Host-Pathogen Interactions

Abstract

Resistance of soybean (Glycine mar L.) seedlings to Phytophthora megasperma var. sojac (Pms) is in part due to the accumulation in infected tissue of a compound which is toxic to Pins. The accunmulation of this compound, a phytoalexin called glyceolin, is triggered by infection, but it can also be triggered by molecules, "elicitors," present in cultures of Pms. The ability of the Pms elicitor to stimulate phytoalexin accumulation in soybean tissues has been used as the basis for biological assays of elicitor activity. Two bioassays were developed and characterized in this study of the Pms elicitor. These bioassays use the cotyledons and the hypocotyls of soybean seedlings. The cotyledon assay was used to characterize the extracellular Pms elcitor. This elicitor was isolated from Pms cultures and purified by ion exchange and molecular sieving chromatography. The extraceflular Pms elicitor was determined to be a predominantly 3-linked glucan, which is similar in composition and structure to a polysaccharide component of Pms mycelial walls.One of the most common reactions of a plant in response to invasion by a pathogen involves the precipitous death or hypersensitive response of those plant cells which have come in contact with a pathogen (9,25,32,33). The direct effects of plant cell death on pathogen development are not known. It has been suggested that cell death may result in the disruption of lysozymes and other cellular structures, releasing enzymes and compounds which may interfere with the growth of the pathogen (37). Compounds which are toxic to the pathogen are present in the area of localized necrosis, but these compounds, called phytoalexins, are thought to be synthesized by the healthy cells neighboring the necrotic area (25,29).It is not known how hypersensitive cell death and phytoalexin accumulation are initiated. The first clues to the nature of the molecules which trigger the accumulation of phytoalexins came from studies of pathogens grown in defined media. Cell-free filtrates from cultures of several pathogens (1-3, 12, 13, 19, 22, 28, 30, 35, 36) were demonstrated to elicit phytoalexin accumulation in their hosts. One of the first of these reports was in 1958 by Uehara (35) West Germany. 4 To whom reprint requests should be addressed at the Department of Chemistry.sp. to seed pods of soybeans, the host of this fungal pathogen, and observed that a fungitoxic compound accumulated in the bean tissue. Similar observations of phytoalexin accumulation in green bean tissue have been made after application of culture filtrates of Monilinia fructicola (7), Penicillium expansum (30), and Colletotrichum lindemuthianum (1, 2). The cell-free culture filtrates of the fungal pathogen (Phytophthora megasperma var.sojae have also been shown to stimulate phytoalexin accumulation in its host, soybeans (3,12,13,17,19,22,28). Attempts were made to purify from some of these pathogen cultures the molecules responsible for triggering phytoalexin accumulation; with the exception of the glucan produced...

Host-Pathogen Interactions

“…A comparison of the relative elicitor activity of fraction I and fraction IV with the sugar composition of these fractions (3,4) suggests that the activity of each fraction is proportional to its glucan content. These data confirm results obtained with assays of Pms elicitor activity using hypocotyls and cotyledons of soybean seedlings (2)(3)(4).…”

“…(1,7,21,25,(27)(28)(29). In previous publications, we have reported that the elicitors from Pms, which are polysaccharide fractions related to the mycelial walls of the soybean pathogen, stimulate the synthesis of glyceollin in soybean seedlings (2)(3)(4). An elicitor of similar nature has been isolated from Colletotrichum lindemuthianum, a fungal pathogen of beans (Phaseolus vulgaris) (1).…”

The structure and function of a soybean β-glucan-elicitor-binding protein

“…GEs released from the cell wall of the fungus P. megasperma initiate the accumulation of phytoalexins in soybean (40). Two groups have previously reported apparent dissociation constants of 1 nM (20) (1997) values which are of the same order of magnitude as those of the present study.…”

Signaling in Soybean Phenylpropanoid Responses (Dissection of Primary, Secondary, and Conditioning Effects of Light, Wounding, and Elicitor Treatments)

“…Although it has been partially purified and characterized (Ayers et al, 1976a(Ayers et al, , 1976b, it has not been subjected to acidic (Ayers et al, 1976b;Sharp et al, 1984) or enzymatic (Cline and Albersheim, 1981;Keen and Yoshikawa, 1983;Ham, et al, 1991) hydrolysis or fractionation. Acid-released fragments have been purified and synthesized accordingly and are being used to characterize glucan-binding activities in soybean plasma membranes (Cheong and Hahn, 1991;Frey et al, 1993).…”