Characterization of the glucose-6-phosphate isomerase gene in Phytophthora infestans reveals the presence of multiple alleles

Glucose-6-phosphate isomerase (GPI) plays a key role in both glycolysis and gluconeogenesis. Isoforms of GPI are common, and therefore, its isozyme pattern is widely used to characterize isolates of Phytophthora infestans. Despite the importance of GPI in P. infestans studies, the gene encoding this enzyme has not yet been characterized. Furthermore, it has been suggested that P. infestans contains multiple copies of the gene but this hypothesis remains to be demonstrated. We have cloned and characterized GPI in various isolates of P. infestans as well as in several species of the genus Phytophthora. The gene contains 1671bp and encodes a protein with a predicted molecular weight of 60.8kDa. Multiple different alleles were identified and Southern analysis indicated certain P. infestans isolates carry several copies of the gene. Phylogenetic analysis revealed that P. infestans GPI is most closely related to sequences from Toxoplasma gondii, Arabidopsis thaliana, and Clarkia lewisii.

Diversity of the Phytophthora infestans population in Flanders, Belgium

A total of 94 isolates of Phytophthora infestans were collected from disease outbreaks in commercial potato crops and private gardens in 2002 and 2003. The isolates were recovered successfully from single lesions of diseased potato foliage. Not from all isolates pure cultures were obtained due to contaminations with Fusarium species and bacteria. The structure of the population was analysed phenotypically. Characteristics of the isolates included in vitro growth rate, mating type, in vitro sensitivity to the phenylamide fungicide metalaxyl-M and allozyme genotype at glucose-6-phosphate isomerase (Gpi) and peptidase (Pep) loci. Significant differences in in vitro growth rate were observed among the 52 isolates by comparing the main radial growth of the isolates after 7 days. Forty seven from the isolates tested were the Al mating type. Only one isolate was characterized as A2 mating type. Isolates with sensitive, intermediate and resistant responses to metalaxyl-M were detected in the populations. Forty isolates had a growth of less then 40 % at 5 ppm metalaxyl-M. Three isolates had a growth of less then 40 % at 100 ppm metalaxyl-M. Eight isolates had a growth of more then 40 % at 5 and 100 ppm metalaxyl-M. Cellulose acetate electrophoresis was used to examine Gpi and Pep banding pattern of the population of P. infestans attacking potato in Flanders. All the isolates tested produced the 100/100 Gpi isozyme electromorph. Five different allozyme genotypes of the Pep loci were identified: 92/92, 96/96, 100/100, 92/100, 83/100.

Effects of inoculum density and substrate type on saprophytic survival of Phytophthora drechsleri, the causal agent of gummosis (crown and root rot) on pistachio in Rafsanjan, Iran

In this study, sampling was carrid out on soils around pistachio trees in various regions of Rafsanjan, Iran. Following isolation and identification of Phytophthora isolates, the predominant species was found to be P. drechsleri and used for further investigation. For studying saprophytic survival of the fungus, soils collected from different areas were combined and autoclaved. Sterile soil was divided into 10 parts and mixed with fungal inoculum at various concentrations of 0-9% (w/w) separately. Each soil part (100g) was placed in 15cm diameter plastic pot. Some soils in pots were supplemented with sterile wheat straw whereas others were mixed with pistachio leaves surface sterilized with 5% (v/v) sodium hypochlorite. After 3, 6 and 9 weeks of incubation, five leaves or straws samples were taken from each replicate and cultured on CMA-PARPH medium and the fungal colony formation was monitored. The experiment was performed using completely randomized design with factorial experiments including three factors (substrate type, inoculum density and time), 10 treatments (0-9 g inoculum levels) and nine replicates. The results showed that the type of substrate (wheat straw and pistachio leaf) was very important for the fungal saprophytic survival in that this was significantly greater for the pistachio leaves. Time was also considered another critical factor for the fungal survival. With passing incubation time, saprophytic survival of the fungus declined. Further, it was demonstrated that increasing inoculum density would result in longer survivability of P. drechsleri and maximum fungal survival on substrate was obtained when inoculum density was at 9% (w/w).

