Pathogen Phytophthora Sojae Research Articles

Cultured parsley cells, a model system for the rapid testing of abiotic and natural substances as inducers of systemic acquired resistance

In cultured parsley (Petroselinum crispumL.) cells, chemical and natural agents that are known to induce systemic acquired resistance (SAR) in different plant species were checked for their ability to condition the cells for an enhanced response to low doses of an elicitor from the cell wall of the fungal pathogenPhytophthora sojae. Use of the fluorescence properties of elicitor-induced furanocoumarin phytoalexins in the culture fluids of parsley cells allows rapid testing of many substances over a broad range of concentrations for their SAR-inducing capacity. Except for probenazole, a number of agents which have been described as effective inducers of disease resistance in plants were able to activate cells for an enhanced elicitor-mediated furanocoumarin accumulation. Among the compounds tested, benzo[1,2,3]thiadiazole-7-carbothioic acid-S-methyl ester, the active ingredient of the commercially available SAR activator Bion®, was found to have the highest potential for conditioning parsley cells. On the other hand, compounds which are known to be inactive as inducers of disease resistance failed to enhance the elicitor response. The results suggest that cultured parsley cells provide a suitable system for the identification of SAR-inducing agents.Using the parsley test system, the artificial sweetener saccharin, a substance not previously reported to induce SAR in plants, was found to be very effective in conditioning the cells for an enhanced accumulation of coumarin derivatives. Furthermore, when applied to cucumber, tobacco and bean plants, saccharin was found to induce resistance against both fungal and viral diseases.

A copia-like retrotransposon Tgmr closely linked to the Rps1-k allele that confers race-specific resistance of soybean to Phytophthora sojae.

We have isolated and characterized Tgmr, a copia-like retrotransposon, linked tightly to the Rps1-k allele that confers race-specific resistance of soybean to the the fungal pathogen Phytophthora sojae. Southern analysis followed by PCR and sequence analyses, using primers based on sequences flanking the insertion site confirmed that the element was inserted in the neighboring region of Rps1-k but not in that of the other four Rps1 alleles. This implies that Tgmr was transposed into the Rps1-k flanking site after the divergence of Rps1 alleles. Southern analysis of a series of diverse soybean cultivars revealed a high level of polymorphism of Tgmr-related sequences. These results indicate that this low copy retroelement family could have been active in the soybean genome in the recent past. Tgmr contains long terminal repeats (LTR) and four non-overlapping open reading frames (ORF), presumably originating from mutations leading to stop codons of a single ORF. The conserved domains for gag, protease, integrase, reverse transcriptase and RNaseH are present in the internal portion of the element. However, the protease, reverse transcriptase and RNaseH of this element are non-functional due to the presence of several stop codons. Possible transactivation of Tgmr and application of this element in insertional mutagenesis for soybean are discussed.

Pod Inoculation Technique with Phytophthora Sojae to Evaluate Soybean Populations for RPS Alleles in Field Plantings

The pathogen Phytophthora sojae (M.J. Kaufmann and J.W. Gerdemann) can be injected into soybean pods [Glycine max (L.) Merr.] determine presence of phytophthora root and stem rot resistance. The objective of this study was to use pod inoculation in the field to screen soybean F2 populations segregating for Rps alleles. The potential exists to inject different races of the pathogen into pods at different nodes of a single plant, allowing for multiple screenings at a time. Controlling the amount of Phytophthora sojae pathogen injected into each pod was necessary for accurate scoring of disease progression. Injecting 25 μL of Race 1 or Race 3 mycelium inoculum gave optimal symptom differences. The amount of disease progression up the pod to the point of attachment or the complete lack of rotting was determined to distinguish between susceptible and resistant plants, respectively. The data obtained from scoring pods conformed well to ratios of expected models. This technique provides a rapid and inexpensive field screening for identifying resistant or susceptible progeny early in the selection process.

