Vascular wilts are among the most destructive plant diseases that occur in annual crops as well as in woody perennials. These diseases are generally caused by soil-borne bacteria, fungi, and ...oomycetes that infect through the roots and enter the water-conducting xylem vessels where they proliferate and obstruct the transportation of water and minerals. As a consequence, leaves wilt and die, which may lead to impairment of the whole plant and eventually to death of the plant. Cultural, chemical, and biological measures to control this group of plant pathogens are generally ineffective, and the most effective control strategy is the use of genetic resistance. Owing to the fact that vascular wilt pathogens live deep in the interior of their host plants, studies into the biology of vascular pathogens are complicated. However, to design novel strategies to combat vascular wilt diseases, understanding the (molecular) biology of vascular pathogens and the molecular mechanisms underlying plant defense against these pathogens is crucial. In this review, we discuss the current knowledge on interactions of vascular wilt pathogens with their host plants, with emphasis on host defense responses against this group of pathogens.
Sexual recombination drives genetic diversity in eukaryotic genomes and fosters adaptation to novel environmental challenges. Although strictly asexual microorganisms are often considered as ...evolutionary dead ends, they comprise many devastating plant pathogens. Presently, it remains unknown how such asexual pathogens generate the genetic variation that is required for quick adaptation and evolution in the arms race with their hosts. Here, we show that extensive chromosomal rearrangements in the strictly asexual plant pathogenic fungus Verticillium dahliae establish highly dynamic lineage-specific (LS) genomic regions that act as a source for genetic variation to mediate aggressiveness. We show that such LS regions are greatly enriched for in planta-expressed effector genes encoding secreted proteins that enable host colonization. The LS regions occur at the flanks of chromosomal breakpoints and are enriched for retrotransposons and other repetitive sequence elements. Our results suggest that asexual pathogens may evolve by prompting chromosomal rearrangements, enabling rapid development of novel effector genes. Likely, chromosomal reshuffling can act as a general mechanism for adaptation in asexually propagating organisms.
Fungal plant pathogens secrete effector molecules to establish disease on their hosts, and plants in turn use immune receptors to try to intercept these effectors. The tomato immune receptor Ve1 ...governs resistance to race 1 strains of the soil-borne vascular wilt fungi Verticillium dahliae and Verticillium albo-atrum, but the corresponding Verticillium effector remained unknown thus far. By high-throughput population genome sequencing, a single 50-Kb sequence stretch was identified that only occurs in race 1 strains, and subsequent transcriptome sequencing of Verticillium-infected Nicotiana benthamiana plants revealed only a single highly expressed ORF in this region, designated Ave1 (for Avirulence on Ve1 tomato). Functional analyses confirmed that Ave1 activates Ve1-mediated resistance and demonstrated that Ave1 markedly contributes to fungal virulence, not only on tomato but also on ARABIDOPSIS: Interestingly, Ave1 is homologous to a widespread family of plant natriuretic peptides. Besides plants, homologous proteins were only found in the bacterial plant pathogen Xanthomonas axonopodis and the plant pathogenic fungi Colletotrichum higginsianum, Cercospora beticola, and Fusarium oxysporum f. sp. lycopersici. The distribution of Ave1 homologs, coincident with the presence of Ave1 within a flexible genomic region, strongly suggests that Verticillium acquired Ave1 from plants through horizontal gene transfer. Remarkably, by transient expression we show that also the Ave1 homologs from F. oxysporum and C. beticola can activate Ve1-mediated resistance. In line with this observation, Ve1 was found to mediate resistance toward F. oxysporum in tomato, showing that this immune receptor is involved in resistance against multiple fungal pathogens.