Distribution of zoosporic fungi in the mud of major Egyptian lakes

Twenty-six identified and three unidentified species belonging to eight genera of zoosporic fungi were recovered from seventy-six mud samples of Egyptian lakes using sesame seeds as baits. Nineteen samples were collected along the borders of four major lakes. Pythium, Phytophthora and Allomyces, yielded the highest spectra of zoosporic species, while Aqualinderella, Blastocladia and Dictyuchus, had one species. Pythium, Aphanomyces, Allomyces, Aqualinderella, Phytophthora and Saprolegnia were the dominant zoosporic genera while Blastocladia and Dictyuchus were rare. The most prevalent species of zoosporic fungi were Aqualinderella fermentans, Aphanomyces laevis and unidentified Pythium species while the other isolated species in this study were rare. Menzalah was the richest lake with zoosporic species followed by Borolous and Karoon. Some zoosporic fungal genera were recovered from the mud of the four tested lakes (participator genera) and others from three lakes or from only two lakes. Physicochemical characteristics of the mud samples (pH, total soluble salts and organic matter) revealed no correlation with the fungal occurrence and distribution in these lakes.

Detection of resistance to fungicides, mating types and fitness of Phytophthora infestans in Hebei, China

In vitro, isolates resistant to metalaxyl (M) and oxadixyl (O) of Phytophthora infestans were 11.2% of 62 isolates from potato and tomato in Hebei Province, mean resistance factor was 15,022 fold and 24,733 fold, respectively, no isolates resistant either to cymoxanil (C) or to dimethomorph (D) were detected. On the other hand, in vivo, isolates resistant to M and O were 29.0% and 32.7%, respectively, among 217 isolates from potato plants in Weichang and Chongli, Hebei, only one isolate resistant to M and O was found among 88 isolates from tomato plants in Xushui, Hebei. Among 73 isolates from potato in Weichang and Chongli, 6 isolates with A2 mating type were 8.2%, 3 A2 isolates resistant to metalaxyl and oxadixyl. 3 A2 isolates were 6.8% of 44 isolates from tomato in Xushui. Cross resistance in Phytophthora infestans was confirmed between M and O, but no cross resistance between C or D and M or O. The field isolates resistant to M and O were outstandingly fitter than sensitive or intermediate ones, mainly due to stronger sporulation capacity of the field isolates.

Spatial and temporal analysis of the microbial community in slow sand filters used for treating horticultural irrigation water

An experimental slow sand filter (SSF) was constructed to study the spatial and temporal structure of a bacterial community suppressive to an oomycete plant pathogen, Phytophthora cryptogea. Passage of water through the mature sand column resulted in complete removal of zoospores of the plant pathogen. To monitor global changes in the microbial community, bacterial and fungal numbers were estimated on selective media, direct viable counts of fungal spores were made, and the ATP content was measured. PCR amplification of 16S rRNA genes and denaturing gradient gel electrophoresis (DGGE) were used to study the dynamics of the bacterial community in detail. The top layer (1 cm) of the SSF column was dominated by a variable and active microbial population, whereas the middle (50 cm) and bottom (80 cm) layers were dominated by less active and diverse bacterial populations. The major changes in the microbial populations occurred during the first week of filter operation, and these populations then remained to the end of the study. Spatial and temporal nonlinear mapping of the DGGE bands provided a useful visual representation of the similarities between SSF samples. According to the DGGE profile, less than 2% of the dominating bands present in the SSF column were represented in the culturable population. Sequence analysis of DGGE bands from all depths of the SSF column indicated that a range of bacteria were present, with 16S rRNA gene sequences similar to groups such as Bacillus megaterium, Cytophaga, Desulfovibrio, Legionella, Rhodococcus rhodochrous, Sphingomonas, and an uncharacterized environmental clone. This study describes the characterization of the performance, and microbial composition, of SSFs used for the treatment of water for use in the horticultural industry. Utilization of naturally suppressive population of microorganisms either directly or by manipulation of the environment in an SSF may provide a more reproducible control method for the future.

