Microscopic Studies on Attachment and Differentiation of Zoospores of the Phytopathogenic Fungus Aphanomyces cochlioides

Abstract

The mode of aggregation, attachment and differentiation of zoospores of the phytopathogenic fungus Aphanomyces cochlioides when interacting with the host and a host-specific attractant and a G-protein activator, mastoparan, was studied by light and scanning electron microscopy. When a zoospore approached very close to the host root, it seemed to halt, then coiled its anterior flagellum on its body. The halted zoospore appeared to contact the host surface with its posterior flagellum, which gradually drew the encysting zoospore onto the root surface. The spore then docked precisely on the root surface at its ventral face with the help of the posterior flagellum and anchored itself by releasing some adhesive materials. The adherent spore became a spherical after shedding its flagella and rapidly turned into an expanded cyst forming a smooth cyst coat around it, and finally changed into a smaller cystospore covered with a wrinkled surface. In contrast, the mastoparan- or cochliophilin A-stimulated zoospores on artificial membranes aggregated by using their posterior flagella before encystment. These contrasting phenomena suggest that A. cochlioides zoospores may use their posterior flagella for successful docking on the host surface or for aggregation of encysting spores in the absence of the host.