Plant pathogen culture collections: it takes a village to preserve these resources vital to the advancement of agricultural security and plant pathology

ABSTRACT Plant pathogen culture collections are essential resources in our fight against plant disease and for connecting discoveries of the present with established knowledge of the past. However, available infrastructure in support of culture collections is in serious need of improvement, and we continually face the risk of losing many of these collections. As novel and reemerging plant pathogens threaten agriculture, their timely identification and monitoring depends on rapid access to cultures representing the known diversity of plant pathogens along with genotypic, phenotypic, and epidemiological data associated with them. Archiving such data in a format that can be easily accessed and searched is essential for rapid assessment of potential risk and can help track the change and movement of pathogens. The underexplored pathogen diversity in nature further underscores the importance of cataloguing pathogen cultures. Realizing the potential of pathogen genomics as a foundation for developing effective disease control also hinges on how effectively we use the sequenced isolate as a reference to understand the genetic and phenotypic diversity within a pathogen species. In this letter, we propose a number of measures for improving pathogen culture collections.

Treatment of cervical pregnancy with cerclage, curettage and balloon tamponade. A report of three cases

BACKGROUND

Cervical pregnancy is a rare form of ectopic pregnancy, historically requiring hysterectomy to control life-threatening hemorrhage. However, with early diagnosis, conservative procedures can preserve the uterus and fertility.

CASES

Three cases of cervical pregnancy were treated successfully with curettage. Two cases utilizing precurettage and postcurettage hemostatic techniques were treated with curettage alone. A third case, not employing precurettage hemostatic maneuvers, required uterine artery embolization to control bleeding and avoid hysterectomy.

CONCLUSION

Curettage alone, when preceded by cervical infiltration with a vasoconstricting agent, placement of a cerclage suture high on the cervix and followed by cervical canal tamponade with a Foley catheter balloon, can successfully treat early cervical pregnancy.

Cosegregation of Avr4 and Avr6 in Phytophthora sojae

The F2 progeny resulting from a cross of Phytophthora sojae race 2 (avirulent on Rps4 and Rps6) and race 7 (virulent on Rps4 and Rps6) were tested for their ability to cause disease on soybean plants carrying the Rps4 or the Rps6 resistance genes. Of 55 F2 progeny analyzed, 41 individuals were avirulent on both of these genes and 14 were virulent on Rps4 and Rps6, indicating that avirulence on Rps4 and Rps6 is dominant and linked. These results support the suggestion that the soybean–Phytophthora relationship is a gene for gene interaction and that the presumptive Avr4 and Avr6 genes are either tightly linked or identical. Keywords: avirulence, genetic mapping, Glycine max, incomplete dominance, Phytophthora megasperma f.sp. glycinea.

In vitro cell injury by oxidized low density lipoprotein involves lipid hydroperoxide-induced formation of alkoxyl, lipid, and peroxyl radicals

Mounting evidence supports current theories linking lipoprotein oxidation to atherosclerosis. We sought the cellular biochemical mechanism by which oxidized LDL inflicts cell injury. Inhibitors of candidate pathways of cell death were used to treat human fibroblast target cells exposed to oxidized LDL.. Ebselen, which degrades lipid hydroperoxides, inhibited oxidized LDL toxicity, consistent with our recent report that 7 beta-hydroperoxycholesterol (7 beta-OOH chol) is the major cytotoxin of oxidized LDL. Intracellular chelation of metal ions inhibited, while preloading cells with iron enhanced, toxicity, Inhibition of oxidized LDL and 7 beta-OOH chol toxicity by 2-keto-4-thiolmethyl butyric acid, a putative alkoxyl radical scavenger and by vitamin E, probucol and diphenylphenylenediamine, putative scavengers of peroxyl radicals was consistent with the involvement of these radicals in the lethal sequence. Cell death was thus postulated to occur due to lipid peroxidation via a sequence involving lipid hydroperoxide-induced, iron-mediated formation of alkoxyl, lipid, and peroxyl radicals. Pathways involving other reactive oxygen species, new protein synthesis, or altered cholesterol metabolism were considered less likely, since putative inhibitors failed to lessen toxicity. Understanding the mechanism of cell injury by oxidized LDL and its toxic moiety, 7 beta-OOH chol, may indicate specific interventions in the cell injury believed to accompany vascular lesion development.