Cocoa-based media for culturing Phytophthora palmivora (Butl.) Butl., causal agent of black pod disease of cocoa

Green cocoa pod husk agar (GCPA), ripe cocoa pod husk agar (RCPA), green cocoa bean agar (GCBA), ripe cocoa bean agar (RCBA), green cocoa mucilage agar (GCMA) and ripe cocoa mucilage agar (RCMA) were prepared and assessd for their clarity and for potential to support mycelial growth and sporulation of P. palmivora. Oatmeal agar (OMA), potato-dextrose agar (PDA), vegetable 8 juice agar (V8JA) and pineapple crown agar (PCA) were included for comparison. The highest radial growth rates of 8.3 and 7.2 mm/day were recorded, respectively, on OMA and GCPA but these were not significantly different (P < 0.05) from each other. The two media also supported good aerial mycelial growth but were not clear. Radial mycelial growth rates of 6.5, 7.0 and 6.6 mm/day were obtained on GCMA, RCPA and V8JA, respectively, and these rates were also not significantly different from each other. Of the three media, only the GCMA was clear and supported the best aerial mycelial growth. In comparison, the RCMA supported a significantly lower radial growth (4.6 mm/day) of P. palmivora than the three media. Growth rates were least on RCBA, PCA and PDA but sporulation was poorest on PDA, PCA and V8JA. GCMA was found to be the best medium based on all the growth parameters and media characteristics. GCMA has been used effectively to isolate/detect P. palmivora from infected cocoa pod tissues. Apart from differences in radial growth rate, both the GCMA and RCMA were similar in all other respects and are recommended for culturing P. palmivora.

Multiple new phenotypic taxa from trees and riparian ecosystems in Phytophthora gonapodyides-P. megasperma ITS Clade 6, which tend to be high-temperature tolerant and either inbreeding or sterile

Phytophthora isolates associated with Phytophthora major ITS Clade 6 were grouped into 11 phenotypic taxa. These comprised the described morphospecies P. gonapodyides, P. megasperma s. str. and P. humicola; four previously identified but so far undescribed taxa, informally designated here P. sp. O-group, P. sp. Apple-cherry, P. taxon Pgchlamydo, and P. taxon Walnut; and four previously unknown taxa, designated P. taxon Oaksoil, P. taxon Raspberry, P. taxon Forestsoil, and P. taxon Riversoil. With the exception of P. gonapodyides, each phenotypic taxon represented an unique ITS lineage. Two isolates morphologically identical to P. gonapodyides comprised a separate lineage and probably represent another taxon, designated here P. taxon Salixsoil, P. humicola, P. sp. O-group, P. sp. Apple-cherry and P. taxon Walnut grouped together as subclade I. Within subclade II, P. taxon Oaksoil, P. taxon Raspberry, P. taxon Forestsoil, P. taxon Riversoil and P. taxon Pgchlamydo formed a cluster of closely related but phenotypically distinct lineages basal to P. gonapodyides and P. megasperma, P. taxon Salixsoil being the most basal member. The taxonomy, adaptation and breeding systems of Clade 6 taxa are discussed. They show a strong association with forests and riparian ecosystems, only a limited association with agriculture and an ability to tolerate high temperatures. Also, in contrast to most other Phytophthora clades, Clade 6 taxa are predominantly sterile or inbreeding in culture. Only one taxon, P. sp. O-group, appears classically A1/A2 heterothallic.

Alder phytophthora in Poland: occurrence and plants colonization

Common alder (Alnus glutinosa) decline has been observed in Poland during the last 6 years. Alder Phytophthora was recorded, however, only from one sampling area in the middle of the country. Species of Armillaria, Fusarium, Mucor, Penicillium and Trichoderma were also isolated from diseased trees. Inoculation of alder stem parts, leaves and seedlings with Phytophthora isolates resulted in the development and spread of necrosis. Studies will be continued in the nearest years.

