Morphological Studies on Zoospores of Aphanomyces cochlioides and Changes during Interaction with Host Materials

The behavioral and morphological diversity of intact zoospores of Aphanomyces cochlioides and their dynamic morphological changes when interacting with the host or a host-specific zoospore attractant were studied by light and electron microscopy. The reniform-ovate zoospore has two heterokont flagella, both inserted in a ventral groove. The anterior flagellum possesses two rows of tripartite tubular hairs (TTHs) distributed throughout its length, whereas the posterior flagellum was ornamented with two rows of highly dense fine tubular hairs, except on its tapered terminal part. The tip of the posterior flagellum has a bunch of similar fine hairs like those on the flagellum shaft. The biflagellated zoospores quickly aggregated on the host (spinach) root, adhered to the root surface, encysted by shedding or retracting the flagella, germinated at a fixed point to form germ tubes, and finally invaded the root tissues via the appressoria within 50–60 min. This precise homing response of A. cochlioides zoospores reflects guidance by a host-specific signal(s) for locating potential infection sites, differentiation of zoospores to cystospores, formation of infection structures and/or germ-tube tropism for completing pre-infection events in haste. Prior to the encystment, the posterior flagellum may be involved in successful docking on the root surface by tip contact followed by shedding or retraction.