Targeted Cell Wall Degradation at the Penetration Site of Cowpea Rust Basidiosporelings

Basidiospore germlings of the cowpea rust fungus (Uro- myces vignae) penetrate the epidermal cell wall of the nonhost plant Vicia faba. In order to characterize the wall structure of the penetration site, leaves were high pressure frozen, freeze substituted, and embedded in appropriate resins. With antibodies against epitopes present in pectin, polygalacturonic acid, xyloglucan, and callose, we studied the modification of these wall components during infec- tion. The density of epitopes was determined at the pene- tration site and compared with noninfected areas of the epidermal wall. Along the fungal penetration hypha, a zone of the plant wall, 0.2 µm wide, exhibited a reduced density of pectin and xyloglucan epitopes. A similar reduc- tion of epitope density was also found for xyloglucan after treatment of sections from noninoculated plants with cel- lulase and xylanase and for pectin after treatment with pectinase. The density of polygalacturonic acid epitopes remained unchanged 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 along the pene- tration hypha. These results indicate that the fungus de- grades the plant cell wall at the penetration site and that the plant cell secretes new wall material into this area to form the wall apposition.