Cellular localization of nonhost resistance reactions of parsley (Petroselinum crispum) to fungal infection.

The response of parsley seedlings (Petroselinum crispum) inoculated with zoospores of the soybean-pathogenic fungus, Phytophthora megasperma f. sp. glycinea, ranged from "immunity" to "physiological susceptibility" depending on the post-inoculation environmental conditions. Typical nonhost resistance reactions, hypersensitive cell death and the formation of small local lesions, occurred under high relative humidity and 16 h illumination per day. Localized biochemical reactions were monitored using fluorescence microscopy combined with histochemical and immunohistochemical methods. The rapid accumulation of furanocoumarin phytoalexins, wall-bound phenolics and callose was detected around infection sites. Indirect antibody staining of frozen tissue sections demonstrated the local accumulation of phenylalanine ammonia-lyase, a key enzyme of general phenylpropanoid metabolism, and S-adenosyl-L-methionine: bergaptol O-methyltransferase, a specific enzyme of the furanocoumarin pathway. The results indicate that phenylpropanoid derivatives are synthesized de novo at infection sites.