High-affinity binding of fungal beta-glucan fragments to soybean (Glycine max L.) microsomal fractions and protoplasts

We have recently reported the existence of binding sites in soybean membranes for a beta-glucan fraction derived from the fungal pathogen Phytophthora megasperma f. sp. glycinea, which may play a role in the elicitor-mediated phytoalexin response of this plant [Schmidt, W. E. & Ebel, J. (1987) Proc. Natl Acad. Sci. USA 84, 4117-4121]. The specificity of beta-glucan binding to soybean membranes has now been investigated using a variety of competing polyglucans and oligoglucans of fungal origin. P. megasperma beta-glucan binding showed high apparent affinity for branched glucans with degrees of polymerization greater than 12. Binding affinity showed good correlation with elicitor activity as measured in a soybean cotyledon bioassay. Modification of the glucans at the reducing end with phenylalkylamine reagents had no effect on binding affinity. This characteristic was used to synthesize an oligoglucosyl tyramine derivative suitable for radioiodination. The 125I-glucan (15-30 Ci/mmol) provided higher sensitivity and lower detection limits for the binding assays while behaving in a manner identical to the [3H]glucan used previously. More accurate determinations of the Kd value for glucan binding indicated a higher affinity than previously shown (37 nM versus 200 nM). The 125I-glucan was used to provide the first reported evidence of specific binding of a fungal beta-glucan fraction in vivo to soybean protoplasts. The binding affinity to protoplasts proved identical to that found in microsomal fractions.

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.

Further investigations of race:cultivar-specific induction of enzymes related to phytoalexin biosynthesis in soybean roots following infection with Phytophthora megasperma f.sp. glycinea

The activities of the following enzymes in soybean roots were determined at early times after infection of the roots with zoospores of an incompatible or a compatible race of Phytophthora megasperma f.sp. glycinea: dimethylallyl-diphosphate : 3,6a,9-trihydroxypterocarpan dimethylallyltransferase (prenyltransferase), an enzyme specific for glyceollin biosynthesis; NADPH-cytochrome reductase and hydroxymethylglutaryl-CoA reductase, enzymes related to the glyceollin pathway; and isocitrate dehydrogenase. Already at 4 h after infection there was a higher activity of the prenyltransferase in the incompatible interaction than in the compatible interaction, and enzyme activity in the incompatible interaction increased considerably between 4 and 8 h after infection. In the compatible interaction prenyltransferase activity was only slightly higher than in uninfected roots. The activity of the other enzymes in infected roots was not significantly different from that in the uninfected roots. No qualitative differences could be detected between the two-dimensional patterns of unlabelled proteins or proteins labelled with L-[35S]methionine of infected and uninfected roots at early times after infection. We conclude from these and earlier results (A. Bonhoff et al. (1986) Arch. Biochem. Biophys. 246, 149-154) that infection of the soybean roots with an incompatible race of the fungus leads to selective induction of the phytoalexin pathway and presumably to induction of other as yet unknown defense mechanisms.

Race:cultivar-specific induction of enzymes related to phytoalexin biosynthesis in soybean roots following infection with Phytophthora megasperma f. sp. glycinea

Primary roots of soybean [Glycine max (L.), cv Harosoy 63] seedlings were inoculated with zoospores from either race 1 (incompatible, host resistant) or race 3 (compatible, host susceptible) of Phytophthora megasperma f. sp. glycinea (Pmg) and the activities of phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS), isoflavone synthase, and dihydroxypterocarpan 6a-hydroxylase related to phytoalexin (glyceollin) biosynthesis, and of glucose-6-phosphate dehydrogenase (Glc-6-PDH) and glutamate dehydrogenase (Glu-DH) were determined at various times after inoculation. About 2-4 h after inoculation with race 1, the activities of PAL, CHS, and pterocarpan 6a-hydroxylase were higher than after inoculation with race 3 and increased considerably thereafter. In contrast, activities of these enzymes in the compatible interaction were equal to or only slightly higher than in the controls over the entire infection period investigated (2-8 h). Isoflavone synthase did not increase until 7 h after inoculation with race 1. There were no significant differences in activities for Glc-6-PDH and Glu-DH between inoculated roots and controls. The results show that infection of soybean roots with zoospores of Pmg race 1 causes a race:cultivar-specific early induction of enzymes involved in glyceollin synthesis, whereas such an induction does not occur with zoospores of race 3. These findings are in agreement with the race:cultivar-specific accumulation of glyceollin in soybean roots reported previously [M. G. Hahn, A. Bonhoff, and H. Grisebach (1985) Plant Physiol. 77, 591-601].

