ABSTRACT
Immunity in higher plants consists of an immune state of
systemic acquired resistance (SAR)
that follows a local infection by pathogenic microorganisms that leads to lesions with death of host cells. SAR encompasses a broad spectrum of bacterial, viral, and fungal agents, as well as the infective pathogen. Various SAR genes encode numerous microbicidal proteins induced by endogenous chemicals that include salicylic acid, which combines with a catalase to increase H
O
, which may facilitate defense. (Figure 26.1)
Phytoimmunity
encompasses both active and passive immune-like phenomena in plants. Plant substances active in phytoimmunity include phytonicides and phytoalexins. Plant resistance to many diseases is associated with the presence of antibiotic substances in plant tissues. Antibiotic substances are inherent in both susceptible and resistant varieties of plant species. They may be constitutional inhibitors present in a plant before contact with a parasite or induced antibiotic substances that arise in plants after contact with a parasite. Defense reactions in plants that are associated with the formation and conversion of antibiotic substances include reactions to wounding and the necrotic reaction. Plant resistance to a specific disease is determined by the various antibiotic substances they contain and the synergistic action of these agents with differing roles in phytoimmunity. Plant varieties differ in the quantity of antibiotic substances present in intact tissues and the intensity of their generation in response to infection. They also differ in the nature of subsequent conversions that may produce a marked increase in antibiotic activity.
