Evolution of Plant Defense

The array of physical and chemical defenses that plants exhibit reflects a diverse set of enemies and variable abiotic conditions. In a general sense, these defenses exerted selective pressure on mammalian herbivores, with the result that many developed mouthparts and digestive tracks facilitating use of particular plant types that were abundant enough to support such animals (e.g., grasses, woody plants), and on insect herbivores, with the result that many became feeding specialists in handling their host plant's toxins and chemical taxonomists adept at finding their host plant in a sea of green plants.

But the role of such coevolution between plants and herbivores, especially insect herbivores, only provides a partial explanation for the defensive pattern of plants. For example, the array of constitutive chemical defenses of terrestrial plants reflects in part the biochemical evolution of early land plants and, thus, the basic problems those plants encountered. A major problem was reducing water loss. Plants evolved cell walls of layered cellulose encrusted with lignin, which necessitated an enzyme that happened to provide other substances needed for biosynthesis of flavonoids. Flavonoids help screen against ultraviolet light, especially important for early land plants when the ozone layer was less developed. Flavonoids also have a variety of negative effects on herbivores (e.g., deter feeding, slow growth). Flavonoids are derived from the shikimic acid and malonic acid pathways; such combination of two pathways probably led to a rapid radiation in diversity of chemical products.

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