Sequence analysis of duplicated actin genes in Lagenidium giganteum and Pythium irregulare (Oomycota)

Southern analysis of genomic DNA identified multiple-copy actin gene families in Lagenidium giganteum and Pythium irregulare (Oomycota). Polymerase chain reaction (PCR) protocols were used to amplify members of these actin gene families. Sequence analysis of genomic coding regions demonstrated five unique actin sequences in L. giganteum (Lg-Ac 1, 2, 3, 4, 5) and four unique actin sequences in P. irregulare (Pi-Acl, 2, 3, 4); none were interrupted by introns. Maximum parsimony analysis of the coding regions demonstrated a close phylogenetic relationship between oomycetes and the chromophyte alga Costaria costata. Three types of actin coding regions were identified in the chromophyte/oomycete lineage. The type 1 actin is the single-copy coding region found in C. costata. The type 2 and type 3 actins are found in the oomycetes and are the result of a gene duplication which occurred soon after the divergence of the oomycetes from the chromophyte algae. The type 2 coding regions are the single-copy sequence of Phytophthora megasperma, the Phytophthora infestans actB gene, Lg-Ac5 and Pi-Ac2. The type 3 coding regions are the single-copy sequence of Achlya bisexualis, the P. infestans actA gene, Lg-Ac1, 2, 3, 4 and Pi-Acl, 3, 4. Correspondence to: D. Bhattacharya     

Expression of the Phytophthora infestans ipiB and ipi0 genes in planta and in vitro

The ipiB and ipiO genes of the potato late blight fungus Phytophthora infestans (Mont.) de Bary were isolated from a genomic library in a screen for genes induced in planta. Expression of these genes was studied during pathogenesis on various host tissues and different host plants, some of which show specific resistance against P. infestans infection. During pathogenesis on leaves and tubers of the fully susceptible potato cultivar (cv.) Ajax and on leaves of the fully susceptible tomato cv. Moneymaker, the P. infestans ipiB and ipiO genes show a transient expression pattern with highest mRNA levels in the early stages of infection. During the interaction with leaves of the partially resistant potato cv. Pimpernel, the expression is also transient but accumulation and disappearance of the mRNAs is delayed. Also in P. infestans inoculated onto a race-specific resistant potato cultivar and onto the nonhost Solanum nigrum, ipiB and ipiO mRNA is detectable during the initial stages of infection. Apparently, the expression of the ipiB and the ipiO genes is activated in compatible, incompatible and nonhost interactions. In encysted zoospores, ipiB and ipiO mRNA accumulation was not detectable, but during cyst germination and appressorium formation on an artificial surface the genes are highly expressed. Expression studies in mycelium grown in vitro revealed that during nutrient starvation the expression of the ipiB and ipiO genes is induced. For ipiO gene expression, carbon deprivation appeared to be sufficient. The ipiO gene promoters contain a sequence motif that functions as a glucose repression element in yeast and this motif might be involved in the regulation of ipiO gene expression.     

Atkinsiella parasitica sp. nov. isolated from a rotifer,Brachionus plicatilis

A new species ofAtkinsiella (Lagenidiales, Oomycetes),Atkinsiella parasitica is described and illustrated. It was isolated from the eggs and bodies of a rotifer,Brachionus plicatilis, with fungal infection in 1992. The rotifer was reared in a hatchery as the first food supply for seed production of crustaceans and fishes. The fungus is characterized by producing monoplanetic, lateral biflagellate zoospores, and infrequently branched discharge tubes. Optimum temperature for the fungus was 25°C. The fungus was considered an obligate marine fungus, because its growth was observed only on PYGS medium including seawater.     

Characterization of cell surface carbohydrates on asexual spores of the water moldSaprolegnia ferax

