Identification of Novel Virulence-Associated Proteins Secreted to Xylem by Verticillium nonalfalfae During Colonization of Hop Plants.

Plant pathogens employ various secreted proteins to suppress host immunity for their successful host colonization. Identification and characterization of pathogen-secreted proteins can contribute to an understanding of the pathogenicity mechanism and help in disease control. We used proteomics to search for proteins secreted to xylem by the vascular pathogen Verticillium nonalfalfae during colonization of hop plants. Three highly abundant fungal proteins were identified: two enzymes, α-N-arabinofuranosidase (VnaAbf4.216) and peroxidase (VnaPRX1.1277), and one small secreted hypothetical protein (VnaSSP4.2). These are the first secreted proteins so far identified in xylem sap following infection with Verticillium spp. VnaPRX1.1277, classified as a heme-containing peroxidase from Class II, similar to other Verticillium spp. lignin-degrading peroxidases, and VnaSSP4.2, a 14-kDa cysteine-containing protein with unknown function and with a close homolog in related V. alfalfae strains, were further examined. The in planta expression of VnaPRX1.1277 and VnaSSP4.2 genes increased with the progression of colonization, implicating their role in fungal virulence. Indeed, V. nonalfalfae deletion mutants of both genes exhibited attenuated virulence on hop plants, which returned to the level of the wild-type pathogenicity in the knockout complementation lines, supporting VnaPRX1.1277 and VnaSSP4.2 as virulence factors required to promote V. nonalfalfae colonization of hop plants.

The secretome of vascular wilt pathogen Verticillium albo-atrum in simulated xylem fluid.

Verticillium albo-atrum is a vascular wilt pathogen capable of infecting many important dicotyledonous plant species. Fungal isolates from hop differ in aggressiveness, causing either mild or lethal symptoms in infected plants. As in other plant pathogenic fungi, extracellular proteins, such as cell wall-degrading enzymes and effectors, are thought to be crucial in the pathogenesis process. In this study, mild and lethal isolates from three countries were grown in simulated xylem medium and secretome analysis by 2D-DIGE showed low qualitative and high quantitative variability among the isolates. Functional classification of 194 identified proteins representing 100 unique protein accessions revealed an arsenal of cell wall-degrading enzymes and potential effectors. The set of proteins that were more abundant in at least two lethal isolates included enzymes acetylcholinesterases, lipases, polygalacturonases, pectate lyase, rhamnogalacturonan acetylesterases, acetylxylan esterase, endoglucanase, xylanases, mannosidases, and a protein similar to alginate lyase and also potential effectors necrosis- and ethylene-inducing protein, small basic 14 kDa hypothetical protein and 79 kDa hypothetical proteins. Other proteins associated with virulence showed different expression profiles between mild and lethal isolates. The results suggest that the increased virulence of lethal isolates has little background shared by all three lethal isolates and that upregulation of isolate specific sets of proteins may be most important.

Different Gene Expressions of Resistant and Susceptible Hop Cultivars in Response to Infection with a Highly Aggressive Strain of Verticillium albo-atrum.

Verticillium wilt has become a serious threat to hop production in Europe due to outbreaks of lethal wilt caused by a highly virulent strain of Verticillium albo-atrum. In order to enhance our understanding of resistance mechanisms, the fungal colonization patterns and interactions of resistant and susceptible hop cultivars infected with V. albo-atrum were analysed in time course experiments. Quantification of fungal DNA showed marked differences in spatial and temporal fungal colonization patterns in the two cultivars. Two differential display methods obtained 217 transcripts with altered expression, of which 84 showed similarity to plant proteins and 8 to fungal proteins. Gene ontology categorised them into cellular and metabolic processes, response to stimuli, biological regulation, biogenesis and localization. The expression patterns of 17 transcripts with possible implication in plant immunity were examined by real-time PCR (RT-qPCR). Our results showed strong expression of genes encoding pathogenesis-related (PR) proteins in susceptible plants and strong upregulation of genes implicated in ubiquitination and vesicle trafficking in the incompatible interaction and their downregulation in susceptible plants, suggesting the involvement of these processes in the hop resistance reaction. In the resistant cultivar, the RT-qPCR expression patterns of most genes showed their peak at 20 dpi and declined towards 30 dpi, comparable to the gene expression pattern of in planta detected fungal protein and coinciding with the highest fungal biomass in plants at 15 dpi. These expression patterns suggest that the defence response in the resistant cultivar is strong enough at 20 dpi to restrict further fungus colonization.