Knowledge of pathogen biology and genetic diversity is a cornerstone of effective disease management, and accurate identification of the pathogen is a foundation of pathogen biology. Species names ...provide an ideal framework for storage and retrieval of relevant information, a system that is contingent on a clear understanding of species boundaries and consistent species identification. Verticillium, a genus of ascomycete fungi, contains important plant pathogens whose species boundaries have been ill defined. Using phylogenetic analyses, morphological investigations and comparisons to herbarium material and the literature, we established a taxonomic framework for Verticillium comprising ten species, five of which are new to science. We used a collection of 74 isolates representing much of the diversity of Verticillium, and phylogenetic analyses based on the ribosomal internal transcribed spacer region (ITS), partial sequences of the protein coding genes actin (ACT), elongation factor 1-alpha (EF), glyceraldehyde-3-phosphate dehydrogenase (GPD) and tryptophan synthase (TS). Combined analyses of the ACT, EF, GPD and TS datasets recognized two major groups within Verticillium, Clade Flavexudans and Clade Flavnonexudans, reflecting the respective production and absence of yellow hyphal pigments. Clade Flavexudans comprised V. albo-atrum and V. tricorpus as well as the new species V. zaregamsianum, V. isaacii and V. klebahnii, of which the latter two were morphologically indistinguishable from V. tricorpus but may differ in pathogenicity. Clade Flavnonexudans comprised V. nubilum, V. dahliae and V. longisporum, as well as the two new species V. alfalfae and V. nonalfalfae, which resembled the distantly related V. albo-atrum in morphology. Apart from the diploid hybrid V. longisporum, each of the ten species corresponded to a single clade in the phylogenetic tree comprising just one ex-type strain, thereby establishing a direct link to a name tied to a herbarium specimen. A morphology-based key is provided for identification to species or species groups.
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.
Since 1992 elm trees have been treated with a biological control product Dutch Trig super( registered ) to protect them from infection by Ophiostoma novo-ulmi causing Dutch elm disease (DED). The ...active ingredient of the biocontrol product consists of the fungus Verticillium albo-atrum strain WCS850. A conidiospore suspension of this fungus is injected into the vascular system of elm trees at a height of 1.3 m. This biocontrol product prevents healthy elm trees from fungal infection transmitted by elm bark beetles. Dutch Trig super( registered ), however, does not protect already infected trees or trees connected with diseased trees via root grafts. Since 2010, only 0.1 % of the injected elms became infected with DED through beetle transmission and an additional 0.4 % of the treated elms were infected through root contact in the Netherlands. Regression analysis considering all injected elm trees in the Netherlands since 1992 indicated that DED infection through beetle transmission had significantly decreased during the 24 years application of Dutch Trig super( registered ). In 2015, approximately 28,300 trees in five countries (Netherlands, USA, Germany, Canada and Sweden) were treated with Dutch Trig super( registered ) to protect valuable susceptible elm trees, mainly in urban environments.
Vascular wilts caused by soil-borne fungal species of the Verticillium genus are devastating plant diseases. The most common species, Verticillium dahliae and Verticillium albo-atrum, have broad host ...ranges and are notoriously difficult to control. Therefore, genetic resistance is the preferred method for disease control. Only from tomato (Solarium lycopersicum) has a Verticillium resistance locus been cloned, comprising the Ve1 gene that encodes a receptor-like protein-type cell surface receptor. Due to lack of a suitable model for receptor-like protein (RLP)-mediated resistance signaling in Arabidopsis (Arabidopsis thaliana), so far relatively little is known about RLP signaling in pathogen resistance. Here, we show that Ve1 remains fully functional after interfamily transfer to Arabidopsis and that Ve1-transgenic Arabidopsis is resistant to race 1 but not to race 2 strains of V. dahliae and V. albo-atrum, nor to the Brassicaceae-specific pathogen Verticillium longisporum. Furthermore, we show that signaling components utilized by Ve1 in Arabidopsis to establish Verticillium resistance overlap with those required in tomato and include SERK3/BAK1, EDS1, and NDR1, which strongly suggests that critical components for resistance signaling are conserved. We subsequently investigated the requirement of SERK family members for Vel resistance in Arabidopsis, revealing that SERK1 is required in addition to SERK3/BAK1. Using virus-induced gene silencing, the requirement of SERK1 for Vel-mediated resistance was confirmed in tomato. Moreover, we show the requirement of SERK1 for resistance against the foliar fungal pathogen Cladosporium fulvum mediated by the RLP Cf-4. Our results demonstrate that Arabidopsis can be used as model to unravel the genetics of Vel-mediated resistance.