Potential Role of Elicitins in the Interaction between Phytophthora Species and Tobacco

The potential role of extracellular elicitor proteins (elicitins) from Phytophthora species as avirulence factors in the interaction between Phytophthora and tobacco was examined. A survey of 85 Phytophthora isolates representing 14 species indicated that production of elicitin is almost ubiquitous except for isolates of Phytophthora parasitica from tobacco. The production of elicitins by isolates of P. parasitica correlated without exception with low or no virulence on tobacco. Genetic analysis was conducted by using a cross between two isolates of P. parasitica , segregating for production of elicitin and virulence on tobacco. Virulence assays of the progeny on tobacco confirmed the correlation between production of elicitin and low virulence.

A gene encoding a host-specific elicitor protein of Phytophthora parasitica

Extracellular elicitor proteins (elicitins) from Phytophthora species induce local and distal defense responses specifically in plants of the Solanaceae and Cruciferae. Based on elicitin amino acid sequences, elicitin-coding sequences from P. parasitica were amplified by the polymerase chain reaction. A genomic clone containing a complete elicitin gene, parA1, was isolated and sequenced. Elicitin was confirmed to be encoded as a precursor protein containing a 20-amino acid signal peptide that is processed before secretion. Bacterial expression of the cloned elicitin gene as a translational fusion protein containing glutathione S-transferase yielded a biologically active protein capable of inducing a hypersensitive response in tobacco, suggesting that fungus-specific postranslational modifications of elicitin are not required for its activity. Southern blot analysis indicated that elicitin genes occur as a multigene family (at least two to 10 copies) in P. parasitica, P. capsici, P. citricola, P. citrophthora, P. cryptogea, P. drechsleri, P. megasperma, and P. palmivora. Some isolates of P. parasitica that did not produce elicitins still contained elicitin-coding sequences but did not accumulate elicitin mRNA.

Transformation of the oomycete pathogen Phytophthora megasperma f. sp. glycinea occurs by DNA integration into single or multiple chromosomes

A procedure for stable transformation was developed for Phytophthora megasperma f. sp. glycinea, an oomycete pathogen of soybean. Transformants were obtained using a bacterial hygromycin resistance gene fused to a promoter and terminator from the ham34 gene of another oomycete, Bremia lactucae. Vector DNA, alone or complexed to cationic liposomes, was introduced into protoplasts using polyethylene glycol and CaCl2. DNA and RNA hybridization, and phosphotransferase assays, confirmed the presence and expression of vector DNA in the transformants. Hybridization to electrophoretically separated chromosomes of P. m. glycinea showed that vector DNA had integrated into only one chromosome in four transformants, and into multiple chromosomes in one transformant.

Characterization of microorganisms involved in accelerated biodegradation of metalaxyl and metolachlor in soils

The metabolism of the chemically related fungicide metalaxyl and the herbicide metolachlor was studied in soils and in liquid culture. Experiments with [14C]metalaxyl demonstrated that it was readily metabolized in a soil (E9) with a history of fungicide treatment, but not in a control soil. The respective half-lives of metalaxyl and metolachlor in the E9 soil were 14 and 56 days. Fourteen soil microorganisms were isolated, identified, and tested for their relative ability to degrade [14C]metalaxyl and [14C]metolachlor. With single microorganisms, breakdown of metalaxyl ranged from 36–52% after 25 days, and up to 75% with mixtures of either fungi or bacteria. Four products of metalaxyl metabolism, isolated by two-dimensional thin-layer chromatography, were tested for their antifungal activity using Phytophthora boehmeriae as a sensitive bioassay organism. All four breakdown products were inactive at concentrations up to 2.5 μg/mL, contrasting with an effective concentration for 50% inhibition of growth of P. boehmeriae for metalaxyl of 0.02 μg/mL. Rates of breakdown of metolachlor ranged from 4 to 86% with different microorganisms. Products of metolachlor breakdown possessed no herbicidal activity at concentrations up to 10 μg/mL. Cell-free extracts of four species of soil bacteria were also active in metalaxyl degradation.

