Host-Pathogen Interactions : XXI. Extraction of a Heat-Labile Elicitor of Phytoalexin Accumulation from Frozen Soybean Stems

An extract of frozen and thawed soybean (Glycine max L. Merr. cv. Wayne) stems is active, in wounded soybean cotyledons, as a heat-labile elicitor of phytoalexins. The elicitor activity of the extract is destroyed by heating to 95°C for 10 minutes. The fraction that contains heat-labile elicitor activity releases heat-stable elicitor-active molecules from purified soybean cell walls. Heat-labile elicitor activity voids a Bio-Gel P-6 column and can be absorbed onto and eluted from a DEAE Sephadex ion exchange column. Using the cotyledon phytoalexin elicitor assay, maximum heatlabile elicitor activity was obtained when soybean stems were extracted with acetate buffer at pH 6.0. Addition of 1 millimolar CaCl₂ increased apparent heat-labile elicitor activity. The heat-labile elicitor stimulated maximum phytoalexin accumulation when applied to cotyledons immediately after the cotyledons were cut. Partially purified stem extracts lost heat-labile elicitor activity during storage for several days at 3°C. The possible role of a heat-labile elicitor in stimulation of phytoalexin accumulation by both abiotic and biotic elicitors is discussed.     

Host-Pathogen Interactions: XII. Response of Suspension-cultured Soybean Cells to the Elicitor Isolated from Phytophthora megasperma var. sojae, a Fungal Pathogen of Soybeans

The glucan elicitor isolated from the mycelial walls of Phytophthora megasperma var. sojae, the fungus which causes stem and root rot in soybeans, stimulates the activity of phenylalanine ammonia-lyase and the accumulation of glyceollin in suspension-cultured soybean cells. Nigeran, a commercially available fungal wall glucan, was the only other compound tested which has any activity in this system. Glyceollin is a phenylpropanoid-derived phytoalexin which is toxic to P. megasperma var. sojae. Evidence is presented to support the hypothesis that the action of elicitors in stimulating phytoalexin synthesis is not species or variety specific but, rather, is part of a general defensive response of plants.     

Release of a Soluble Phytoalexin Elicitor from Mycelial Walls of Phytophthora megasperma var. sojae by Soybean Tissues

A soluble elicitor of glyceollin accumulation was released from insoluble mycelial walls of Phytophthora megasperma var. sojae after incubation with soybean cotyledon tissue for as little as 2 minutes. Various enzymic and chemical treatments of the released elicitor indicated that the activity resided in a carbohydrate moiety, and gel filtration disclosed the presence of at least two active molecular species. Cell-free extracts from soybean cotyledons or hypocotyls also released soluble elicitors from fungal cell walls that were similar to those released by living cotyledon tissue. These results may suggest that contact of fungal pathogens with host tissues is required to release fungal wall elicitors which then initiate phytoalexin accumulation in the plant.     

Purification and Properties of an Elicitor of Castor Bean Phytoalexin from Culture Filtrates of the Fungus Rhizopus stolonifer

Evidence has been obtained for the presence in filtrates of 3-day-old cultures of the fungus Rhizopus stolonifer grown on potato-dextrose medium of both high molecular weight and low molecular weight elicitors of the production of the phytoalexin casbene in cell-free extracts of castor bean (Ricinus communis L.) seedlings. The high molecular weight elicitor activity was purified by means of gel filtration chromatography. Both protein and carbohydrate are associated with the most purified fraction containing elicitor activity. The elicitor is inactivated by treatments at 60 C or higher temperatures for 15 minutes. The molecular weight of the purified elicitor was estimated from gel filtration chromatography in 10 mm Na-phosphate (pH 7) to be 30,000 ± 5,000. Treatments of the purified elicitor fraction with either sodium periodate or the nonspecific protease preparation, pronase, substantially reduced its activity as an elicitor of casbene production. On the basis of these properties it is concluded that the elicitor is most likely a protein and may be a glycoprotein. It is estimated that 2 × 10⁻⁸ m elicitor gives about a 14-fold increase in casbene synthetase activity in extracts of treated split seedlings in comparison with controls. This corresponds to about 50% of the maximal activity obtainable in this assay system developed to measure elicitor activity.     

