CAVITY SPOT OF CARROTS .2. CELL-WALL-DEGRADING ENZYMES SECRETED BY PYTHIUM AND PATHOGEN-RELATED PROTEINS PRODUCED BY THE ROOT-CELLS

This study investigates the biochemical relationships between carrot r oots and Pythium violae, the pathogen responsible for cavity spot (CS) disease. P. violae isolates obtained from CS lesions, cultured in Pet ri dishes on agar were used for inoculation of uninfected mature carro ts. The fungus secreted a wide spectrum of enzymes that degraded the c ellulose and pectic substances of the carrot cell walls. Cellulase and polygalacuronase (pg) showed the highest activity during the first da y post-inoculation. subsequently declining. Pectin lyase (PnL), pectat e lyase (Pet) and pectin methylesterase (PME) gradually increased to t heir highest levels of activity 14 to 30 days post-inoculation. This p attern of activity enables establishment of the hyphae. Several plant pathogen-related substances such as peroxidase, chitinase, glucanase a nd polyphenol oxidase were produced in the infected tissue. Peroxidase activity rose in the inoculated roots from day 1 postinoculation. Chi tinase, glucanase and polyphenol oxidase activities first appeared 3-4 days post-inoculation. At this time, two bands corresponding to chiti nase at about 26 and 33 KDa and one band corresponding to glucanase at about 24 KDa could be resolved by SDS-PAGE.