Department of Plant Pathology, Physiology and Weed Science, Virginia Polytechnic Institute and State University, 1444 Diamond Springs Road, Virginia Beach 23455-3363.
Phytophthora species are destructive fungus-like plant pathogens that use asexual single-celled flagellate zoospores for dispersal and plant infection. Many of the zoospore behaviors are density-dependent although the underlying mechanisms are poorly understood. Here, we use P. nicotianae as a model and demonstrate autoregulation of some zoospore behaviors using signal molecules that zoospores release into the environment. Specifically, zoospore aggregation, plant targeting, and infection required or were enhanced by threshold concentrations of these signal molecules. Below the threshold concentration, zoospores did not aggregate and move toward a cauline leaf of Arabidopsis thaliana (Col-0) and failed to individually attack annual vinca (Catharanthus roseus cv. Little Bright Eye). These processes were reversed when supplemented with zoospore-free fluid (ZFF) prepared from a zoospore suspension above threshold densities but not with calcium chloride at a concentration equivalent to extracellular Ca2+ in ZFF. These results suggest that Ca2+ is not a primary signal molecule regulating these communal behaviors. Zoospores coordinated their communal behaviors by releasing, detecting, and responding to signal molecules. This chemical communication mechanism raises the possibility that Phytophthora plant infection may not depend solely on zoospore number in the real world. Single zoospore infection may take place if it is signaled by a common molecule available in the environment which contributes to the destructiveness of these plant pathogens.