Summary In asexual reproduction of the water mold,Saprolegnia ferax, four distinct and sequentially produced spores are involved in dispersal, two of which are motile and two of which are nonmotile. Composition of cell surface glycoproteins may be important in dispersal strategies for each of these stages. Binding patterns of fluorescently labelled lectins were investigated to identify differences in glycoproteins of asexually produced dispersal stages. The pattern of lectin binding to zoospores was diverse. FITC-Con A bound to surfaces of zoospores and membranes of the water expulsion vacuole system, indicating the prescence of mannosyl and glucosyl residues. In zoospores incubated for more than 30 min in FITC-WGA and FITC-GS II. which bind N-acetyl glucosamine, fluorescence was sometimes localized in peripheral, intracellular patches. In shorter incubations, secondary zoospores bound these lectins along the groove region where K-bodies were located. Surfaces of cystospores typically bound FITC-WGA, but not FITC-GS II. FITC-GS II, however, bound to empty cystospore walls, probably because reactive sugars were available at the inner surface of the wall. Germ tubes emerging from cystospores bound labelled WGA and GS II, but not Con A. The same lectin binding pattern was found along discharge papilla of primary cystospores, indicating that modifications in cystospore walls associated with direct germination and zoospore discharge were similar. Thus, glycoproteins involved in early establishment of the hyphal system differ from those forming the cell surface of cystospores. Differences in the binding pattern of lectins to zoospores and cystospores highlight differences between cell surface carbohydrates of motile and nonmotile asexual stages.Abbreviations BPA lectin fromBauhinia purpurea - C₁ primary cystospore - C₂ secondary cystospore - Con A concanavalin A, lectin fromCanavalia ensiformis - DBA lectin fromDolichos biflorus - DIC Nomarski differential interference contrast optics - DS dilute salts - FITC fluorescein isothiocyanate - FUC fucose - Gal galactose - GalNAc N-acetyl galactosamine - Glc glucose - GlcNAc N-acetyl glucosamine - GS I Griffonia simplicifolia lectin I - GS II G. simplicifolia lectin II - Man mannose - MPA lectin fromMaclura pomifera - PC phase contrast optics - PNA lectin fromArachis hypogaea - SBA soybean agglutinin, lectin fromGlycine max - UEA-1 lectin fromUlex europaeus - WGA wheat germ agglutinin fromTriticum vulgare - WV water expulsion vacuole     

Soil pathogen communities associated with native and non‐native Phragmites australis populations in freshwater wetlands

Soil pathogens are believed to be major contributors to negative plant–soil feedbacks that regulate plant community dynamics and plant invasions. While the theoretical basis for pathogen regulation of plant communities is well established within the plant–soil feedback framework, direct experimental evidence for pathogen community responses to plants has been limited, often relying largely on indirect evidence based on above‐ground plant responses. As a result, specific soil pathogen responses accompanying above‐ground plant community dynamics are largely unknown. Here, we examine the oomycete pathogens in soils conditioned by established populations of native noninvasive and non‐native invasive haplotypes of Phragmites australis (European common reed). Our aim was to assess whether populations of invasive plants harbor unique communities of pathogens that differ from those associated with noninvasive populations and whether the distribution of taxa within these communities may help to explain invasive success. We compared the composition and abundance of pathogenic and saprobic oomycete species over a 2‐year period. Despite a diversity of oomycete taxa detected in soils from both native and non‐native populations, pathogen communities from both invaded and noninvaded soils were dominated by species of Pythium. Pathogen species that contributed the most to the differences observed between invaded and noninvaded soils were distributed between invaded and noninvaded soils. However, the specific taxa in invaded soils responsible for community differences were distinct from those in noninvaded soils that contributed to community differences. Our results indicate that, despite the phylogenetic relatedness of native and non‐native P. australis haplotypes, pathogen communities associated with the dominant non‐native haplotype are distinct from those of the rare native haplotype. Pathogen taxa that dominate either noninvaded or invaded soils suggest different potential mechanisms of invasion facilitation. These findings are consistent with the hypothesis that non‐native plant species that dominate landscapes may “cultivate” a different soil pathogen community to their rhizosphere than those of rarer native species.     

Oomycetes along a 120,000 year temperate rainforest ecosystem development chronosequence

The occurrence of plant-associated oomycetes in natural ecosystems and particularly during long-term ecosystem development is largely unknown. Using DNA sequencing, we investigated the frequency and host relationships of plant-root-associated oomycete communities along a 120 000 y glacial chronosequence, comprising site ages with rapid compositional change (“early succession”; 5–70 y old soil); relatively stable higher-diversity sites (“mature”, 280–12000 y); and ancient, nutrient-limited soils with declining plant biomass and stature (“retrogression”, 60 000, 120 000 y). Plant-associated oomycetes were frequent in early successional sites, occurring in 38–65% of plant roots, but rare (mean 3%) in all older ecosystems. Oomycete OTUs occurred non-randomly with plant host species, and were more frequent on those plant species that declined most strongly in abundance between ecosystem ages. While oomycetes were common in early succession, their absence in older sites suggests a limited role in later stages of ecosystem development.     