Elicitor-induced phytoalexin synthesis in soybean cells: changes in the activity of chalcone synthase mRNA and the total population of translatable mRNA

Rapid changes in the mRNA activity encoding chalcone synthase, a central enzyme involved in isoflavonoid phytoalexin synthesis, were induced in cultured cells of soybean (Glycine max) after treatment with a glucan elicitor from the cell walls of the fungus, Phytophthora megasperma f. sp. glycinea, a soybean pathogen. Two-dimensional gel electrophoresis of the in vitro- and in vivo-synthesized chalcone synthase showed that it consisted of a group of proteins of similar molecular weights of about 41,000, but with differing isoelectric points between pH 6.1 and pH 7.1. Total activity of chalcone synthase mRNA increased as early as 40 to 60 min after the onset of elicitor induction, and reached a peak at about 4 h. Treatment with the fungal elicitor caused major changes in the population of total translatable RNA as indicated by two-dimensional electrophoresis of the translation products. The mRNA activities for at least 16 proteins were increased and for at least 4 proteins were decreased. The elicitor-induced changes in the population of translatable mRNA occurred at a rate similar to that observed for chalcone synthase mRNA activity. Our results suggest that soybean cells respond to the glucan elicitor by major metabolic changes at the RNA level including the enhanced capacity for phytoalexin synthesis.

Phytoalexin synthesis in soybean cells: elicitor induction of phenylalanine ammonia-lyase and chalcone synthase mRNAs and correlation with phytoalexin accumulation

A glucan elicitor from cell walls of the fungus Phytophthora megasperma f. sp. glycinea, a pathogen of soybean (Glycine max), induced large and rapid increases in the activities of enzymes of general phenylpropanoid metabolism, phenylalanine ammonia-lyase, and of the flavonoid pathway, acetyl-CoA carboxylase and chalcone synthase, in suspension-cultured soybean cells. The changes in phenylalanine ammonia-lyase and chalcone synthase activities were correlated with corresponding changes in the mRNA activities encoding these enzymes, as determined by enzyme synthesis in vitro in a mRNA-dependent reticulocyte lysate. The time courses of the elicitor-induced changes in mRNA activities for both enzymes were very similar with respect to each other. Following the onset of induction, the two mRNA activities increased significantly at 3 h, reached highest levels at 5 to 7 h, and subsequently returned to low values at 10 h. Similar degrees of induction of mRNA activities and of the catalytic activities of phenylalanine ammonia-lyase and chalcone synthase were observed in response to three diverse microbial compounds, the glucan elicitor from P. megasperma, xanthan, an extracellular polysaccharide from Xanthomonas campestris, and endopolygalacturonase from Aspergillus niger. However, whereas the glucan elicitor induced the accumulation of large amounts of the phytoalexin, glyceollin, in soybean cells, endopolygalacturonase induced only low, albeit significant, amounts; xanthan did not enhance glyceollin accumulation under the conditions of this study. This result might imply that enzymes other than phenylalanine ammonia-lyase or chalcone synthase exert an important regulatory function in phytoalexin synthesis in soybean cells.

Improved purification and further characterization of acetyl-CoA carboxylase from cultured cells of parsley (Petroselinum hortense)

Acetyl-CoA carboxylase from irradiated cell-suspension cultures of parsley (Petroselinum hortense) has been purified to apparent homogeneity. The procedure included affinity chromatography of the enzyme on avidinmonomer--Sepharose 4B. Molecular weights of about 420000 for the native enzyme and about 220000 for the enzyme subunit were determined respectively by gel filtration or sucrose-density-gradient sedimentation and by electrophoresis in the presence of dodecyl sulfate. The purified enzyme showed an isoelectric point of 5. The enzyme carboxylated the straight-chain acyl-CoA esters of acetate, propionate, and butyrate at decreasing rates in this order. The catalytic efficiency of the carboxylase was highest when ATP existed largely as MgATP2- complex. At the optimum pH of 8 the apparent Km values for the substrates were: acetyl-CoA, 0.15 mmol/1; bicarbonate, 1 mmol/1; MgATP2-, 0.07 mmol/1. The carboxylase was inhibited by greater than 50 mmol/l NaCl, KCl, or Tris/HCl buffer. The putative allosteric activator, citrate, stimulated the enzyme only slightly at concentrations below 2 mmol/l, but strongly inhibited the carboxylase at higher concentrations. The results of these studies demonstrate that several properties of the light-inducible acetyl-CoA carboxylase of parsley cells, an enzyme of the flavonoid pathway, are remarkably similar to those of acetyl-CoA carboxylases from a variety of other organisms.

