Evidence that thaxtomin C is a pathogenicity determinant of Streptomyces ipomoeae, the causative agent of streptomyces soil rot disease of sweet potato

Streptomyces ipomoeae is the causal agent of Streptomyces soil rot of sweet potato, a disease marked by highly necrotic destruction of adventitious roots, including the development of necrotic lesions on the fleshy storage roots. Streptomyces potato scab pathogens produce a phytotoxin (thaxtomin A) that appears to facilitate their entrance into host plants. S. ipomoeae produces a less-modified thaxtomin derivative (thaxtomin C) whose role in pathogenicity has not been examined. Here, we cloned and sequenced the thaxtomin gene cluster (txt) of S. ipomoeae, and we then constructed targeted txt mutants that no longer produced thaxtomin C. The mutants were unable to penetrate intact adventitious roots but still caused necrosis on storage-root tissue. These results, taken in context with previous histopathological study of S. ipomoeae infection, suggest that thaxtomin C plays an essential role in inter- and intracellular penetration of adventitious sweet potato roots by S. ipomoeae. Once inside the plant host, the pathogen uses one or more yet-tobe- determined factors to necrotize root tissue, including that of any storage roots it encounters. © 2012 The American Phytopathological Society

A ubiquitin carboxyl extension protein secreted from a plant-parasitic nematode Globodera rostochiensis is cleaved in planta to promote plant parasitism

Nematode effector proteins originating from esophageal gland cells play central roles in suppressing plant defenses and in formation of the plant feeding cells that are required for growth and development of cyst nematodes. A gene (GrUBCEP12) encoding a unique ubiquitin carboxyl extension protein (UBCEP) that consists of a signal peptide for secretion, a mono-ubiquitin domain, and a 12 amino acid carboxyl extension protein (CEP12) domain was cloned from the potato cyst nematode Globodera rostochiensis. This GrUBCEP12 gene was expressed exclusively within the nematode\u27s dorsal esophageal gland cell, and was up-regulated in the parasitic second-stage juvenile, correlating with the time when feeding cell formation is initiated. We showed that specific GrUBCEP12 knockdown via RNA interference reduced nematode parasitic success, and that over-expression of the secreted GrΔSPUBCEP12 protein in potato resulted in increased nematode susceptibility, providing direct evidence that this secreted effector is involved in plant parasitism. Using transient expression assays in Nicotiana benthamiana, we found that GrΔSPUBCEP12 is processed into free ubiquitin and a CEP12 peptide (GrCEP12) in planta, and that GrCEP12 suppresses resistance gene-mediated cell death. A target search showed that expression of RPN2a, a gene encoding a subunit of the 26S proteasome, was dramatically suppressed in GrΔSPUBCEP12 but not GrCEP12 over-expression plants when compared with control plants. Together, these results suggest that, when delivered into host plant cells, GrΔSPUBCEP12 becomes two functional units, one acting to suppress plant immunity and the other potentially affecting the host 26S proteasome, to promote feeding cell formation

Genome content and phylogenomics reveal both ancestral and lateral evolutionary pathways in plant-pathogenic Streptomyces species

© 2016, American Society for Microbiology. All Rights Reserved. Streptomyces spp. are highly differentiated actinomycetes with large, linear chromosomes that encode an arsenal of biologically active molecules and catabolic enzymes. Members of this genus are well equipped for life in nutrient-limited environments and are common soil saprophytes. Out of the hundreds of species in the genus Streptomyces, a small group has evolved the ability to infect plants. The recent availability of Streptomyces genome sequences, including four genomes of pathogenic species, provided an opportunity to characterize the gene content specific to these pathogens and to study phylogenetic relationships among them. Genome sequencing, comparative genomics, and phylogenetic analysis enabled us to discriminate pathogenic from saprophytic Streptomyces strains; moreover, we calculated that the pathogen-specific genome contains 4,662 orthologs. Phylogenetic reconstruction suggested that Streptomyces scabies and S. ipomoeae share an ancestor but that their biosynthetic clusters encoding the required virulence factor thaxtomin have diverged. In contrast, S. turgidiscabies and S. acidiscabies, two relatively unrelated pathogens, possess highly similar thaxtomin biosynthesis clusters, which suggests that the acquisition of these genes was through lateral gene transfer

Common scab disease: structural basis of elicitor recognition in pathogenic Streptomyces species.

