Fungal elicitors of the phytoalexin response in higher plants

Roles of small RNAs in soybean defense against Phytophthora sojae infection

The genus Phytophthora consists of many notorious pathogens of crops and forestry trees. At present, battling Phytophthora diseases is challenging due to a lack of understanding of their pathogenesis. We investigated the role of small RNAs in regulating soybean defense in response to infection by Phytophthora sojae, the second most destructive pathogen of soybean. Small RNAs, including microRNAs (miRNAs) and small interfering RNAs (siRNAs), are universal regulators that repress target gene expression in eukaryotes. We identified known and novel small RNAs that differentially accumulated during P. sojae infection in soybean roots. Among them, miR393 and miR166 were induced by heat-inactivated P. sojae hyphae, indicating that they may be involved in soybean basal defense. Indeed, knocking down the level of mature miR393 led to enhanced susceptibility of soybean to P. sojae; furthermore, the expression of isoflavonoid biosynthetic genes was drastically reduced in miR393 knockdown roots. These data suggest that miR393 promotes soybean defense against P. sojae. In addition to miRNAs, P. sojae infection also resulted in increased accumulation of phased siRNAs (phasiRNAs) that are predominantly generated from canonical resistance genes encoding nucleotide binding-leucine rich repeat proteins and genes encoding pentatricopeptide repeat-containing proteins. This work identifies specific miRNAs and phasiRNAs that regulate defense-associated genes in soybean during Phytophthora infection.

Insect-induced daidzein, formononetin and their conjugates in soybean leaves

In response to attack by bacterial pathogens, soybean (Gylcine max) leaves accumulate isoflavone aglucones, isoflavone glucosides, and glyceollins. In contrast to pathogens, the dynamics of related insect-inducible metabolites in soybean leaves remain poorly understood. In this study, we analyzed the biochemical responses of soybean leaves to Spodoptera litura (Lepidoptera: Noctuidae) herbivory and also S. litura gut contents, which contain oral secretion elicitors. Following S. litura herbivory, soybean leaves displayed an induced accumulation of the flavone and isoflavone aglycones 4’,7-dihyroxyflavone, daidzein, and formononetin, and also the isoflavone glucoside daidzin. Interestingly, foliar application of S. litura oral secretions also elicited the accumulation of isoflavone aglycones (daidzein and formononetin), isoflavone 7-O-glucosides (daidzin, ononin), and isoflavone 7-O-(6’-O-malonyl-β-glucosides) (malonyldaidzin, malonylononin). Consistent with the up-regulation of the isoflavonoid biosynthetic pathway, folair phenylalanine levels also increased following oral secretion treatment. To establish that these metabolitic changes were the result of de novo biosynthesis, we demonstrated that labeled (¹³C₉) phenylalanine was incorporated into the isoflavone aglucones. These results are consistent with the presence of soybean defense elicitors in S. litura oral secretions. We demonstrate that isoflavone aglycones and isoflavone conjugates are induced in soybean leaves, not only by pathogens as previously demonstrated, but also by foliar insect herbivory.

A polypeptide from tomato leaves induces wound-inducible proteinase inhibitor proteins

Defensive genes in plants can be activated by several different types of nonpeptide signaling molecules. An endogenous polypeptide, consisting of 18 amino acids, was isolated from tomato leaves and was able at very low concentrations to induce the synthesis of two wound-inducible proteinase inhibitor proteins when supplied to young tomato plants. The sequence of the polypeptide was determined, and an identical polypeptide was synthesized that possessed full inducing activity. These data establish that a polypeptide factor can initiate signal transduction to regulate the synthesis of defensive proteins in plant tissues.

Rapid switching of plant gene expression induced by fungal elicitor

The pattern of messenger RNA synthesis in suspension-cultured bean cells (Phaseolus vulgaris L.) was analyzed by blot hybridization and in vitro translation of newly synthesized messenger RNA. The RNA was separated from preexisting RNA by organomercurial affinity chromatography after in vivo labeling with 4-thiouridine. The elicitor induced the synthesis of messenger RNA's encoding phenylalanine ammonia-lyase, chalcone synthase, and chalcone isomerase, three enzymes of phenylpropanoid metabolism involved in the synthesis of isoflavonoidderived phytoalexins. This is part of a rapid and extensive change in the pattern of messenger RNA synthesis directing production of a set of proteins associated with expression of disease resistance.

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.

