Specific attraction to and infection of cotton root cap cells by zoospores of Pythium dissotocum

Abstract

Root cap cells of two cotton species (Gossypium barbadense L. and G. hirsutum L.) elicited a specific chemotactic response in zoospores of Pythium dissotocum. When roots of cotton seedlings were placed into a suspension of Pythium dissotocum zoospores, there was immediate attraction, accumulation, and encystment exclusively in the root cap region. Seedlings which attracted zoospores were killed within 24 h. Furthermore, root cap cells remained attractive when isolated nondestructively from the root and placed into a zoospore suspension; attraction, accumulation, and encystment on individual root cap cells occurred within seconds after contact. Penetration and death of isolated cells occurred within 15–30 min. After 30 min, approximately 25% of living cells were directly colonized by zoospores. Root cap cells killed by freezing or drying remained attractive but at a reduced level; approximately half as many killed cells as living cells were directly colonized by zoospores. The number of root cap cells directly colonized by zoospores did not increase with time. In contrast, zoospores of Pythium catenulatum that exhibited a chemotactic response to Agrostis palustris (Bentgrass) were not attracted to and did not infect cotton seedlings or isolated root cap cells.