Inactivation of Arabidopsis WAT1 (Walls Are Thin1), a gene required for secondary cell‐wall deposition, conferred broad‐spectrum resistance to vascular pathogens, including the bacteria Ralstonia ...solanacearum and Xanthomonas campestris pv. campestris, and the fungi Verticillium dahliae and Verticillium albo‐atrum. Introduction of NahG, the bacterial salicylic acid (SA)‐degrading salicylate hydroxylase gene, into the wat1 mutant restored full susceptibility to both R. solanacearum and X. campestris pv. campestris. Moreover, SA content was constitutively higher in wat1 roots, further supporting a role for SA in wat1‐mediated resistance to vascular pathogens. By combining transcriptomic and metabolomic data, we demonstrated a general repression of indole metabolism in wat1‐1 roots as shown by constitutive down‐regulation of several genes encoding proteins of the indole glucosinolate biosynthetic pathway and reduced amounts of tryptophan (Trp), indole‐3‐acetic acid and neoglucobrassicin, the major form of indole glucosinolate in roots. Furthermore, the susceptibility of the wat1 mutant to R. solanacearum was partially restored when crossed with either the trp5 mutant, an over‐accumulator of Trp, or Pro35S:AFB1‐myc, in which indole‐3‐acetic acid signaling is constitutively activated. Our original hypothesis placed cell‐wall modifications at the heart of the wat1 resistance phenotype. However, the results presented here suggest a mechanism involving root‐localized metabolic channeling away from indole metabolites to SA as a central feature of wat1 resistance to R. solanacearum.
In the course of taxonomic changes, Verticillium albo-atrum sensu lato, a causal agent of Verticillium wilt, was split into the three different species, V. albo-atrum Reinke & Berthold, V. alfalfae ...Inderb. et al., and V. nonalfalfae Inderb. et al. (Inderbitzin et al. 2011), which questions all prior morphology-based identifications of this ascomycete as a pathogen on multiple hosts worldwide. Thus, genetically confirmed new reports of the newly described species will provide essential information on the geographical distribution and host range of these pathogens. In order to investigate the natural distribution of Verticillium spp. on Tree of Heaven, Ailanthus altissima(Mill.) Swingle, and to obtain isolates that might have potential as biological control agents for this nonnative, invasive tree species (Schall and Davis 2009, Kasson et al. 2014), a survey of Verticillium wilt and its causal agents on A. altissima was conducted from May to August 2011 in the distribution range of the tree in eastern and southern Austria. During this survey, symptomatic A. altissima trees exhibiting foliar wilt symptoms, yellowish vascular discoloration, epicormic shoots on the stem, dieback, and premature death were observed at 12 out of the 22 investigated sites. Similar symptoms were reported on A. altissima in southern Styria in autumn 1997, attributed to Verticillium spp. and other fungi causing bark canker (Cech 1998). To identify the causal organism, 50-cm-long branch samples were taken from symptomatic A. altissima and cut into 5-cm pieces, surface sterilized with 96% ethanol for 1 min, and then rinsed with sterile water. After removing the bark, tissue samples were excised from discolored sapwood and placed onto 2% malt extract agar plates supplemented with 100 mg/liter streptomycin sulfate. Plates were incubated at 22degreesC in the dark for 10 days. At two out of the 22 sites (Bad Radkersburg, province Styria, and Ganserndorf, province Lower Austria) with symptomatic A. altissima, cultures morphologically resembling Verticillium albo-atrum sensu lato were isolated, whereas V. dahliae or no Verticillium spp. were obtained from the other sites. Morphological characteristics of the isolates corresponded to the description of V. nonalfalfae(Inderbitzin et al. 2011), which is, however, morphologically indistinguishable from V. alfalfae. Thus, identification was confirmed by DNA sequencing, using primer pair ITS1-F and ITS4, and a BLAST search against reference ITS sequences at GenBank, which revealed a 100% homology between the two Austrian isolates and the type specimen of V. nonalfalfae(GenBank Accession No. JN187973). Aligned sequences from both isolates (G1/5 and I3/2), were deposited into GenBank (Accession Nos. KT223526 and KT223527). To confirm pathogenicity, 20 canopy trees were stem-inoculated with a conidial suspension (1 x 10 super(7) spores/ml, 3 ml per tree) of V. nonalfalfae isolate G1/5 in August 2011; six canopy trees treated with sterile water served as controls. Four weeks after inoculation, all fungus-inoculated trees developed wilting symptoms and were dead by 2013, whereas all six control trees remained asymptomatic. V. nonalfalfae was consistently reisolated from symptomatic trees. To our knowledge, this is the first report of V. nonalfalfae causing Verticillium wilt on A. altissima in Europe.
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