption.140... 

While there are only a few cases of induction of chemical defenses among seaweeds, the phenomenon is very common among terrestrial plants." This discrepancy in the prevalence of inducible defenses in terrestrial vs. marine plants could be due to the different biologies of vascular terrestrial plants vs. nonvascular seaweeds (e.g., the induction stimulus from localized damage may not be efficiently translocated in seaweeds133), complex interactions between grazing damage and environmental conditions,161 or herbivore taxa studied,5 but the lesser amount of research on seaweeds relative to terrestrial plants may also explain much of the disparity. 

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