Phytoalexin Elicitor Activity of Carbohydrates from Phytophthora megasperma f.sp. glycinea and Other Sources

Three unique classes of carbohydrates were isolated from the hyphal cell walls of Phytophthora megasperma f.sp. glycinea (Pmg) and compared with other substances for their activity as elicitors of the phytoalexin glyceollin in soybean tissues. Glucomannans extracted from cell walls with soybean β-1,3-endoglucanase were purified and proved to be the most active elicitors yet reported. They were approximately 10 times more active in soybean cotyledons than the heterogeneous β-glucan elicitor fraction extracted from Pmg walls. In addition, the glucomannan fraction gave race-specific elicitor activity in soybean hypocotyls. Pronase was found to be a suitable reagent for the mild extraction of glycopeptides from Pmg cell walls. All of the carbohydrates isolated from Pmg cell walls possessed significant elicitor activity, but other glucans, a glucomannan and mannan from other sources, were much less active. Chitin and chitosan, reported to function as elicitors in other plants, had low activity in soybean cotyledons. Arachidonic acid was inactive, despite its previously observed elicitor activity in potato tubers. The results indicated that, for Pmg, the carbohydrate elicitor most probably involved in the initiation of phytoalexinmediated defense during fungus infection of soybean plants is the glucomannan fraction liberated by endoglucanase.     

Elicitor-induced accumulation of glyceollin and callose in soybean roots and localized resistance against Phytophthora megasperma f.sp. glycinea

Immersion of roots of 2-day-old soybean seedlings (Glycine max cv. Harosoy 63) into solutions of several glucan elicitors caused the accumulation to various degrees of the soybean phytoalexin glyceollin. Laminarin and polytran proved to be more effective elicitors in this system than the glucan elicitor from Phytophthora megasperma f.sp. glycinea (Pmg). Digitonin and tomatin caused, in addition to glyceollin accumulation, the deposition of callose in the rhizodermis. Pretreatment of the soybean roots with laminarin effected an increase in resistance of the seedlings against a compatible race of Pmg.     

Host-Pathogen Interactions: X. Fractionation and Biological Activity of an Elicitor Isolated from the Mycelial Walls of Phytophthora megasperma var. sojae

An elicitor of phytoalexin production in soybean (Glycine max L.) tissues was isolated from purified Phytophthora megasperma var. sojae mycelial walls by a heat treatment similar to that used to solubilize the surface antigens from the cell walls of Saccharomyces cerevisiae. The wall-released elicitor is a discrete, minor portion of the P. megasperma var. sojae mycelial walls. The elicitor released from the mycelial walls was divided by diethylaminoethylcellulose and concanavalin A-Sepharose chromatography into four fractions, each having different chemical characteristics. The four fractions were obtained from each of the three races of P. megasperma var. sojae. The corresponding fractions from each of the three races are very similar in composition and elicitor activity. The results suggest that the elicitor activity of each fraction resides in the glucan component of the fraction. Evidence is presented to demonstrate that the elicitors are not race-specific and that the accumulation of glyceollin is not sufficient to account for race-specific resistance.     

Host-Pathogen Interactions : XVII. HYDROLYSIS OF BIOLOGICALLY ACTIVE FUNGAL GLUCANS BY ENZYMES ISOLATED FROM SOYBEAN CELLS

The ability of β-glucosylase I, a soybean cell wall β-glucosyl hydrolase, to degrade elicitors of phytoalexin accumulation was studied. Extensive β-glucosylase I treatment of the glucan elicitor isolated from the mycelial walls of Phytophthora megasperma var. sojae results in hydrolysis of 77% of the glucosidic bonds of the elicitor and destruction of 94% of its activity. Soybean cell walls contain some additional factor, probably one or more additional enzymes, which can assist β-glucosylase I in hydrolyzing the glucan elicitor. This was demonstrated by the more rapid hydrolysis of the glucan elicitor by a mixture of soybean cell wall enzymes (containing β-glucosylase I). In a single treatment, the mixture of cell wall enzymes hydrolyzed 91% of the glucosidic bonds and destroyed 85% of the activity of the elicitor. The enzymes from soybean cell walls will also hydrolyze elicitor-active oligoglucosides prepared from the mycelial walls of Phytophthora megasperma var. sojae. The active oligoglucosides are more susceptible than the glucan elicitor to hydrolysis by these enzymes. The mixture of cell wall enzymes or β-glucosylase I, by itself, hydrolyzes more than 96% of the glucosidic bonds and destroys more than 99% of the activity of the oligoglucoside elicitor. Two possible advantages for the existence of these enzymes in the walls of soybean cells are discussed.     

