THE HOST‐PARASITE INTERFACE OF PERONOSPORA MANSHURICA ON GLYCINE MAX

PEYTON, G. A., and C. C. BOWEN. (Iowa State U., Ames.) The host-parasite interface of Peronospora manshurica on Glycine max. Amer. Jour. Bot. 50(8): 787-797. Illus. 1963.-The fine structure of the vegetative intercellular hyphae, intracellular haustoria, and invaded host cells is described. Perinuclear Golgi apparatus and extensive lomasomes are characteristic of thehyphae and haustoria of this fungus. The invading haustoria do not penetrate the plasma membrane of the host. Except for a sheath near the point of penetration, there is no evidence of true host wall around the haustorium. However, a "zone of apposition," with staining properties different from those of normal host cell wall, forms around the haustorial wall between the host and parasite plasma membranes. Special modifications of the host cytoplasm in the vicinity of haustoria are described, including formation of "secretory bodies" and their apparent discharge through the host plasma membrane into the zone of apposition. This phenomenon, together with an apparent increase in the number of ribosomes in the host, suggests highly specific reactions of the host cytoplasm to the invading haustorium. AN UNDERSTANDING of the organization of the interface between host and parasite at the cellular level is basic to the study of biochemical interactions between the 2 organisms and to a better understanding of parasitism and pathogenesis. This is especially true of obligate parasitism where infection often exists for some time without causing a great deal of apparent harm to the host and where unique structural and biochemical relationships must occur. The anatomical relationships between host and parasite have been investigated in a number of fungus plant diseases with the light microscope. Investigators were hampered by the small size of the fungal cells involved and by the limited resolving power of the light microscope. Despite these limitations, many astute observations were made, and many details of host-parasite relationships have been described. The work of Rice (1927) and Fraymouth (1956) is representative. The application of the greater resolving power of the electron microscope to the investigation of plant diseases caused by fungi has been limited to a brief study of Uromyces caladii by Moore and McAlear (1961b) and a somewhat more extensive investigation of Puccinia graminis by Ehrlich and Ehrlich (1963). A careful study of the fine structure of a host-parasite relationship involving a Phycomycete would add significantly to the understanding of parasitism and pathogenesis. 1 Received for publication January 20, 1963. This paper is based on a portion of a thesis submitted by the senior author to the Graduate Faculty of Iowa State University in partial fulfillment of the requirements of the degree of Master of Science. This investigation was supported in part by research grant C-3982 from the National Institutes of Health, United States Public Health Service. The authors appreciate the advice and assistance given by Dr. John M. Dunleavy. 2 Present address: Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan. Peronospora manshurica (Naom.) Syd. ex Gaum., causal agent of downy mildew of soybeans, is an obligate parasite and a typical member of the large group of downy mildew fungi. This study is limited to the vegetative aspects of infection; the many aspects of parasite reproduction, both sexual and asexual, will be left for future work. MATERIALS AND METHODs-Production of diseased and control material Throughout this study, soybeans, Glycine max (L.) Merr. cultivar 'Illini,' were used for both infected and control material. Race 8 of P. manshurica, to which 'Illini' soybeans are highly susceptible (Grabe and Dunleavy, 1959) was used throughout the investigation. Potted seedlings were infected by conidia applied in suspension. Subsequently, infected plants and healthy controls of the same age were placed in a constant-temperature growth chamber at 21 C under 450-750 ft-c of illumination, with 18 hr light and 6 hr darkness. Leaves were removed for study from infected and control plants at intervals ranging from 7 to 60 days after infection. Specimen preparation for electron microscopyA circular punch was fashioned from a no. 18 Yale BD hypodermic needle. The needle was attached to a syringe containing a small amount of water and used to punch out uniform circular pieces of leaf tissue 0.8 mm in diameter. The pieces of tissue were then expelled into the fixative and promptly evacuated to insure penetration of the fixative. Fixation was accomplished by one of the following procedures: (1) 1% osmic acid in 0.2 M potassium phosphate buffer (Ledbetter, M. C., Cambridge, Massachusetts, private communication, 1961) at pH 7 for 30 min at room temperature; (2) 4% unbuffered potassium permanganate (Mollenhauer, 1959) for 10 min at room temperature; (3) and (4) 4% formaldehyde and 7.55% sucrose in