TARGETED CELL-WALL DEGRADATION AT THE PENETRATION SITE OF COWPEA RUSTBASIDIOSPORELINGS

Basidiospore germlings of the cowpea rust fungus (Uromyces vignae) pen etrate the epidermal cell wall of the nonhost plant Vicia faba. In ord er to characterize the wall structure of the penetration site, leaves were high pressure frozen, freeze substituted, and embedded in appropr iate resins. With antibodies against epitopes present in pectin, polyg alacturonic acid, xyloglucan, and callose, we studied the modification of these wall components during infection. The density of epitopes wa s determined at the penetration site and compared with noninfected are as of the epidermal wall. Along the fungal penetration hypha, a zone o f the plant wall, 0.2 mu m wide, exhibited a reduced density of pectin and xyloglucan epitopes. A similar reduction of epitope density was a lso found for xyloglucan after treatment of sections from noninoculate d plants with cellulase and xylanase and for pectin after treatment wi th pectinase. The density of polygalacturonic acid epitopes remained u nchanged in the outer layer of the epidermal wall, but increased over the inner layer. A high density of polygalacturonic acid epitopes was found over a collarlike wall apposition produced by the plant cell alo ng the penetration hypha. These results indicate that the fungus degra des the plant cell wall at the penetration site and that the plant cel l secretes new wall material into this area to form the wail appositio n.