An ultrastructural study of the late-blight fungus Phytophthora infestans and its interaction with the foliage of two potato cultivars possessing different levels of general (field) resistance

In the susceptible reaction of foliage of the cultivar Majestic there was extensive colonization of the host tissue prior to the onset of necrosis. The development of intracellular hyphae was confined to the initially parasitized palisade cells. Thereafter the fungus grew as intercellular hyphae, which penetrated host cells and formed haustoria. Haustorial morphology was highly variable, ranging in type from small spherical to much larger digitlike structures. Haustorial formation was preceded by the laying down of a moderately electron-dense penetration matrix bounded by the host plasmalemma. This material probably constituted the extrahaustorial matrix once haustorial development had taken place. The matrical material stained with silver proteinate reagent, and this reaction was blocked by dimedone, indicating that it was carbohydrate in nature. In the resistant reaction of foliage of the cultivar Shamrock, the epidermal cells rapidly became necrotic. Quantities of exceedingly electron-dense granules appeared in the necrotic host cells. Host organelles were no longer recognizable, but the fungal cytoplasm remained intact. In adjoining host cells, thin cell wall appositions were formed, which had a heterogeneous composition. On occasions when the fungus attempted to invade an underlying mesophyll cell, papillae usually formed in that cell at the site of incipient penetration.

A quantitative histological and ultrastructural analysis of interactions between the flax rust and near-isogenic host lines varying in their degree of incompatibility

Histological differences were evident in the leaves of eight near-isogenic lines of flax infected with the rust fungus Melampsora lini. Compared with the compatible L9 and M1 interactions, fungal growth was progressively more restricted in the moderately incompatible K and M4, incompatible M and P, and highly incompatible L and M3 interactions. This restriction took place in advance of appreciable necrosis of host cells in K, M4, M, and P. At 72 h the proportion of haustoria-containing cells which were necrotic was only 10–15% in K and M4. In M at 72 h necrosis was 80% or more at infection sites with small colonies but was negligible at sites with large colonies. In P, by contrast, a similar proportion of necrosis, 40% at 72 h, was present at all infection sites. However, in L and M3 host necrosis was much more rapid and the fungus was restricted to a few host cells. An early ultrastructural event was the appearance of extensive fibrillar deposits in the initially electron-lucent extrahaustorial matrices of both the incompatible M and P and moderately incompatible M4 and K interactions. A positive reaction with silver proteinate indicated that these matrical deposits contained carbohydrate, possibly a mucopolysaccharide or glycoprotein, but they were not extracted by either cellulase, pectinase, or chitinase. The extrahaustorial membrane, surrounding the haustoria in the compatible L9 and M1 interactions and the moderately incompatible K interaction was not stained by the periodic acid – phosphotungstic acid – chromic acid (PACP) procedure believed specific for the plasmalemma. In incompatible reactions clusters of electron-dense particles sometimes replaced starch in plastids of infected host cells. This event usually coincided with the appearance of extensive matrical deposits around fungal haustoria. At the same time particles were also found in plastids in uninfected host cells immediately adjacent to infection sites, particularly in the M and P interactions. These particles were extracted from thin sections by using pullulanase followed by α-amylase, indicating that they consisted of highly branched amylopectin.