Induction of enzymes of phytoalexin synthesis in cultured soybean cells by an elicitor from Phytophthora megasperma f. sp. glycinea

The glucan elicitor from cell walls of the fungal pathogen, Phytophthora megasperma f. sp. glycinea, induced rapid but transient increases in enzyme activities of general phenylpropanoid metabolism (phenylalanine ammonia-lyase and 4-coumarate: CoA ligase) and of the flavonoid pathway (chalcone synthase) in cell suspension cultures of soybean (Glycine max). After transferring cells into fresh medium, two peaks of inducibility for the enzymes by elicitor were observed, one shortly after transfer (stage I), and one at the end of the linear growth phase (stage II). Only one of the two isoenzymes of 4-coumarate: CoA ligase ("isoenzyme 2"), for which a specific involvement in flavonoid biosynthesis has been postulated, was affected by the elicitor. For two of the induced enzymes, phenylalanine ammonia-lyase and chalcone synthase, the changes in activity at stage I were shown to be preceded by large changes in their rates of synthesis, as determined by in vivo labelling with [(35)S] methionine and immunoprecipitation.

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.The changes in phenylalanine ammonia-lyase activity in parsley cells were caused by corresponding changes in the mRNA activity for this enzyme. Phenylalanine ammonia-lyase mRNA activity increased much faster and transiently reached a much higher level in elicitor-treated than in irradiated cell cultures. In contrast to irradiation, treatment of the cells with the elicitor did not induce the enzymes of the flavonoid glycoside pathway, as demonstrated for acetyl-CoA carboxylase and chalcone synthase. Induction of these enzymes by light was abolished by simultaneous application of the elicitor.

Enzymes of General Phenylpropanoid Metabolism and of Flavonoid Glycoside Biosynthesis in Parsley: Differential Inducibility by Light during the Growth of Cell Suspension Cultures

Several enzymes of phenylpropanoid metabolism showed large changes in their inducibility by light during the growth cycle of cell suspension cultures from parsley (Petroselinum hortense Hoffm.). Two of the three enzymes of general phenylpropanoid metabolism (group I) and six of the approximately 13 enzymes of the flavone and flavonol glycoside pathways (group II) were investigated. Both enzymes of group I (phenylalanine ammonia-lyase and 4-coumarate:coenzyme A ligase) were most efficiently induced at two different stages: first, soon after starting a new culture, and second, near the beginning of the stationary phase. In contrast, the enzymes of group II (acetyl-coenzyme A carboxylase, flavanone synthase, chalcone isomerase, UDP-apiose synthase, and at least one of two malonyltransferases) were maximally induced during exponential growth of the culture. This result supports the conclusions drawn from previous data that the two groups are regulated differentially and that the enzymes within each group are regulated in a coordinated manner.

Enzymic synthesis of lignin precursors. Purification and properties of a cinnamoyl-CoA: NADPH reductase from cell suspension cultures of soybean (Glycinemax)

A cinnamoyl-coenzyme A reductase catalyzing the NADPH-dependent reduction of substituted cinnamoyl-CoA thiol esters to the corresponding cinnamaldehydes was isolated from cell suspension cultures of soybean (Glycine max L. var. Mandarin). A 1660-fold purification of the enzyme was achieved by (NH4)2SO4 fractionation, chromatography on DEAE-cellulose, hydroxyapatite and Sephadex G-100 and affinity chromatography on 5'-AMP-Sepharose. The apparent molecular weight of the reductase was found to be about 38 000 on the basis of the elution volume from a Sephadex G-100 column. Maximum rate of reaction was observed between pH 6.0 and 6.2 in 0.1-0.2 M citrate buffer at 30 degrees C. The enzyme was markedly inhibited by thiol reagents. The reductase showed a high degree of specificity for cinnamoyl-CoA esters. Feruloyl-CoA was the substrate with the lowest Km value (73 muM) and highest V (230 nkat/mg) followed by 5-hydroxy-feruloyl-CoA, sinapoyl-CoA, p-coumaroyl-CoA, caffeoyl-CoA and cinnamoyl-CoA. No reaction took place with acetyl-CoA. The Km value for NADPH varied with the type of substrate. Km values of 28, 120, and 290 muM were found with feruloyl-CoA, sinapoyl-CoA, and p-coumaroyl-CoA, respectively. The rate of reaction observed with NADH was only about 5% of that found with NADPH. The reaction products CoASH and NADP+ inhibited the reaction. The Ki values were in the range of 0.5-1 mM and the inhibition was of a noncompetitive (mixed) type. The role of the reductase in the biosynthesis of lignin precursors is discussed.

Host-Pathogen Interactions: IX. Quantitative Assays of Elicitor Activity and Characterization of the Elicitor Present in the Extracellular Medium of Cultures of Phytophthora megasperma var. sojae

Resistance of soybean (Glycine max L.) seedlings to Phytophthora megasperma var. sojae (Pms) is in part due to the accumulation in infected tissue of a compound which is toxic to Pms. The accumulation of this compound, a phytoalexin called glyceollin, 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 elicitor. This elicitor was isolated from Pms cultures and purified by ion exchange and molecular sieving chromatography. The extracellular 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.

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.