A genome-scale metabolic model of potato late blight suggests a photosynthesis suppression mechanism

BACKGROUND

Phytophthora infestans is a plant pathogen that causes an important plant disease known as late blight in potato plants (Solanum tuberosum) and several other solanaceous hosts. This disease is the main factor affecting potato crop production worldwide. In spite of the importance of the disease, the molecular mechanisms underlying the compatibility between the pathogen and its hosts are still unknown.

RESULTS

To explain the metabolic response of late blight, specifically photosynthesis inhibition in infected plants, we reconstructed a genome-scale metabolic network of the S. tuberosum leaf, PstM1. This metabolic network simulates the effect of this disease in the leaf metabolism. PstM1 accounts for 2751 genes, 1113 metabolic functions, 1773 gene-protein-reaction associations and 1938 metabolites involved in 2072 reactions. The optimization of the model for biomass synthesis maximization in three infection time points suggested a suppression of the photosynthetic capacity related to the decrease of metabolic flux in light reactions and carbon fixation reactions. In addition, a variation pattern in the flux of carboxylation to oxygenation reactions catalyzed by RuBisCO was also identified, likely to be associated to a defense response in the compatible interaction between P. infestans and S. tuberosum.

CONCLUSIONS

In this work, we introduced simultaneously the first metabolic network of S. tuberosum and the first genome-scale metabolic model of the compatible interaction of a plant with P. infestans.

Survival of Phytophthora cinnamomi as oospores, stromata, and thick-walled chlamydospores in roots of symptomatic and asymptomatic annual and herbaceous perennial plant species

Studies were conducted to determine how Phytophthora cinnamomi survives during hot and dry Mediterranean summers in areas with limited surviving susceptible hosts. Two Western Australian herbaceous perennials Chamaescilla corymbosa and Stylidium diuroides and one Western Australian annual Trachymene pilosa were collected weekly from a naturally infested site from the Eucalyptus marginata (jarrah) forest from winter to spring and less frequently during summer 2011/2012. Selfed oospores, thick-walled chlamydospores, and stromata of P. cinnamomi were observed in each species. Oospores and thick-walled chlamydospores germinated in planta confirming their viability. This is the first report of autogamy by P. cinnamomi in naturally infected plants. Stromata, reported for the first time for P. cinnamomi, were densely aggregated inside host cells, and germinated in planta with multiple germ tubes with hyphae capable of producing oospores and chlamydospores. Trachymene pilosa was completely asymptomatic, S. diuroides did not develop root lesions but some plants wilted, whilst C. corymbosa remained asymptomatic above ground but lesions developed on some tubers. The presence of haustoria suggests that P. cinnamomi grows biotrophically in some hosts. Asymptomatic, biotrophic growth of P. cinnamomi in some annual and herbaceous perennials and the production of a range of survival structures have implications for pathogen persistence over summer and its management.

Gene profiling of a compatible interaction between Phytophthora infestans and Solanum tuberosum suggests a role for carbonic anhydrase

Late blight of potato, caused by the oomycete pathogen Phytophthora infestans, is a devastating disease that can cause the rapid death of plants. To investigate the molecular basis of this compatible interaction, potato cDNA microarrays were utilized to identify genes that were differentially expressed in the host during a compatible interaction with P. infestans. Of the 7,680 cDNA clones represented on the array, 643 (12.9%) were differentially expressed in infected plants as compared with mock-inoculated control plants. These genes were classified into eight groups using a nonhierarchical clustering method with two clusters (358 genes) generally down-regulated, three clusters (241 genes) generally up-regulated, and three clusters (44 genes) with a significant change in expression at only one timepoint. Three genes derived from two down-regulated clusters were evaluated further, using reverse transcription real-time polymerase chain reaction analysis. For these analyses, both incompatible and compatible interactions were included to determine if suppression of these genes was specific to compatibility. One gene, plastidic carbonic anhydrase (CA), was found to have a very different expression pattern in compatible vs. incompatible interactions. Virus-induced gene silencing was used to suppress expression of this gene in Nicotiana benthamiana. In CA-silenced plants, the pathogen grew more quickly, indicating that suppression of CA increases susceptibility to P. infestans.

