herpotrichoides. Resistant cultivars, which were introduced in 1988 and are now grown on nearly 1,000,000 hectares in the Pacific Northwest United States, have reduced midseason fungicide treatments to roughly half of that needed with susceptible cultivars (Jones et al. 1995). Estimates from 1994 indicate that genetic protection from eyespot disease reduced growers' production costs by $40 per hectare.

Plants with pest-protection properties can inhibit growth, reproduction, or survival of a particular pest or group of pests, or they may tolerate a pest infestation with minimal or acceptable levels of damage. Pest-protected plants that reduce pest populations can exhibit pest-protection characteristics through structural mechanisms. Trichomes on leaf surfaces, for example, present a structural barrier that reduces feeding activity of some insects. Pest-defense systems can also involve intracellular or biochemical mechanisms. These defense mechanisms can work through the action of preformed defensive compounds, and through induced defensive compounds, reactions, and signaling pathways that are triggered specifically or nonspecifically by an invading pest.

To understand the rationale of current and future directions of transgenic breeding for pest-protection and to assess risks of transgenic pest-protected plants relative to those that may be posed by conventional pest-protected plants, this section reviews mechanisms of conventional and transgenic resistance to insects and pathogens.

2.3.1 Natural Pest-protection Mechanisms
Preformed Chemical Defenses

Plants constitutively produce a variety of antimicrobial or insecticidal chemicals that are known or suspected to provide pest-protection (Mansfield 1983; Rosenthal and Berenbaum 1991). The chemicals are often sequestered in specialized cells or expressed in particular organs. Chemicals having antibiotic or suppressive activities against pathogens and insects include saponins, glycoalkaloids, terpenoids, and phenolic compounds. They can have acute or chronic toxic effects and some compounds can have behavioral effects on insects that reduce insect feeding, reproduction, or colonization. The saponin avenacin A-1, for example, is a glycosylated triterpene that is toxic to fungi by perturbing membrane structure and function (Osbourn 1996). It is found in the roots of some cereals. Avenacin A-1 in oats confers resistance to a number of root-infecting fungal pathogens, such as Gaeumannomyces graminis. Like other chemical defenses, avenacin A-1 is effective as an antibiotic in proportion to its accumulation in roots, the inherent sensitivity of the fungus, and the ability of the pathogen to detoxify the compound. Some compounds have relatively broad

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