Structural defense mechanisms of pepper and cactus against anthracnose pathogens

Abstract

Pepper and cactus were used to study the structural defense responses of host plant against anthracnose pathogen. Fruit tissues of pepper plants, Capsicum annuum cv. Jejujaerae (susceptible) and C. baccatum cv. PBC80 (resistant) inoculated with Colletotrichum gloeosporioides, were examined to compare cytological differences between compatible and incompatible interactions of peppers. The susceptible pepper showed extensive colonization resulting in degeneration of cytoplasm, disruption of chloroplasts, and dissolution of host cell walls, while the resistant pepper showed resistant responses characterized by cuticle thickening, hypersensitive cell death, and formation of a periderm-like boundary layer. Stem discs of two-year-old cactus species, Cereus tetragonus (susceptible) and C. peruvianus (resistant), were inoculated on the center (pith) with the mycelium of Glomerella cingulata. The susceptible cactus was extensively colonized, whereas colonization was limited to a small area in the resistant cactus in which an abscission layer (AL) was prominently formed in parenchyma internal to the inoculation site. Cell division was noted 2-4 days after treatment, forming the AL consisting of several cells 7 days after treatment. After 10 days, the outer layers were sloughed off from the inner layers. Ethanol extracts of the fungal culture also stimulated AL formation. Our results suggest that the AL formation is induced by fungal metabolites, and that it serves as a histological barriers in the resistant cactus against the anthracnose pathogen.