Ultrastructure of the host–pathogen interface in Arabidopsis thaliana leaves infected by the downy mildew Hyaloperonospora parasitica

Transmission electron microscopy was used to examine the host–pathogen interface in Arabidopsis thaliana (L.) Heynh. leaves infected by the biotrophic downy mildew pathogen Hyaloperonospora parasitica (Pers.:Fr.) Constant. Both conventionally fixed as well as high-pressure frozen samples were examined. Excellent preservation of the host–pathogen interface was obtained in many of our high-pressure frozen samples and provided information not available in conventionally fixed samples. Mature haustoria of H. parasitica were distinctly pyriform in shape. A small collar of host cell wall material surrounded the neck of each haustorium near the host cell wall penetration site. The presence of callose in collars was demonstrated using immunogold labeling with a monoclonal antibody specific for (1→3)-β-glucans. The body of each haustorium was ensheathed by an invaginated portion of the invaded host-cell plasma membrane known as the extrahaustorial membrane. Lying between this membrane and the haustorial wall was a layer of electron-dense material known as the extrahaustorial matrix (EHM). The EHM typically was thicker at the distal end of a haustorium than at the proximal end. The surface of the EHM covered by the extrahaustorial membrane was highly irregular in outline. Considerable vesicular activity was observed in association with the extrahaustorial membrane.Key words: transmission electron microscopy, high-pressure freezing, haustoria, Peronospora parasitica.

Oomycetes and fungi: similar weaponry to attack plants

Fungi and Oomycetes are the two most important groups of eukaryotic plant pathogens. Fungi form a separate kingdom and are evolutionarily related to animals. Oomycetes are classified in the kingdom Protoctista and are related to heterokont, biflagellate, golden-brown algae. Fundamental differences in physiology, biochemistry and genetics between fungi and Oomycetes have been described previously. These differences are also reflected in the large variations observed in sensitivity to conventional fungicides. Recently, more pronounced differences have been revealed by genomics approaches. However, in this review we compare the mode of colonization of the two taxonomically distinct groups and show that their strategies have much in common.

Ultrastructure of the Penetration and Infection of Pansy Roots by Thielaviopsis basicola

ABSTRACT Transmission electron microscopy was used to study the penetration and infection of pansy roots by Thielaviopsis basicola. Events observed in 7- to 10-day-old roots produced on moist filter paper differed slightly from those in roots from 4-week-old plants washed free of potting media prior to inoculation. By 3 h postinoculation (PI), epidermal cells of roots produced on filter paper exhibited aggregated cytoplasm and papilla formation in response to germ tube tips. The presence of callose in papillae was demonstrated using immunogold labeling. Papilla formation was not effective in preventing host cell penetration. A slender infection hypha emerged from a germ tube tip and grew through a papilla. Its tip then expanded to form a globose infection vesicle. By 6 h PI, infection hyphae emerged from infection vesicles, and invaded host cells showed signs of necrosis. By 8 h PI, infection hyphae had grown into cortical cells in spite of papilla formation in these cells. By 24 h PI, distinctive intracellular hyphae were present in necrotic cortical cells. In washed roots, most epidermal cells failed to respond to invasion. Hyphae simply grew through these cells and contacted cortical cells that exhibited aggregated cytoplasm and papillae formation. Infection structures similar to those produced in epidermal cells from roots grown on filter paper then formed in cortical cells of washed roots. The fact that T. basicola formed infection structures only in cells that responded to invasion suggests that T. basicola has a more complex relationship with its host than would be expected in a nectrotrophic pathogen. We believe that T. basicola is best described as a necrotrophic hemibiotroph.

Ultrastructure of compatible and incompatible interactions of soybean roots infected with the plant pathogenic oomycete Phytophthora sojae