Inhibition of elicitor-induced phytoalexin formation in cotton and soybean cells by citrate

Addition of an elicitor preparation from Verticillium dahliae to soybean or cotton cell suspension cultures induces the formation of the phytoalexins, glycelollin or sesquiterpene aldehydes, respectively. Recent work (PS Low, PF Heinstein 1986 Arch Biochem Biophys 249: 472-479) has shown that the induction of phytoalexin biosynthesis in these cells is preceded by rapid changes in the plant cell membrane which can be conveniently monitored by membrane associated fluorescent probes. Using this elicitation assay, we have found that citrate, a common metabolite of higher plants, acts as a potent inhibitor of elicitation when added prior to treatment with elicitor. The citrate concentration required to obtain a 50% inhibition of the elicitor-induced fluorescence transition in cultured cotton cells was found to be about 2 millimolar, while the concentration of citrate observed to inhibit elicitor-induced sesquiterpene aldehyde formation in the same cell suspensions was also 2 millimolar. Curiously, in the presence of elicitor, citrate at less than ID₅₀ concentrations increased cell mass accumulation significantly above control incubations without elicitor. A similar inhibition of glyceollin formation with an increase in cell mass accumulation was also observed upon addition of 1 to 5 millimolar citrate to soybean cell suspension cultures. The physiological significance of the inhibition by citrate of phytoalexin formation in plant cell suspensions was supported by the observation that a similar inhibition of sesquiterpene aldehyde formation occurs in cotton plantlets elicited by cold shock or V. dahliae stress. The specificity of citrate as an inhibitor of phytoalexin formation was demonstrated by data showing that other di- and tricarboxylic-hydroxy acids did not inhibit, with the exception of malate which inhibited phytoalexin formation in soybean cells with roughly half the potency of citrate. These experiments not only demonstrate that citrate can act as a specific inhibitor of elicitation, but they further confirm the validity of monitoring elicitation and its modulation with fluorescent probes.     

Host-Pathogen Interactions : XXV. Endopolygalacturonic Acid Lyase from Erwinia carotovora Elicits Phytoalexin Accumulation by Releasing Plant Cell Wall Fragments

Heat-labile elicitors of phytoalexin accumulation in soybeans (Glycine max L. Merr. cv Wayne) were detected in culture filtrates of Erwinia carotovora grown on a defined medium containing citrus pectin as the sole carbon source. The heat-labile elicitors were highly purified by cation-exchange chromatography on a CM-Sephadex (C-50) column, followed by agarose-affinity chromatography on a Bio-Gel A-0.5m gel filtration column. The heat-labile elicitor activity co-purified with two α-1,4-endopolygalacturonic acid lyases (EC 4·2·2·2). Endopolygalacturonic acid lyase activity appeared to be necessary for elicitor activity because heat-inactivated enzyme preparations did not elicit phytoalexins. The purified endopolygalacturonic acid lyases elicited pterocarpan phytoalexins at microbial-inhibitory concentrations in the soybean-cotyledon bioassay when applied at a concentration of 55 nanograms per milliliter (1 × 10⁻⁹ molar). One of these lyases released heat-stable elicitors from soybean cell walls, citrus pectin, and sodium polypectate. The heat-stable elicitor-active material solubilized from soybean cell walls by the lyase was composed of at least 90% (w/v) uronosyl residues. These results demonstrate that endopolygalacturonic acid lyase elicits phytoalexin accumulation by releasing fragments from pectic polysaccharides in plant cell walls.     

Host-Pathogen Interactions : XIX. THE ENDOGENOUS ELICITOR, A FRAGMENT OF A PLANT CELL WALL POLYSACCHARIDE THAT ELICITS PHYTOALEXIN ACCUMULATION IN SOYBEANS

An elicitor of phytoalexin accumulation (endogenous elicitor) is solubilized from purified cell walls of soybean (Glycine max [L.] Merr., cv. Wayne) by extracting the walls with hot water or by subjecting the walls to partial acid hydrolysis. The endogenous elicitor obtained from soybean cell walls binds to an anion exchange resin. The elicitor-active material released from the resin contains oligosaccharides rich in galacturonic acid; small amounts of rhamnose and xylose are also present. The preponderance of galacturonic acid in the elicitor-active fragments suggests that the elicitor is, in fact, a fragment of a pectic polysaccharide. This possibility is supported by the observation that treatment of the wall fragments with a highly purified endopolygalacturonase destroys their ability to elicit phytoalexin accumulation. This observation, together with other evidence presented in this paper, suggests that galacturonic acid is an essential constituent of the elicitor-active wall fragments. Endogenous elicitors were also solubilized by partial hydrolysis from cell walls of suspension-cultured tobacco, sycamore, and wheat cells.     

