Alternaria alternata f. sp. citri tangerine pathotype induces reactive oxygen species accumulation in susceptible citrus leaves

Abstract

During host-phytopathogen interactions, a series of events can occur, resulting in resistance or susceptibility to the disease. This study investigated the infection process of Alternaria alternata f. sp. citri "tangerine" pathotype on citrus genotypes through microscopy and biochemical assays. Germination of fungal conidia on detached leaves was observed using scanning electron microscopy (SEM). In addition, biochemical changes in the leaves were verified by quantifying peroxidase (POX), catalase (CAT), superoxide dismutase (SOD), and the accumulation of hydrogen peroxide (H2O2). Analyses were performed using transmission electron microscopy (TEM) of plant material at 24 h after inoculation (HAI) to verify changes in leaves at the subcellular level. SEM results indicated no differences regarding fungus germination, appressorium formation, or entrance of the fungus through the stomata among the three genotypes analyzed, regardless of being susceptible or resistant. When enzymatic activities were evaluated, two different patterns were observed. In the susceptible genotypes (Ponkan and Murcott), the increase in SOD and CAT activities were low, and a hypersensitive response (HR) was observed. In contrast, no H2O2 accumulation was observed in the resistant genotype (Fremont), and SOD and CAT activities were higher than in the susceptible genotypes. The TEM results revealed that the inoculated leaves of the susceptible cultivars exhibited cell plasmolysis and plasma membrane fragmentation at 24 HAI. Thus, it is suggested that the resistance of the Fremont genotype is related to its ability to detoxify reactive oxygen species, in addition to not responding to inoculation through H2O2 accumulation and showing no damage to plasma membranes.