Temporal and subcellular localization of PR-1 proteins in tomato stem tissues infected by virulent and avirulent isolates of Phytophthora capsici

Immunoblot analysis and immunogold labeling of PR-1 protein (pathogenesis-related protein 1) in tomato (Lycopersicon esculentum Mill.) were performed to examine the temporal and spatial expression patterns of PR-1 protein induced by Phytophthora capsici infection. Soluble proteins with molecular masses of 10, 17, 25, 27 and 75 kDa were induced and accumulated in P. capsici-infected stem tissues during the compatible and incompatible interactions. Western blot analysis revealed that expression of PR-1 protein (17 kDa), at 12 to 24 h after inoculation, occurred earlier in the incompatible than in the compatible interaction. Immunogold labeling of PR-1 proteins occurred over cell walls and cytoplasm of the host and the oomycete pathogen and at the interface between host and oomycete cell walls at 24 h after inoculation in the compatible interaction. In the incompatible interaction, numerous PR-1 proteins accumulated predominantly over oomycete cell walls and at the interface between host and oomycete cell walls. The quantity of PR-1 proteins deposited in both host and oomycete cells was much less in the compatible than the incompatible interaction. Healthy tomato stem tissue was nearly free of immunogold labeling of PR-1 proteins.

The effect of benomyl on growth and ultrastructure of two isolates of Phytophthora infestans from Egypt

The effect of benomyl as a fungicide on the growth rate and ultrastructure of two isolates (P1319 and P623) of Phytophthora infestans is compared. Benomyl caused a concentration-dependent inhibition of the mycelial growth of both isolates. The isolate P1319 was found to be more sensitive to benomyl than the isolate P623. Ultarstructural studies confirmed these observations. The hyphae of isolate P1319 subjected to 100 and 500 ppm benomyl showed more severe changes in the cytoplasm than those of isolate P623. An increase in lipid bodies and vacuoles in the hyphal cytoplasm was the characteristic phenomenon after treatment with benomyl, particularly at a concentration of 500 ppm.

Extensive reprogramming of primary and secondary metabolism by fungal elicitor or infection in parsley cells

The transcription rates of numerous plant genes have previously been shown to be strongly affected by pathogen infection or elicitor treatment. Here we estimate the extent and complexity of this response by analyzing the patterns of mRNA induction in fungal elicitor-treated parsley cells (Petroselinum crispum) for several representatives from various primary and secondary metabolic pathways, cytosolic as well as plastidic. As a reference, we use the biphasic accumulation curve for the coordinately induced mRNAs encoding the three core enzymes of general phenylpropanoid metabolism, phenylalanine ammonia-lyase, cinnamate 4-hydroxylase and 4-coumarate:CoA ligase. Coincidence with this curve was observed for the mRNA induction kinetics of several, but not all, phenylpropanoid branch pathway-related reactions, whereas seven selected mRNAs from the pentose phosphate, glycolytic and shikimate pathways, including various cytosolic and plastidic isoforms, were induced with great differences in timing. Likewise unique and dissimilar from the reference curve were the induction patterns for various mRNAs encoding enzymes or proteins that are either more distantly or not at all related to phenylpropanoid metabolism. None of over 40 mRNAs tested so far remained unaffected. Using one strongly elicitor-responsive mRNA from carbohydrate metabolism, encoding a cytosolic glucose 6-phosphate dehydrogenase, for in situ RNA/RNA hybridization in fungus-infected parsley leaf tissue, we observed again the previously reported, close simulation of metabolic changes in true plant/fungus interactions by elicitor treatment of cultured cells. In addition to demonstrating extensive, highly complex functional, temporal and spatial patterns of changes in gene expression in infected plant cells, these results provide valuable information for the identification of pathogen-responsive promoters suitable for gene technology-assisted resistance breeding.

Mating type, nuclear DNA content and isozyme analysis of Egyptian isolates of Phytophthora infestans

Nineteen isolates of Phytophthora infestans from potatoes growing in the Nile Delta were analyzed. All isolates were of the A1 mating type. Intraspecific isozyme variation was evaluated for all isolates. Using 15 selected enzymes, 215 electrophoretic phenotypes were identified among 19 isolates examined. Dihydrolipoamide dehydrogenase yielded the greatest number of electrophoretic phenotypes. Based on isozyme analysis, these isolates were subsequently divided into three distinct groups. Feulgen cytophotometry was used to measure the DNA content of individual zoospore nuclei. The distribution of DNA content in the Egyptian isolates was similar to that previously observed in isolates from other regions of the world.