Metabolic Regulation during Glyceollin Biosynthesis in Green Soybean Hypocotyls

The accumulation of the isoflavonoid phytoalexin, glyceollin, occurs in hypocotyls of green soybean seedlings (Glycine max L. Merr. cv Harosoy 63) in response to the injection of a glucan elicitor isolated from the mycelial walls of the fungus, Phytophthora megasperma f. sp. glycinea. This accumulation, which levels off after 24 hours, is preceded by a dramatic, transient rise in extractable activities of two early enzymes in the biosynthetic pathway, phenylalanine ammonia-lyase (PAL) and p-coumaryl CoA ligase (pCL). The maximum amount of extractable activity occurs 12 to 16 hours after elicitor treatment and is coincident with the most rapid period of glyceollin accumulation. These results suggest a regulatory role for these early enzymes in the biosynthesis of this secondary metabolite. High performance liquid chromatography analysis of the early intermediates in the pathway further corroborates this hypothesis. The relative pool size and rate of turnover of p-coumaric acid, an early intermediate in glyceollin production, increase during the period of rapid increases in enzyme activities. Removal of cotyledons from elicitor-treated seedlings reduces glyceollin accumulation approximately 70%. This limitation of phytoalexin accumulation by cotyledon removal is correlated with a similar cotyledon effect on reduction of extractable activities of both PAL and pCL as well as a decrease in the flux of carbon through the p-coumaric acid pool. This research further supports the hypothesis that early enzymic steps in a biosynthetic pathway diverting carbon from primary to secondary metabolites function as regulatory control points.     

Host-Pathogen Interactions : XXII. A Galacturonic Acid Oligosaccharide from Plant Cell Walls Elicits Phytoalexins

Elicitors of phytoalexin accumulation in soybean (Glycine max L. Merr., cv Wayne) cotyledons were released from soybean cell walls and from citrus pectin by partial acid hydrolysis. These two hydrolysates yielded nearly identical distributions of elicitor activity when fractionated on anion-exchange columns. Chromatography of the pectin elicitor on gel filtration and high-pressure anion-exchange columns did not further purify the elicitor. Elicitor activity of the preparation was lost by treatment with either endo-α-1,4-polygalacturonase or pectate lyase. Glycosyl residue compositions of the purified elicitors from cell walls and pectin were both found to be approximately 98% galacturonosyl residues. Linkage analysis of the pectin elicitor showed that most, if not all, of the galacturonosyl residues were α-1,4-linked. The high-mass molecular ions detected by fast atom bombardment-mass spectrometry of the most active elicitor fractions from cell walls and pectin both corresponded precisely to a molecule composed of 12 galacturonosyl residues. These results suggest that dodeca-α-1,4-d-galacturonide is the active elicitor, but the possibility remains that the active component could be a slightly modified oligogalacturonide present, but not detected, in the purified fractions.     

Effects of fungal elicitor on lignin biosynthesis in cell suspension cultures of soybean

Soybean (Glycine max L.) cells cultured in B5 medium produce extremely low amounts of lignin. However, modification in the growth medium, by lowering the concentration of NO⁻₃ and PO²⁻₄, results in the lignification of these cells without affecting levels of cell wall-esterified 4-coumaric and ferulic acid. The production of an extracellular, macromolecular complex by the cultured soybean cells (Moore TS Jr 1973 Plant Physiol 51: 529-536) allows a rapid, nondestructive solubilization of the lignin which can be estimated by reaction with phloroglucinol in free solution. This system has been used to study the effects of fungal elicitor on the synthesis of lignin in soybean cells. The inclusion of very low levels of an elicitor fraction from the cell walls of Phytophthora megasperma in the medium in which lignification of the soybean cells occurs suppressed both the accumulation of extracellular lignin and phloroglucinol staining of the cell walls without affecting the levels of bound hydroxycinnamic acids. The activity profiles of phenylalanine ammonia-lyase (EC 4.3.1.5) and isoenzymes of 4-coumarate:CoA ligase (EC 6.2.1.12) were compared in lignifying and elicitor-treated cell cultures as was the activity of chalcone synthase, an enzyme of flavonoid biosynthesis. The measured activities of these enzymes in cell cultures treated with elicitor were considerably lower than in untreated cells.     

