Induced resistance in plants and the role of pathogenesis-related proteins

Abstract

The nature of induced resistance Resistance, according to Agrios (1988) is the ability of an organism to exclude or overcome, completely or in some degree, the effect of a pathogen or other damaging factor. Disease resistance in plants is manifested by limited symptoms, reflecting the inability of the pathogen to grow or multiply and spread, and often takes the form of a hypersensitive reaction (HR), in which the pathogen remains confined to necrotic lesions near the site of infection. Induced resistance is the phenomenonthat a plant, once appropriately stimulated, exhibits an enhanced resistance upon 'challenge' inoculation with a pathogen. Although induced resistance has been attracting attention recently (Ryals et al., 1994; Hammerschmidt and Kuc, 1995), the first systematic enquiry into induced resistance was made by Ross (1961a,b). He observed that the inducible resistance response to tobacco mosaic virus (TMV) in N gene-containing, hypersensitively reacting tobacco was not confined to the immediate vicinity of the resulting local necrotic lesions, but extended to other plant parts. A ring of tissue around the developing lesions became fully refractory to subsequent infection (localized acquired resistance; Ross, 1961a), whereas challenge inoculation of distant tissues resulted in much smaller, and occasionally fewer, lesions (systemic acquired resistance (SAR); Ross, 1961b) than in non-induced plants. Even leaves thatweremere initials at the time of the primary inoculation became induced, suggesting that as a result of the initial infection, a signal was generated, transported and propagated, that primed the plant to respond more effectively to subsequent infection (Bozarth and Ross, 1964). Treatments that influenced lesion size after primary infection had similar effects on lesions developing upon challenge inoculation (Ross, 1966), leading to the conclusion that the mechanisms responsible for resistance expression were the same under both conditions. Only upon challenge inoculation, defense mechanisms appeared to be expressed earlier and to a greater extent (De Laat and Van Loon, 1983; Dean and Kuc, 1987).