peer reviewedIn Streptomyces scabiei, the main causative agent of common scab disease of root and tuber crops, the interaction between the substrate-binding protein (SBP) CebE (CebEscab) and cellotriose released by the plant host (KD in the nanomolar range) is the first event for the onset of its pathogenic lifestyle. Here, we report the structure of CebEscab in complex with cellotriose at a resolution of 1.55 Å, adopting a general fold of the B subcluster of SBPs. The interaction between CebEscab and cellotriose involves multiple direct or water-mediated hydrogen bonds and hydrophobic interactions, with the glucose monomer at the non-reducing end occupying the most conserved part of the substrate-binding cleft. As main interactions between the two domains of CebE involve cellotriose itself, the closed conformational state of CebE is performed via an induced-fit ligand binding mechanism where cellotriose binding triggers the domain movement. Analysis of regulon predictions revealed that the signaling pathway from CebE-mediated cellotriose transport to the transcriptional activation of thaxtomin phytotoxin biosynthesis is conserved in Streptomyces spp. causing common scab, except for Streptomyces ipomoeae, which specifically colonizes sweet potatoes and responds to other and yet unknown virulence elicitors. Interestingly, strains belonging to the pathogenic species turgidiscabies and caniscabiei have a cellotriose-binding protein orthologous to the CebE protein of the saprophytic species Streptomyces reticuli with lower affinity for its substrate (KD in the micromolar range), suggesting higher cellotriose concentrations for perception of their host. Our work also provides the structural basis for the uptake of cellobiose and cellotriose by non-pathogenic cellulose-decomposing Streptomyces species.IMPORTANCECommon scab is a disease caused by a few Streptomyces species that affects important root and tuber crops including potato, beet, radish, and parsnip, resulting in major economic losses worldwide. In this work, we unveiled the molecular basis of host recognition by these pathogens by solving the structure of the sugar-binding protein CebE of Streptomyces scabiei in complex with cellotriose, the main elicitor of the pathogenic lifestyle of these bacteria. We further revealed that the signaling pathway from CebE-mediated transport of cellotriose is conserved in all pathogenic species except Streptomyces ipomoeae, which causes soft rot disease in sweet potatoes. Our work also provides the structural basis of the uptake of cellobiose and cellotriose in saprophytic Streptomyces species, the first step activating the expression of the enzymatic system degrading the most abundant polysaccharide on earth, cellulose

Potato Scab

NYS IPM Type: Vegetables IPM Fact SheetPotato scab is a common tuber disease that occurs throughout the potato growing regions of the world. Although scab does not usually affect total yields, significant economic losses result from reduced marketability of the tubers. Economic losses are greatest when tubers intended for table stock are infected, since appearance is important for this market. While superficial scab lesions do not greatly affect the marketability of processing potatoes, deep-pitted lesions, however, do increase peeling losses and detract from the appearance of the processed product. The occurrence of scab and its severity varies by season and from field to field. Cropping history, soil moisture, and soil texture are largely responsible for this variability. Potato scab lesions can be confused with powdery scab, a disease caused by an entirely different pathogen, the fungus Spongospora subterranea

Detection of Potato Tuber Diseases and Defects

The objective of this bulletin is to aid in the diagnosis of tuber diseases and defects that most often result in production problems. Symptoms of 20 selected diseases and disorders as they appear on the tubers are described and illustrated

Silver Scurf of Potato

NYS IPM Type: Vegetables IPM Fact SheetSilver scurf, caused by the fungus Helminthosporium solani, is a common disease of potato and is present in all major production areas in the United States. The incidence and severity of silver scurf in New York, and its associated economic losses, have increased since about 1985. The brown blemishes that develop on the tuber surface lower the market value of the crop. These losses are greatest when the disease occurs on tubers with white or red skin intended for the table-stock market. The increased water loss from infected tubers during storage results in shrinkage that can be economically significant. Silver scurf-infected tubers may be more susceptible to secondary infection by other pathogens. Silver scurf infection has been associated with decreased seed-tuber vigor, but the importance of these effects is not well documented

Rhizoctonia Disease (of Potato)

NYS IPM Type: Vegetables IPM Fact SheetRhizoctonia solani is a fungus that attacks tubers, underground stems, and stolons of potato plants. Although it probably occurs wherever potatoes are grown, it causes economically significant damage only in cool, wet soils. In temperate production areas, losses from R. solani are sporadic and occur only when weather is cold and wet in the weeks following planting. In northern areas, where growers often must plant in cold soils, Rhizoctonia is a more consistent problem. Poor stands, stunted plants, reduced tuber number and size, and misshapen tubers are characteristic of the Rhizoctonia disease

Late Blight, A Serious Disease of Potatoes and Tomatoes

NYS IPM Type: BrochureNYS IPM Type: Vegetables IPM BrochureLate blight is a plant disease that mainly attacks potatoes and tomatoes, although it can sometimes be found on other crops, weeds and ornamentals in the same botanical family (Solanaceae). Other plants that late blight may infect include petunia, nightshades, and tomatillos