PCR amplification of ribosomal DNA for species identification in the plant pathogen genus Phytophthora

We have developed a PCR procedure to amplify DNA for quick identification of the economically important species from each of the six taxonomic groups in the plant pathogen genus Phytophthora. This procedure involves amplification of the 5.8S ribosomal DNA gene and internal transcribed spacers (ITS) with the ITS primers ITS 5 and ITS 4. Restriction digests of the amplified DNA products were conducted with the restriction enzymes RsaI, MspI, and HaeIII. Restriction fragment patterns were similar after digestions with RsaI for the following species: P. capsici and P. citricola; P. infestans, P. cactorum, and P. mirabilis; P. fragariae, P. cinnamomi, and P. megasperma from peach; P. palmivora, P. citrophthora, P. erythroseptica, and P. cryptogea; and P. megasperma from raspberry and P. sojae. Restriction digests with MspI separated P. capsici from P. citricola and separated P. cactorum from P. infestans and P. mirabilis. Restriction digests with HaeIII separated P. citrophthora from P. cryptogea, P. cinnamomi from P. fragariae and P. megasperma on peach, P. palmivora from P. citrophthora, and P. megasperma on raspberry from P. sojae. P. infestans and P. mirabilis digests were identical and P. cryptogea and P. erythroseptica digests were identical with all restriction enzymes tested. A unique DNA sequence from the ITS region I in P. capsici was used to develop a primer called PCAP. The PCAP primer was used in PCRs with ITS 1 and amplified only isolates of P. capsici, P. citricola, and P. citrophthora and not 13 other species in the genus. Restriction digests with MspI separated P. capsici from the other two species. PCR was superior to traditional isolation methods for detection of P. capsici in infected bell pepper tissue in field samples. The techniques described will provide a powerful tool for identification of the major species in the genus Phytophthora.

Development of PCR primers from internal transcribed spacer region 2 for detection of Phytophthora species infecting potatoes

We developed PCR primers and assay methods to detect and differentiate three Phytophthora species which infect potatoes and cause late blight (Phytophthora infestans) and pink rot (P. erythroseptica and P. nicotianae) diseases. Primers based on sequence analysis of internal transcribed spacer region 2 of ribosomal DNA produced PCR products of 456 bp (P. infestans), 136 bp (P. erythroseptica), and 455 bp (P. nicotianae) and were used to detect the pathogens in potato leaf (P. infestans) and tuber (P. infestans, P. erythroseptica, and P. nicotianae) tissue with a sensitivity of 1 to 10 pg of DNA. Leaf and tuber tissue were processed for PCR by a rapid NaOH method as well as a method based on the use of commercially available ion-exchange columns of P. infestans primers and the rapid NaOH extraction method were used to detect late blight in artificially and naturally infected tubers of potato cultivar Red LaSoda. In sampling studies, P. infestans was detected by PCR from artificially infected tubers at 4 days postinoculation, before any visible symptoms were present. The PCR assay and direct tissue extraction methods provide tools which may be used to detect Phytophthora pathogens in potato seedlots and storages and thus limit the transmission and spread of new, aggressive strains of P. infestans in U.S. potato-growing regions.

Application of the PCR technique to detect Phytophthora infestans in potato tubers and leaves

A characterized repetitive sequence from Phytophthora infestans (P. infestans) was used to perform a PCR with the DNA from the four races 1, 3, 4, and 1-11. To obtain amplifiable DNA, all extractions had to be purified by DNA adsorbing spin columns. Only two out of four tested primers were well suited and gave an DNA amplificate of the same size for all four races. After optimization the detection limit of the PCR corresponded to 100 ng of freeze-dried mycelia per ml, specificity was established when testing a collection of the most important potato pathogenic fungi and bacteria. Using P. infestans zoospores to infect tuber slices, the detection threshold was determined to be two days post infection when 3 to 6 zoospores were applied. After infiltrating the tenfold concentration into potato leaves a visible PCR signal was obtained one day post infection. Further improvements of the sensitivity threshold in detecting P. infestans for breeding and prognosis purposes are discussed.

