Activation of defense responses to Fusarium infection in Asparagus densiflorus

Defense responses to Fusarium oxysporum f. sp. asparagi and F. proliferatum were compared after root inoculation of the asparagus fern, Asparagus dens iflorus vars. Myersii and Sprengeri, and cultivated asparagus, A. officinal is cv. Guelph Millennium. Both varieties of A. densiflorus exhibited a hype rsensitive response with rapid death of epidermal cells within 8-24 h and r estricted the fungal growth. In A. officinalis roots, rapid cell death was not found, and necrotic lesions were observed 8-14 d after fungal inoculati on. Peroxidase and phenylalanine ammonia-lyase activities increased signifi cantly in inoculated A. densiflorus but not A. officinalis plants. Local an d systemic induction of peroxidase activity was detected after pathogen ino culation in root and spear tissues, respectively, of A. densiflorus. POX ac tivity decreased in roots of inoculated A. officinalis by 8 d post-inoculat ion. Germination and germ tube growth were inhibited when spores of F. oxys porum f. sp. asparagi were incubated in root exudates and on root segment s urfaces of inoculated A. densiflorus plants exhibiting hypersensitive cell death. Spore germination of F. proliferatum and three fungi non-pathogenic to cultivated asparagus was inhibited as well. Rapid induction of hypersens itive cell death in A. densiflorus was associated with restriction of funga l growth, and activation of peroxidase and phenylalanine ammonia-lyase, two defense enzymes thought to be important for plant disease resistance.