分离自蚊幼虫的腐霉一新种及其rDNA的ITS区段分析

在一次室外蚊虫生物防治实验中,从被病菌感染的蚊幼虫体中分离到一株卵菌。根据其形态特征及其rDNA的ITS区段的碱基系列,鉴定为腐霉属一新种,命名为贵阳腐霉Pythiumguiyangense。对新种作了拉丁文描述。对新种与其近似种卡地腐霉P.carolinianum、奇雄腐霉P.middletonii和多卵腐霉P.multisporum的区别进行了讨论。模式标本(干培养物)保存在中国科学院微生物研究所菌物标本馆(HMAS),活培养物保存在贵阳医学院。     

Rapid wound responses ofSaprolegnia ferax hyphae depend upon actin and Ca2+-involving deposition of callose plugs

Summary Growing hyphae of the oomyceteSaprolegnia ferax wounded by impalement with a ca. 0.2 m diameter glass microelectrode normally respond within seconds with an apically directed cytoplasmic contraction followed by production of a plug which encases the electrode and occludes its recording of transmembrane potentials. This plug contains callose and Ca²⁺-associated membranes. To characterize the rapid wounding response, we disrupted specific filamentous (F) actin populations and Ca²⁺ regulation. Plug formation is inhibited by disruption of F-actin populations and low exogenous Ca²⁺ but not by inhibition of stretch-activated Ca²⁺ channels with Gd³⁺. Therefore, stretch-activated channels are not the immediate sensor. Instead, sensing may involve strain on the actin cytoskeleton which triggers the occlusion response. This wound response is qualitatively similar to the production of septa which isolate developing sporangia and seal severed hyphae, indicating the use of a normal basic cellular developmental system as a protective mechanism against environmental damage. The wound response is essential, since an inability to seal sites of mechanical damage is potentially catastrophic in acellular coenocytic organisms.Abbreviations APW artificial pond water - BAPTA 1,2-bis(orthzo-aminophenoxy)ethane-N,N,N,N-tetrapotassium acetate - CTC chlortetracycline - DIC Nomarski differential interference contrast microscopy - F-actin filamentous actin - LatB latrunculin B - PM plasma membrane - RP rhodamine-labeled phalloidin - SA channels stretch-activated channels     

灭蚊真菌－贵阳腐霉对植物的安全性试验

The aim of this study is to investigate whether Pythium guiyangense, a mosquito-killing fungus isolated in Guiyang, Guizhou Province of China in 1994, is pathogenic to plants. Six common crops, Cucumis sativa, Lycopersicon esculentum, Capsicum annuum, Nicotiana tabacum, Brassica campestris and Oryza sativa were used as subjects for test. Zoospores of the fungus were used to infect the plants with soil inoculation method, caudex injection method and foliage spray method. Both positive control (using P. aphanidermatum) and negative control (using sterile water) were set up in all the experiments. The results showed that no infection was found on the tested plants in soil inoculation experiments. In caudex injection test, callus grew around the wounded tissue in most of the plants. Brownish rottenness could be found only in the injected wounds in a few plants, probably caused by saprophytic bacteria or other fungi, and the germ-carrying plants grew normally. No abnormal appearance was found on the six crops in foliage spray test. It was demonstrated that P. guiyangense could hardly infect plants in nature, and was a safe and promising agent for mosquito biological control.     

A molecular phylogeny of <Emphasis Type="Italic">Haptoglossa</Emphasis> species,terrestrial peronosporomycetes (oomycetes) endoparasitic on nematodes

Phylogenetic relationships of seven Haptoglossa isolates were analyzed by using mitochondrial COII amino acid sequences with a data set of 34 peronosporomycetes. Haptoglossa isolates formed a single clade and appeared to be basal to the clade consisting of all other peronosporomycetes. The Haptoglossa clade was divided into two subclades: one clade consisted of five aplanosporic isolates and the other included one aplanosporic and one zoosporic isolate. These results indicate that the genus Haptoglossa is monophyletic, and patterns of infection cell formation reflect more the phylogenetic relationship between the species than patterns of sporogenesis.