Elicitor stimulation of the defense response in cultured plant cells monitored by fluorescent dyes

Addition of fungal elicitors to plant cells in suspension is known to stimulate biochemical changes in the plant cell leading to production of defense compounds. In this paper we demonstrate that introduction of elicitors from the pathogenic fungus Verticillium dahliae to cultured cotton, tobacco, or soybean cells leads to a rapid, dramatic change in the fluorescence of several membrane-associated potentiometric or pH-sensitive dyes. The fluorescence transitions occur abruptly following a brief (0 to 10 min) lag period in apparently most cells of the suspension simultaneously. Furthermore, both the length of the lag period and the rate of the subsequent fluorescence change were shown to be highly dependent on elicitor concentration. When the crude elicitor extract was separated by gel filtration chromatography into several active fractions, the ability of each fraction to stimulate phytoalexin production in the cotton cell suspension was found to correlate directly with the rate of the fluorescence decrease in the fluorescence assay. Because the assay is rapid, simple to perform, quantitative, and reproducible, it represents an attractive alternative to the more cumbersome and perhaps less quantitative elicitor assays currently in use. The fact that membrane-potential-sensitive dyes of different structure respond to elicitation of plant cells similarly further suggests, but does not prove, that asymmetric ion fluxes into or out of the plant cell are involved in the initial events of elicitor signal transduction.

Several biotic and abiotic elicitors act synergistically in the induction of phytoalexin accumulation in soybean

Plants often respond to microbial infection by producing antimicrobial compounds called phytoalexins. Plants also produce phytoalexins in response to in vitro treatment with molecules called elicitors. Specific elicitors, including a hexa-β-glucosyl glucitol derived from fungal cell walls, the pectin-degrading enzyme endopolygalacturonic acid lyase, and oligogalacturonides obtained by either partial acid hydrolysis or enzymatic degradation of plant cell walls or citrus polygalacturonic acid, induce soybean (Glycine max. L.) cytoledons to accumulate phytoalexins. The experiments reported here demonstrate that the elicitor-active hexa-β-glucosyl glucitol acts synergistically with several biotic and abiotic elicitors in the induction of phytoalexins in soybean cotyledons. At concentrations below 50 ng/ml, the hexa-β-glucosyl glucitol does not induce significant phytoalexin accumulation. When assayed in combination with either endopolygalacturonic acid lyase or with a decagalacturonide released from citrus polygalacturonic acid by this lyase, however, the observed elicitor activity of the hexa-β-glucosyl glucitol is as much as 35-fold higher than the sum of the responses of these elicitors assayed separately. A similar synergism was also demonstrated for the combination of the hexa-β-glucosyl glucitol with dilute solutions of sodium acetate, sodium formate, or sodium propionate buffers. These buffers are thought to damage or kill plant cells, which may cause the release of oligogalacturonides from the plant cell wall. The results suggest that oligogalacturonides act as signals of tissue damage and, as such, can enhance the response of plant tissues to other elicitor-active molecules during the initiation of phytoalexin accumulation.

Autoclaved fungal mycelia increase diosgenin production in cell suspension cultures of Dioscorea deltoidea

The addition of autoclaved mycelia of non-host specific fungi to cell suspension cultures of Dioscorea deltoidea improved diosgenin production by as much as 72% compared to control cultures. Phytoalexin elicitors laminarin, arachidonic acid and chitin added to D. deltoidea cultures had no stimulating effect on the diosgenin level.

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.

Induction and suppression of phytoalexin biosynthesis in cultured cells of safflower, Carthamus tinctorius L., by metabolites of Alternaria carthami Chowdhury

Cell suspension cultures derived from the safflower variety US-10 respond to treatment with cell wall elicitors from either Phytophthora megasperma f.sp.glycinea or Alternaria carthami Chowdhury by producing polyacetylenic phytoalexins. These polyacetylenes were absent from the uninduced cell cultures. Low concentrations of brefeldin A, a toxin produced by A. carthami, when added to suspension-cultured safflower cells, considerably diminished the accumulation of the phytoalexins following elicitor treatment. Suppression of the synthesis of polyacetylenic phytoalexins suggests a role for brefeldin A in limiting the host range of A. carthami, the causal agent of a leaf and head blight disease in safflower.

Rapid effect of an elicitor on uptake and intracellular distribution of phosphate in cultured parsley cells

Cell suspension cultures of parsley (Petroselinum hortense) grown in synthetic medium take up most of the inorganic phosphate supplied with the medium within the initial 5 days after transfer. Nuclear magnetic resonance spectra of intact parsley cells from this growth stage revealed that approximately half of the phosphate was located within the vacuoles, whereas after 7 days of growth phosphate content of the vacuoles was relatively low. At both times, addition of an elicitor preparation from Alternaria carthami, which is not toxic to the cells, led to a temporary increase of vacuolar phosphate at the expense of cytoplasmic phosphate, even when excess phosphate was added to the medium. The rapid decrease of cytoplasmic phosphate might play a role in the redirection of phenylpropanoid metabolism reported for elicitor-treated parsley cells.