A receptor on soybean membranes for a fungal elicitor of phytoalexin accumulation

Soybean membrane preparations specifically bound [¹⁴C]mycolaminaran, a branched β-1,3-glucan produced by Phytophthora sp. which elicits production of the phytoalexin glyceollin in soybean tissues. A Scatchard plot of the binding data disclosed the presence of a single affinity class of binding sites with a K_d value of 11.5 micromolar for the glucan. To assess the physiologic importance of mycolaminaran binding in phytoalexin elicitation, several derivatives of mycolaminaran were prepared. Reduced mycolaminaran had slightly greater elicitor activity and binding affinity than the native substance, while periodinated mycolaminaran was virtually devoid of either elicitor activity orbinding capability. Phosphorylated mycolaminaran, on the other hand, gave values for both elicitor activity and membrane binding which were intermediate between the native and periodinated preparations. No other tested carbohydrates competed with the binding of [¹⁴C]mycolaminaran. Soybean membrane preparations contained β-1,3-endoglucanase activity that degraded mycolaminaran and reduced both its efficiency as a phytoalexin elicitor and its membrane binding at temperatures above 0°C. Once [¹⁴C]mycolaminaran bound to membranes, however, it was not appreciably susceptible to glucanase attack and could not be displaced with excess unlabeled ligand. Taken collectively, the observations suggest that the membrane binding sites are mycolaminaran-specific receptors which are physiologically involved in the initiation of phytoalexin production in soybean cotyledons. Because the binding of mycolaminaran to membranes was abolished by heat and proteolytic enzymes, the receptor is probably a protein(s) or glycoprotein(s).     

Rapid Response of Suspension-cultured Parsley Cells to the Elicitor from Phytophthora megasperma var. sojae: INDUCTION OF THE ENZYMES OF GENERAL PHENYLPROPANOID METABOLISM

Large and rapid increases in the activities of two enzymes of general phenylpropanoid metabolism, phenylalanine ammonia-lyase and 4-coumarate:CoA ligase, occurred in suspension-cultured parsley cells (Petroselinum hortense) treated with an elicitor preparation from Phytophthora megasperma var. sojae. Highest enzyme activities were obtained with an elicitor concentration similar to that required for maximal phenylalanine ammonialyase induction in cell suspension cultures of soybean, a natural host of the fungal pathogen.     

Stress Responses in Alfalfa (Medicago sativa L.) : V. Constitutive and Elicitor-Induced Accumulation of Isoflavonoid Conjugates in Cell Suspension Cultures

The isoflavonoid conjugates medicarpin-3-O-glucoside-6″-O-malonate (MGM), afrormosin-7-O-glucoside (AG), and afrormosin-7-O-glucoside-6″-O-malonate (AGM) were isolated and characterized from cell suspension cultures of alfalfa (Medicago sativa L.), where they were the major constitutive secondary metabolites. They were also found in alfalfa roots but not in other parts of the plant. The phytoalexin medicarpin accumulated rapidly in suspension cultured cells treated with elicitor from Colletotrichum lindemuthianum, and this was subsequently accompanied by an increase in the levels of MGM. In contrast, net accumulation of afrormosin conjugates was not affected by elicitor treatment. Labeling studies with [¹⁴C]phenylalanine indicated that afrormosin conjugates were the major de novo synthesized isoflavonoid products in unelicited cells. During elicitation, [¹⁴C]phenylalanine was incorporated predominantly into medicarpin, although a significant proportion of the newly synthesized medicarpin was also conjugated. Treatment of ¹⁴C-labeled, elicited cells with l-α-aminooxy-β-phenylpropionic acid, a potent inhibitor of PAL activity in vivo, resulted in the initial appearance of labeled medicarpin of very low specific activity, suggesting that the phytoalexin could be released from a preformed conjugate under these conditions. Our data draw attention to the involvement of isoflavone hydroxylases during the constitutive and elicitor-induced accumulation of isoflavonoids and their conjugates in alfalfa cell cultures.     

