Journal of Basic Microbiology 2010, 50, 1–10 1 © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jbm-journal.com Research Paper Ultrastructure of Aphanomyces cochlioides zoospores and changes during their developmental transitions triggered by the host-specific flavone cochliophilin A Tofazzal Islam Graduate School of Agriculture, Hokkaido University, Kita-Ku, Sapporo, Japan Aphanomyces cochlioides is a serious damping-off causing pathogen of sugar beet, spinach and some other members of Chenopodiaceae and Amaranthaceae. The biflagellated motile zoo- spores of the pathogen locate their host roots by perceiving the host-specific flavone cochlio- philin A (5-hydroxy-6,7-methylenedioxyflavone), transiently modify into cystospores that ger- minate prior to penetration. This study for the first time illustrated ultrastructure of the zoospores and morphological modification during their developmental transitions triggered by cochliophilin A using transmission electron microscopy (TEM). TEM revealed that zoospores had two heterokont flagella inserted laterally into a ventral groove of their body where each is attached to a kinetosome. In the cross sections of flagellar axonemes, two single and nine peripheral microtubules in doublets were clearly observed. Mitochondria, the Golgi complexes, finger print vesicles, and vesicles with striated electron opaque inclusion and vesicles con- taining a granular cortex and center were also detected. The latter vesicles disappeared and two flagella were shed when zoospores converted to spherical cystsopores. The shape, size and number of mitochondria were dynamically changed during the encystment of zoospores presumably through fission and fusion processes. The dynamics of mitochondria observed in this study indicated its distinct role in the signal transduction pathway of the zoospore encystment. This study also revealed the transformation of nuclear shape from pyriform in zoospores to spherical in cystospores and lanseolate in the hyphae. Keywords: Aphanomyces cochlioides / Zoospore encystment / Mitochondrial dynamics / Host-specific signal / Oomycetes / Flagella / TEM Received: September 13, 2009; accepted: December 19, 2009 DOI 10.1002/jobm.200900301 Introduction * Peronosporomycetes (Oömycetes in the old classifica- tion) belong to the kingdom Straminipila, are distinct from fungi and more closely related to brown algae and diatoms phylogenetically [1 – 3]. They cause many de- structive diseases of plants, animals, fishes and hu- mans. One of the members of peronosporomycetes, Aphanomyces cochlioides Drechs. in the family Leptoleg- niaceae (order Saprolegniaceae), is a serious pathogen of damping-off diseases in sugar beet, spinach and some other members of Chenopodiaceae and Amaranthaceae [4]. This fungus-like soilborne stramenopile reproduces Correspondence: Tofazzal Islam, School of Agriculture and Rural De- velopment, Bangladesh Open University, Gazipur-1705, Bangladesh E-mail: tofazzalislam@yahoo.com asexually via zoosporangia on the vegetative thallus or sexually via antheridia and oogonia on infected host. The non-motile primary zoospores, released from the cylindrical zoosporangia, immediately encyst and at- tach in clusters on the tip of the sporangia [4]. Laterally biflagellate, uninucleate, naked (no cell wall) and mo- tile secondary zoospores arising from the primary cystospores are thought to be the main infective propagules of seedlings [5 – 8]. The reniform-ovate sec- ondary zoospore of A. cochlioides has two heterokont (dissimilar) flagella, both inserted into a ventral groove [6]. The anterior flagellum possesses two rows of tripar- tite tubular hairs (TTHs) distributed throughout its length, whereas the posterior flagellum is ornamented with two rows of dense fine hairs except on its tapered terminal part [2, 6]. Once released from the primary 2100333 MIK 03/10 MIK00301j.doc Weishaar VMWare: CS3