Elicitin produced by an isolate of Phytophthora parasitica pathogenic to tobacco

Elicitin was prepared from a Phytophthora parasitica culture isolated from tobacco in Australia. The protein with elicitor activity was purified and sequenced. This protein, which contains 98 amino acid residues, shows full homology with an elicitin produced by a completely unrelated isolate from carnation, indicating conservation of elicitin sequence within a single species.

Evidence for outcrossing in Phytophthora sojae and linkage of a DNA marker to two avirulence genes

Two genetically different isolates of the homothallic Oomycete, Phytophthora sojae, were demonstrated to outcross and form hybrid oospores after co-culturing in vitro. Random amplified polymorphic DNA (RAPD) markers revealed ten hybrids among 354 oospores analysed. One F1 hybrid was allowed to self fertilise and produce an F2 population of 247 individuals. Among 53 F2 individuals, selected at random, 18 polymorphic RAPD markers were observed to segregate at near 3:1 Mendelian ratios, consistent with segregation for dominant alleles at single loci. Segregation of virulence against soybean resistance genes Rps1a, 3a, and 5 revealed that the avirulence genes Avrla, 3a and 5 were dominant to virulence. Avirulence against these three resistance genes appeared to be conditioned by one locus for Avr1a and two independent, complementary dominant loci for both Avr3a and Avr5. Segregation of virulence against Rps6 was in the ratio of 1:2:1 (avirulent:mixed reaction:virulent), suggesting a semi-dominant allele at a single locus. Two avirulence genes and one RAPD marker formed one linkage group, in the order Avr3a, OPH4-1, Avr5, each separated by approximately 5 cM. Our results confirm that outcrossing occurred between the parental isolates, and that sexual recombination under field conditions may play an important role in generating and maintaining genetic diversity in populations of P. sojae.

PCR amplification of species-specific DNA sequences can distinguish among Phytophthora species

We used PCR to differentiate species in the genus Phytophthora, which contains a group of devastating plant pathogenic fungi. We focused on Phytophthora parasitica, a species that can infect solanaceous plants such as tomato, and on Phytophthora citrophthora, which is primarily a citrus pathogen. Oligonucleotide primers were derived from sequences of a 1,300-bp P. parasitica-specific DNA segment and of an 800-bp P. citrophthora-specific segment. Under optimal conditions, the primers developed for P. parasitica specifically amplified a 1,000-bp sequence of DNA from isolates of P. parasitica. Primers for P. citrophthora similarly and specifically amplified a 650-bp sequence of DNA from isolates of P. citrophthora. Detectable amplification of these specific DNA sequences required picogram quantities of chromosomal DNA. Neither pair of primers amplified these sequences with DNAs from other species of Phytophthora or from the related genus Pythium. DNAs from P. parasitica and P. citrophthora growing in infected tomato stem tissue were amplified as distinctly as DNAs from axenic cultures of each fungal species. This is the first report on PCR-driven amplification with Phytophthora species-specific primers.

Translocation of cytoplasm and nucleus to fungal penetration sites is associated with depolymerization of microtubules and defence gene activation in infected, cultured parsley cells

We describe a novel system of reduced complexity for analysing molecular plant-fungus interactions. The system consists of suspension-cultured parsley (Petroselinum crispum) cells infected with a phytopathogenic fungus (Phytophthora infestans) which adheres to a coated glass plate and thus immobilizes the plant cells for live microscopy. Conventional light and electron microscopy as well as time-lapse video microscopy confirmed the virtual identity of fungal infection structures and of several characteristic early plant defence reactions in the cultured cells and whole-plant tissue. Using this new system to approach previously unresolved questions, we made four major discoveries: (i) rapid translocation of plant cell cytoplasm and nucleus to the fungal penetration site was associated with local depolymerization of the microtubular network; (ii) the directed translocation was dependent on intact actin filaments; (iii) a typical plant defence-related gene was activated in the fungus-invaded cell; and (iv) simultaneous activation of this gene in adjacent, non-invaded cells did not require hypersensitive death of the directly affected cell.