Effects of Ca2+ on phytoalexin induction by fungal elicitor in soybean cells

A glucan elicitor from the cell walls of the fungus Phytophthora megasperma f.sp. glycinea caused increases in the activities of the phytoalexin biosynthetic enzymes, phenylalanine ammonia-lyase and chalcone synthase, and induced the production of the phytoalexin, glyceollin, in soybean (Glycine max) cell suspension cultures when tested in culture medium containing 1.2 mmol/liter Ca2+. Removal of extracellular Ca2+ by treatment with ethylene glycol bis(beta-aminoethyl ether)-N, N'-tetraacetic acid followed by washing the cells with Ca2+-free culture medium abolished the elicitor-mediated phytoalexin response. This suppression was largely reversed on readdition of Ca2+. Elicitor-mediated enhancement of biosynthetic enzyme activities and accumulation of glyceollin was strongly inhibited by La3+; effective concentrations for 50% inhibition were (mumol/liter) 40 for phenylalanine ammonia-lyase, 100 for chalcone synthase, and 30 for glyceollin. Verapamil caused similar effects only at concentrations higher than 0.1 mmol/liter, whereas trifluoperazine and 8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate did not affect enzyme induction by the elicitor in the concentration range tested. Uptake of alpha-amino isobutyric acid into soybean cells, which was rapidly inhibited in the presence of the glucan elicitor, was not affected by La3+ nor was uptake inhibition by the elicitor relieved by La3+. The Ca2+ ionophore, A23187, enhanced phytoalexin biosynthetic enzyme activities and glyceollin accumulation in a dose-dependent manner, with 50% stimulation (relative to the elicitor) occurring at about 5 mumol/liter. The results suggest that the glucan elicitor causes changes in metabolite fluxes across the plasma membrane of soybean cells, among which changes in Ca2+ fluxes appear to be important for the stimulation of the phytoalexin response.     

Properties of Rhizopus stolonifer Polygalacturonase, an Elicitor of Casbene Synthetase Activity in Castor Bean (Ricinus communis L.) Seedlings

Some properties of the polygalacturonase-elicitor from the filtrates of Rhizopus stolonifer cultures have been examined in an attempt to understand its mode of action as an elicitor of casbene synthetase activity in castor bean seedlings. Both the polygalacturonase activity and the elicitor activity are heat-labile with similar heat-sensitivity profiles. Also, the catalytic activity of the enzyme is lost on treatment with sodium periodate, as had been shown previously for the elicitor activity. The pH optimum of the enzyme activity with polygalacturonic acid as the substrate is 4.9. Exposures of germinating castor bean seedlings to the elicitor for short-term periods of 1 to 10 minutes followed by washing and incubation in sterile, distilled water are partially effective in elicitation in comparison with the continuous exposure of the seedlings over 11 hours to the same amount of the elicitor. The initial rate of reaction catalyzed by the enzyme is about 3 times faster with polygalacturonic acid as a substrate than with partially (50%) methylated polygalacturonic acid (pectin). The K_m value of the enzyme for polygalacturonic acid is about 4.2 millimolar in terms of monomeric units and about 0.07 millimolar in terms of polymer concentration. Examination of the types of products formed by the action of the enzyme suggests that it is an endo-hydrolase. The amino acid composition of this enzyme is similar to those of other extracellular fungal proteins reported. The carbohydrate moiety of the glycoprotein polygalacturonase-elicitor is composed of 92% mannose and 8% glucosamine by gas chromatography-mass spectrometry analysis. The linkage group analysis of the carbohydrate moiety showed that mannosyl residues which are 1,2-linked comprise about 70% of the total glycosyl residues and demonstrated the presence of some 1,3,6- and 1,2,6-linked branching mannosyl residues.     

Different cell-wall components from Phytophthora megasperma f. sp. glycinea elicit phytoalexin production in soybean and parsley

Different components of a crude cell-wall preparation from the phytopathogenic fungus, Phytophthora megasperma f. sp. glycinea, act as elicitors of phytoalexin accumulation in parsley (Petroselinum crispum) and soybean (Glycine max). Treatments of cultured parsley cells and protoplasts or soybean cells and cotyledons with proteinase-digested or deglycosylated elicitor preparations identify proteinaceous constituents as active eliciting compounds in parsley, which are inactive in soybean. The proteinase-treated elicitor as well as a defined heptaglucan are active in soybean but do not stimulate phytoalexin synthesis in parsley. Soybean and parsley cells therefore not only perceive different signals from cell walls of Phytophthora megasperma f. sp. glycinea, but are unable to respond to the fungal compounds primarily recognized by the other plant.Abbreviations Pmg Phytophthora megasperma f. sp. glycinea