Partial Resistance to Root-Borne Infection by Phytophthora sojae in Three Allelic Necrotic Root Mutants in Soybean

Disease lesion mimic mutations that affect aerial parts of plants have been characterized in several species, but root disease mimics have not been previously described. Our objective was to characterize genetically and physiologically three independently derived necrotic root mutants found in soybean. The three mutants were inherited as a single recessive gene, were allelic, and were not linked to any of seven marker traits tested. When inoculated via the hypocotyl, necrotic root plants were susceptible to hyphal infection by a compatible race of the fungal pathogen Phytophthora sojae. In contrast, the roots showed partial resistance to infection by P. sojae zoospores, which correlated with the accumulation of isoflavonoid phytoalexins and group 2 peroxidases under axenic conditions. However, activation of the defense response in roots of the necrotic root mutants was insufficient to prevent nodulation by the symbiotic microorganism Bradyrhizobium japonicum.

Suppression of fungal ?-glucan-induced plant defence in soybean (Glycine max L.) by cyclic 1,3-1,6-?-glucans from the symbiont Bradyrhizobium japonicum

The production of antimicrobial phytoalexins is one of the best-known inducible defence responses following microbial infection of plants or treatment with elicitors. In the legume soybean (Glycine max L.), 1,3-1,6-β-glucans derived from the fungal pathogen Phytophthora sojae have been identified as potent elicitors of the synthesis of the phytoalexin, glyceollin. Recently it has been reported that during symbiotic interaction between soybean and the nitrogen-fixing bacterium Bradyrhizobium japonicum USDA 110 the bacteria synthesize cyclic 1,3-1,6-β-glucans. Here we demonstrate that both the fungal and the bacterial β-glucans are ligands of β-glucan-binding sites which are putative receptors for the elicitor signal compounds in soybean roots. Whereas the fungal β-glucans stimulate phytoalexin synthesis at low concentrations, the bacterial cyclic 1,3-1,6-β-glucans appear to be inactive even at relatively high concentrations. Competition studies indicate that increasing concentrations of the bacterial 1,3-1,6-β-glucans progressively inhibit stimulation of phytoalexin synthesis in a bioassay induced by the fungal 1,3-1,6-β-glucans. Another type of cyclic β-glucan, a 1,2-β-glucan from Rhizobium meliloti, that does not nodulate on soybean, seems to be inactive as elicitor and as ligand of the β-glucan-binding sites. These results may indicate a novel mechanism for a successful plant-symbiont interaction by suppressing the plant's defence response.

Towards standard European measures of outcome

A cross between two different races (race 7 × race 25) of the soybean root and stem rot pathogen Phytophthora sojae was analyzed to characterize the genomic region flanking two cosegregating avirulence genes, Avr4 and Avr6. Both genes cosegregated in the ratio of 82:17 (avirulent:virulent) in an F2 population, suggestive of a single locus controlling both phenotypes. A chromosome walk was commenced from RAPD marker OPE7.1C, 2.0 cM distant from the Avr4/6 locus. Three overlapping cosmids were isolated which included genetic markers that flank the Avr4/6 locus. The chromosome walk spanned a physical distance of 67 kb which represented a genetic map distance of 22.3 cM, an average recombination frequency of 3.0 kb/cM and 11.7-fold greater than the predicted average recombination frequency of 35.3 kb/cM for the entire P. sojae genome. Six genes (cDNA clones) expressed from the Avr4/6 genomic region encompassed by the cosmid contig were identified. Single nucleotide polymorphisms and restriction fragment length polymorphisms showed these six genes were closely linked to the Avr4/6 locus. Physical mapping of the cDNA clones within the cosmid contig made it possible to deduce the precise linkage order of the cDNAs. None of the six cDNA clones appear to be candidates for Avr4/6. We conclude that two of these cDNA clones flank a physical region of approximately 24 kb and 4.3 cM that appears to include the Avr4/6 locus.