Cavity spot of carrots: II. Cell‐wall‐degrading enzymes secreted by Pythium and pathogen‐related proteins produced by the root cells

Abstract

SummaryThis study investigates the biochemical relationships between carrot roots and Pythium violae, the pathogen responsible for cavity spot (CS) disease. P. violae isolates obtained from CS lesions, cultured in Petri dishes on agar were used for inoculation of uninfected mature carrots. The fungus secreted a wide spectrum of enzymes that degraded the cellulose and pectic substances of the carrot cell walls. Cellulase and polygalacuronase (pg) showed the highest activity during the first day post‐inoculation, subsequently declining. Pectin lyase (PnL), pectate lyase (PeL) and pectin methylesterase (PME) gradually increased to their highest levels of activity 14 to 30 days post‐inoculation. This pattern of activity enables the penetration of the fungus through the walls of the host cells and the establishment of the hyphae. Several plant pathogen‐related substances such as peroxidase, chitinase, glucanase and polyphenol oxidase were produced in the infected tissue. Peroxidase activity rose in the inoculated roots from day 1 post‐inoculation. Chitinase, glucanase and polyphenol oxidase activities first appeared 3–4 days post‐inoculation. At this time, two bands corresponding to chitinase at about 26 and 33 KDa and one band corresponding to glucanase at about 24 KDa could be resolved by SDS‐PAGE.