Hydrogen peroxide plays a critical role in the defence response of tomato to Cladosporium fulvum

The presence and localization of hydrogen peroxide (H2O2) in compatible and incompatible interactions of Cladosporium fulvum and tomato were studied by light microscopy using the histochemical stain 3, 3'-diaminobenzidine (DAB). To obtain synchronous development of the fungus, inoculation of tomato was by injection of conidia into the intercellular area of the spongy mesophyll. Conidia were suspended in scavengers of reactive oxygen species (ROS), i.e., catalase (CAT), superoxide dismutase (SOD), and mannitol, or the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor, diphenylene iodonium (DPI), at the time of inoculation. Conidia and initial germ tubes in compatible and incompatible interactions caused a localized host reaction characterized by H2O2 production and a variety of plant responses which included protein cross-linking, callose deposition, and accumulation of phenolic compounds. Further growth of an avirulent race was restricted by similar host responses whereas, hyphae of a virulent race grew as in a normal compatible interaction. In the incompatible interaction, treatment with CAT, SOD, or DPI generally resulted in increased fungal length and a decrease in the percentage of sites with host responses. These results support previous suggestions for this host-pathogen system of a critical role for ROS in limiting colonization, and this appears to be partly as a signal molecule for plant defence responses. (C) 2002 Elsevier Science Ltd. All rights reserved.