Plant defense strategy

The production of toxins is only one aspect of plant defense strategy. As a result of the persistent battle of plants and herbivores, many optimized phenotypes have evolved, such as the preferential accumulation of alkaloids in tissues with a pattern that is consistent with predictions of optimal defense theory,65 i.e., the defense metabolites are allocated preferentially to tissues with a high probability of attack.66 The inducibility of pathways leading to plant secondary compounds as a strategy to minimize the costs of plant defense is a result of permanent optimization. One of a few examples of inducible alkaloid biosynthesis is the different Nicotiana species that exhibit dramatic wound-induced increases of nicotine, nomicotine, or anabasine.67... 

Anti-Juvenile Hormones. A plant defensive strategy targeted to disruption of the endocrine regulation of the early larval stages of metamorphosis was revealed with the discovery of anti-juvenile... 

Other aspects of implementing indirect volatile defenses, as recently discussed in several excellent reviews [17, 94, 95], will include the assessment of timing and synergy of volatile emissions with direct defense responses of the herbivore-damaged plant, especially if indirect defense strategies will be integrated with toxin-based pest control strategies. 

A better understanding of the role of phytoalexins in plant defenses and of the mechanisms of induced resistance may potentially open a powerful new approach to the control of insect pests of cultivated plants. If indeed, in light of the hypothesis of optimal defense strategies (3), a post-attack response is a more efficient line of defense than the attack-independent accumulation of allelochemics, the exploitation of phytoalexin-producing mechanisms may represent a fertile field for future investigations. Several uses of induced resistance may be conceived. Four of these approaches are briefly discussed. 

Phytoecdysones, due to their effects on the behavior and the development of certain species of Insects, appear to be components of multichemical defensive strategies found In some Insect-resistant plant species ... 

The role of phytoecdysones in this scheme is that of an Important component in a rather complex defensive strategy of some plants. Their presence in plants probably serves a limited, yet important, protective role. For example, cotton bolls bred with several ppm of ponasterone A would very likely be resistant to attack by pink bollworm. 

McArthur C, Elagerman A, Robbins CT. 1991. Physiological strategies of mammalian herbivores against plant defenses. In Palo RT, Robbins CT, Eds. Plant Defences against Mammalian Herbivory. Boca, Raton FL CRC Press, pp. 103-114. 

Figure 9.1. Different defense strategies of Nicotiana plants that evolved in response to herbivore attack. Besides a specialized germination-behavior that efficiently reduces the over-all number of potential herbivores, Nicotiana plants evolved the inducible nicotine synthesis as a direct defense and the inducible emission of volatiles to attract parasitoids of the herbivore as an indirect defense. A specialized defense mechanism is triggered in response to attack by the herbivore Mctduca sexta, adapted to Nicotiana plants. Attack results in an ethylene burst, which down regulates nicotine accumulation and results in a fundamental transcriptional re-organization within the plant.

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