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Plants come in frequent contact with potentially pathogenic fungi, bacteria, and viruses, yet disease results from relatively few of these exposures. In many cases an encounter leaves no obvious trace of its occurrence and the microbe fails to establish itself due to a lack in activation of pathogenicity functions or to highly effective plant defense mechanisms. Other encounters leave evidence of an intense plant-microbe interaction that results in the arrest of pathogen development after attempted colonization. In these cases plant tissues often display activated defense functions that produce antimicrobial compounds, enzymes, and structural reinforcements that may limit pathogen growth (Dixon and Lamb, 1990). These reactions may also be associated with the HR, a localized region of plant cell death around infection sites. An HR may involve just a single cell or it can produce death of extensive regions of tissue. The …