Compatible and incompatible interactions of two soybean isolines containing either Rps1a or Rps1b resistance genes with races 2 and 8 of Phytophthora sojae were examined by light and electron microscopy. Phytophthora sojae race 2 is virulent on Rps1b plants and avirulent on Rps1a plants. Race 8 shows the reverse reaction; it is avirulent on Rps1b plants, but virulent on Rps1a plants. All combinations of races and cultivars were examined at times ranging from 30 min to 20 h postinoculation. Zoospore encystment, germination, and infection occurred within 30 min in all interactions. No evidence of appressorium formation was found. Wall appositions in epidermal cells adjacent to hyphae were very frequent by 30 min postinoculation. Differences between compatible and incompatible interactions became evident as early as 4 h postinoculation. The major difference appeared to relate to timing of host responses, which lead to two different types of relationships. In compatible interactions, P. sojae exhibited a short biotrophic phase with the establishment of many haustoria without triggering visible plant responses in cortical cells until approximately 10 h postinoculation. By 15 h postinoculation, almost the entire root was necrotic, wall appositions were abundant, and vascular tissue was colonized. The incompatible interaction was characterized by a nearly complete absence of haustoria, rapid host cell necrosis, and formation of many wall appositions by 4 h postinoculation. The pathogen rarely penetrated beyond the endodermis of the resistant host and colonization of vascular tissue was rare. Overall there were clear ultrastructural differences between compatible and incompatible interactions of soybean with P. sojae. These data support a strong correlation of resistance with host cell death, formation of wall appositions, and absence of root stele colonization. Key words: Phytophthora sojae, Glycine max, host–pathogen interaction, ultrastructure.

Ultrastructural markers and systematics in smut fungi and allied taxa

To identify monophyletic groups within the smut fungi and related taxa, characteristics of hyphal septation and zones of host–parasite interaction were analyzed by serial-section electron microscopy of 139 species belonging to 50 smut and 10 allied genera. Our results support the hypothesis of the existence of two phylogenetically separate lines of smut fungi. The first line, the Microbotryales, is composed of Aurantiosporium, Fulvisporium, Liroa, Microbotryum, Sphacelotheca, and Ustilentyloma, which are distributed into the Ustilentylomataceae and Microbotryaceae. The second monophyletic line, the Ustilaginomycetes, is formed by the remaining smut fungi studied here together with the Exobasidiales, Graphiolales, and Cryptobasidiales (in the traditional sense). The ultrastructural analysis identifies three lineages within the Ustilaginomycetes. The Entorrhizomycetidae are represented by Entorrhiza. The Ustilaginomycetidae consist of the Urocystales and Ustilaginales. The Exobasidiomycetidae are composed of the Doassansiales, Entylomatales, Exobasidiales, Georgefischeriales, Graphiolales, Microstromatales, and Tilletiales. The Entorrhizomycetidae, Exobasidianae, Entorrhizales, Entylomatales, Doassansiales, Georgefischeriales, Microbotryales, Microstromatales, Tilletiales, Urocystales, Entorrhizaceae, Entylomataceae, Georgefischeriaceae, Mycosyringaceae, Rhamphosporaceae, and Ustilentylomataceae are proposed as new taxa. The descriptions of the Exobasidiomycetidae, Ustilaginomycetidae, Exobasidiales, Ustilaginales, Doassansiaceae, Tilletiaceae, and Ustilaginaceae are emended. Some species of Ustilago are transferred to Microbotryum. Key words: basidiomycetes, classification, Exobasidiales, Graphiolales, phylogeny, smut fungi, ultrastructure.

Cellular interaction of the smut fungus Ustacystis waldsteiniae

The cellular interaction between the smut fungus Ustacystis waldsteiniae and its host Waldsteinia geoides was analyzed by serial-section electron microscopy using chemically fixed and high-pressure frozen – freeze-substituted samples. After penetration, each haustorium extends a short distance into the host cell where it often forms up to three short lobes. The haustorium is wholly ensheathed by a prominent matrix. The matrix is a complex structure, differing significantly from that known of other fungal plant parasites: it is filled with amorphous, electron-opaque material in which membrane-bounded, coralloid vesicles are embedded. During the contact phase of the hypha with the host cell wall, vesicles with electron-opaque contents accumulate in the contact area of the hypha where they appear to fuse with the fungal plasma membrane and extrude their contents. Subsequently, the host cell wall increases in electron opacity and matrix material becomes deposited between host plasma membrane and host cell wall exactly at the ends of the altered areas in the host cell wall. The coralloid vesicles within the matrix, however, are of host origin: exocytosis of Golgi products into the matrix results in the formation of coralloid vesicular buds in the host plasma membrane. Subsequently, the buds seem to detach from the host plasma membrane to flow as coralloid vesicles into the matrix. Matrix development continues during penetration and after penetration at the haustorial tips. After host wall penetration, the fungal cell wall comes in contact with the matrix. The fungal component of the matrix may play a key role in the inducement of these transfer cell-like compartments in host cells responding to infection. Key words: freeze substitution, haustoria, high-pressure freezing, host–parasite interaction, smut fungi, Ustacystis waldsteiniae.