The Evolution of Plant Defense
5.1 Introduction
All living organisms are a potential source of food. It seems plausible therefore that defense mechanisms in autotrophic organisms would have arisen soon after the origin of heterotrophy. As a result, the appearance of defense mechanisms against parasitism and predation probably pre-dates the move of green plants to the land . Although the defenses that arose initially are not necessarily those existing today, as we have seen in Chapter 2, the array of defenses available to plants is staggering. Some of these defenses are constitutive, that is, always expressed in the plant, while others are induced following attack.
Many of the defenses present in plants are active against more than one attacker. Indeed, many of the individual defense components play other roles in the plant that might originally have been, and perhaps still are, more important than their role in defense against attackers. Thus, defenses might be effective against pathogens and insect herbivores, but might also be important in plant responses to abiotic stress. For example, a waxy plant surface, capable of deterring fungal invasion or insect attack, might have originally evolved in response to environmental pressures such as drought. Other defenses exploit processes that occur during normal plant development. Deposition of hydroxyproline-rich glycoproteins might strengthen cell walls against internal mechanical stresses during morphogenesis , but their involvement in plant defense might be the result of secondary linking of their induction to stress signals arising from damage caused by fungal pathogens . It seems plausible, therefore, that if any plant defense component is used against more than one stressor, and also has a role in plant growth and development, that component is likely to be subject to multiple selective pressures, of which a particular attacker might only be a minor part.
5.2 Hypotheses of plant defense
For many decades, plant biologists have been intrigued by the interactions between plants and herbivores, and in particular by the fact that plants seem very well defended against