Cytochemical specialization at the haustorial interface of a biotrophic fungal parasite, Albugo Candida

The haustorial apparatus of Albugo candida consisted of a narrow neck attached to a spherical body, which was packed with mitochondria. Covering the haustorial body was an electron-dense matrix which was bounded by the extrahaustorial membrane. This membrane, a continuation of the host cell plasmalemma, was highly convoluted with many short tentacles extending into the surrounding host cytoplasm. Single cisternae of host rough endoplasmic reticulum were always closely associated with the distal portion of the haustorial neck. Silver proteinate staining indicated that both the penetration jacket, a distinct zone surrounding the proximal part of the haustorial neck, and the extrahaustorial matrix were polysaccharide in nature, possibly complexed to other macromolecules such as proteins. The haustorial cell wall reacted only slightly with the silver proteinate, and in the distal region of the neck, there was no reaction. The fungal plasmalemma reacted strongly with the stain, except in the distal neck region, where there was no staining. With PACP, a staining procedure believed specific for the plasmalemma, the extrahaustorial membrane did not stain, whilst the host plasmalemma continuous with it did stain. The distal region of the haustorial neck wall also stained intensely with PACP clearly differentiating it cytochemically from the much lighter staining proximal neck and haustorial body walls. The penetration jacket stained more intensely than the extrahaustorial matrix with PACP, which might suggest a different chemical composition. Following incubation in 0.8 M sucrose osmoticum, the extrahaustorial membrane pulled away from the proximal neck wall, but remained tightly affixed to the distal neck wall, indicating that a tight junction existed between that membrane and the fungal cell wall in that region. Enzymic digestion with either cellulase or an endo-β-1,3-glucanase removed neither the haustorial body wall nor the extrahaustorial matrix. The significance of this finding is briefly discussed.

Plasmolysis of rusted flax: a fine-structural study of the host–pathogen interface

Plasmolysis of rust-infected flax tissue after treatment with sucrose, mannitol, or calcium chloride solutions caused withdrawal of the host plasmalemma from the cell wall and movement of the host protoplast towards the haustorium. The extrahaustorial membrane, however, was not displaced from its position along the haustorial neck or around the haustorial body. In sucrose and mannitol solutions the extrahaustorial matrix was unchanged in volume, which suggests that it is either a more rigid structure than has usually been suggested or an artifact of fixation. After calcium chloride treatment, the matrix was frequently slightly distended and some plasmolysis of the haustorial protoplast was generally observed; this did not occur in the sucrose or mannitol solutions. The possible significance of these findings is briefly discussed.

A fine-structural study of the pea downy mildew fungus Peronospora pisi in its host Pisum sativum

Downy mildew disease of the cultivated pea Pisum sativum L. caused by the fungus Peronospora pisi Sydow was studied in mature leaves and young shoots of the host plant. Particularly in systemic infections of young shoot tissue, a common occurrence was an extremely electron-opaque membrane-bound, hemispherical deposit extending through the host cell wall into the host cytoplasm. This material which abutted directly onto the intercellular hyphal wall was termed the penetration matrix. Its formation was apparently the result of a specific interaction between the host and obligate fungal parasite. Similar apparently solid or gellike material constituted the matrix surrounding the digitlike intracellular haustorium. This membrane-bound extrahaustorial matrix was present through the penetrated host cell wall and formed a relatively thick layer around haustoria in young shoot tissue, but was much thinner distally around haustoria in mature leaf mesophyll cells. An unusual, regularly arranged, tubular network of ribosome-free endoplasmic reticulum was occasionally found in the host cytoplasm in systemically infected shoot tissue adjacent to haustoria.

Acid phosphatase and protease release by the insectivorous plant Drosera rotundifolia

The leaves of the insectivorous plant Drosera rotundifolia L. produced extracellular hydrolytic enzymes in response to feeding with gelatin. Enzyme release was first detected 1 to 2 days after feeding, reached a maximum on day 4, and then gradually declined. Optimal activity of both acid phosphatase (orthophosphoric-monoester phosphohydrolase (acid optimum), EC 3.1.3.2) and protease enzymes was in the acidic range of pH. The acid phosphatase attacked a range of phosphorylated compounds but its p-nitrophenylphosphatase (PNPPase) and ribonucleoside triphosphatase activities were highest. It displayed relatively low phosphomonoesterase (EC 3.1.3.1, 3.1.3.2) activity. Both acid phosphatase and protease enzymes were insensitive to the sulphydryl inhibitor N-ethylmaleimide. The acid phosphatase was strongly inhibited by fluoride and orthophosphate. The nature of the apparent induction of hydrolase enzyme activity is briefly discussed.