Involvement of peroxidases in the coffee resistance to orange rust (Hemileia vastatrix)

Coffea arabica plants with the SH5 and SH4SH5 genotypes were inoculated with isolates of the coffee orange rust (Hemileia vastatrix) races II and VI, in order to establish two incompatible interactions (I1 and I2) and a compatible one. Both incompatible interactions were characterized by restricted fungal growth associated with rapid hypersensitive plant cell death, monitored by cell autofluorescence and/or browning. Cytological and biochemical studies were performed to investigate the association of peroxidases (PODs) with coffee resistance to rust. In both incompatible interactions, in contrast with the compatible one, investigations revealed a peak of POD activity prior or at the same time, as the beginning of cell death. During the first peak, the isoenzyme pattern for peroxidases obtained by IEF (isoelectric focusing electrophoresis) showed an increase in activity of anionic and cationic isoenzymes. Cytochemically, POD and H2O2 were localized at the interface between the cuticle and fungal pre-penetration structures, and at infection sites. In both incompatible interactions, a later increase in POD activity was determined which can be related to host cell wall lignification. This peak coincided with the one observed in the compatible interaction. Treatments of coffee leaves (I2 incompatible interaction) with 2,4-dichlorophenol, an activator of peroxidases and other oxidases, significantly increased cell autofluorescence. On the contrary, salicyl hydroxamic acid, an inhibitor of the same enzymes, and diphenyleneiodonium chloride, an inhibitor of NADPH oxidases, decreased cell autofluorescence. These results suggested that POD, NADPH oxidases and eventually other oxidases are involved in the coffee resistance to H. vastatrix.