Genome Sequence of a Lethal Strain of Xylem-Invading Verticillium nonalfalfae.

Verticillium nonalfalfae, a soilborne vascular phytopathogenic fungus, causes wilt disease in several crop species. Of great concern are outbreaks of highly aggressive V. nonalfalfae strains, which cause a devastating wilt disease in European hops. We report here the genome sequence and annotation of V. nonalfalfae strain T2, providing genomic information that will allow better understanding of the molecular mechanisms underlying the development of highly aggressive strains.

Xylem Sap Extraction Method from Hop Plants.

Verticillium wilt is one of the most important diseases on hop that significantly influence continuation of production on affected areas. It is caused by the soil borne vascular pathogen Verticillium nonalfalfae, which infects plants through the roots and then advances through the vascular (xylem) system. During infection, V. nonalfalfae secretes many different virulence factors. Xylem sap of infected plants is therefore a rich source for investigating the molecules that are involved in molecular interactions of Verticillium - hop plants. This protocol provides instructions on how to infect hop plants with V. nonalfalfae artificially and how to obtain xylem sap from hop plants.

Hop (Humulus lupulus L.) response mechanisms in drought stress: Proteomic analysis with physiology.

Drought is one of the major environmental devastating stressors that impair the growth and productivity of crop plants. Despite the relevance of drought stress, changes in physiology and resistance mechanisms are not completely understood for certain crops, including hop (Humulus lupulus L.). In this research the drought response of hop was studied using a conventional physiological approach (gas exchange techniques, fluorescence, relative water content measurements) and proteomic analysis (2D-DIGE). Plants of two cultivars (Aurora and Savinjski golding) were exposed to progressive drought in a pot experiment and analysed at different stress stages (mild, moderate and severe). Measurements of relative water content revealed a hydrostable water balance of hop. Photosynthesis was decreased due to stomatal and non-stomatal limitation to the same extent in both cultivars. Of 28 identified differentially abundant proteins, the majority were down regulated and included in photosynthetic (41%) and sugar metabolism (33%). Fifteen % of identified proteins were classified into the nitrogen metabolism, 4% were related to a ROS related pathway and 7% to other functions.

Comparative proteomic profiling in compatible and incompatible interactions between hop roots and Verticillium albo-atrum.

Verticillium wilt, caused by the soil borne fungal pathogen Verticillium albo-atrum, is a serious threat to hop (Humulus lupulus L.) production in several hop-growing regions. A proteomic approach was applied to analyse the response of root tissue in compatible and incompatible interactions between hop and V. albo-atrum at 10, 20 and 30 days after inoculation, using two-dimensional difference gel electrophoresis (2D-DIGE) coupled with de novo sequencing of derivatized peptides. Approximately 1200 reproducible spots were detected on the gels, of which 102 were identified. In the compatible interaction, 252 spots showed infection-specific changes in spot abundance and an accumulation of defence-related proteins, such as chitinase, ß-glucanase, thaumatin-like protein, peroxidase and germin-like protein, was observed. However, no significant infection-specific changes were detected in the incompatible interaction. The results indicate that resistance in this pathosystem may be conferred by constitutive rather than induced defence mechanisms. The identification and high abundance of two mannose/glucose-specific lectin isoforms present only in the roots of the resistant cultivar suggests function of lectins in hop resistance against V. albo-atrum.

Proteomic profile of dry-cured ham relative to PRKAG3 or CAST genotype, level of salt and pastiness.

Two-dimensional electrophoresis was used to compare dry-cured biceps femoris insoluble protein fraction according to genotype (PRKAG3Ile199Val and CASTLys249Arg/Ser638Arg) as well as salt and pastiness level. The PRKAG3 affected mainly muscle metabolic enzymes, indicating its possible influence on muscle metabolism with heterozygotes Ile/Val appearing different from both homozygous genotypes. The effect of CAST was smaller, affecting the quantity of one actin fragment. Dry-cured ham salt and pastiness level affected a wide variety of protein spots including metabolic enzymes, plasma proteins, chaperones and myofibrillar proteins, including protein fragments, indicating the connection with proteolysis. Pastiness was associated with salt content, reflected also by the fact that many spots were affected by both factors. Despite the absence of extreme pastiness (or low salt samples), some protein spots (actin, MHC fragment, desmin fragment) exhibited important differences in intensity according to pastiness (and salt level) suggesting they could be used as potential quality markers.