PhD Candidate, Memorial University of Newfoundland
Potato common scab (CS) is an economically important crop disease that is primarily caused by the soil bacterium Streptomyces scabies. Current control practices for managing this disease include crop rotation, irrigation and chemical fumigation; however, such strategies often fail, are environmentally unfriendly or produce inconsistent results. In order to develop better control strategies for CS, we must first understand the molecular mechanisms used by S. scabies to infect the plant host and to induce disease symptoms. Recent research has shown that the ability of S. scabies to cause disease is due to the production of virulence factors that play different roles during the infection process. Among these factors is a family of plant toxins called the coronafacoyl phytotoxins, which are thought to allow the bacterium to suppress plant immune responses. The purpose of this study is to better understand how these phytotoxins are produced by S. scabies by characterizing three enzymes (called Sdr, Cyp450 and Cfl) that are known to be required for phytotoxin biosynthesis. Each enzyme is thought to catalyze a specific step in the biosynthetic pathway for the coronafacoyl phytotoxins, and the exact function of each will be investigated. This will be accomplished either by purifying the enzyme and characterizing the specific biochemical reaction that it catalyzes, or by characterizing biosynthetic intermediates that accumulate in strains of S. scabies that are unable to produce the enzyme. The results of this study will provide important insights into the production of an important virulence factor and could potentially lead to the development of strategies aimed at shutting down phytotoxin production for management of CS disease.
Abstract: Bacteria and fungi produce a remarkable array of bioactive small molecules. Many of these have found use in medicine as chemotherapies to treat diseases ranging from infection and cancer to hyperlipidemia and autoimmune disorders. The applications may or may not reflect the actual targets for these compounds. Through careful studies of microbes, their associated molecules and their targets, a growing understanding of the ecology of microbial secondary metabolism is emerging that exposes the central role of secondary metabolites in many complex biological systems.
Pub.: 19 Apr '11, Pinned: 07 Jun '17
Abstract: Among the multitude of soil-inhabiting, saprophytic Streptomyces species are a growing number of plant pathogens that cause economically important diseases, including potato scab. Streptomyces scabies is the dominant pathogenic species worldwide, but is only one of many that cause very similar disease symptoms on plants. Molecular genetic analysis is beginning to identify the mechanisms used by plant pathogenic species to manipulate their hosts. The nitrated dipeptide phytotoxin, thaxtomin, inhibits cellulose biosynthesis in expanding plant tissues, stimulates Ca2+ spiking, and causes cell death. A secreted necrogenic protein, Nec1, contributes to virulence on diverse plant species. The thaxtomin biosynthetic genes and nec1 lie on a large mobilizable PAI, along with other putative virulence genes including a cytokinin biosynthetic pathway and a saponinase homolog. The PAI is mobilized during conjugation and site-specifically inserts in the linear chromosome of recipient species, accounting for the emergence of new pathogens in agricultural systems. The recently available genome sequence of S. scabies will accelerate research on host-pathogen interactions.
Pub.: 25 May '06, Pinned: 07 Jun '17
Abstract: Plant-pathogenic Streptomyces spp. cause scab disease on economically important root and tuber crops, the most important of which is potato. Key virulence determinants produced by these species include the cellulose synthesis inhibitor, thaxtomin A, and the secreted Nec1 protein that is required for colonization of the plant host. Recently, the genome sequence of Streptomyces scabies 87-22 was completed, and a biosynthetic cluster was identified that is predicted to synthesize a novel compound similar to coronafacic acid (CFA), a component of the virulence-associated coronatine phytotoxin produced by the plant-pathogenic bacterium Pseudomonas syringae. Southern analysis indicated that the cfa-like cluster in S. scabies 87-22 is likely conserved in other strains of S. scabies but is absent from two other pathogenic streptomycetes, S. turgidiscabies and S. acidiscabies. Transcriptional analyses demonstrated that the cluster is expressed during plant-microbe interactions and that expression requires a transcriptional regulator embedded in the cluster as well as the bldA tRNA. A knockout strain of the biosynthetic cluster displayed a reduced virulence phenotype on tobacco seedlings compared with the wild-type strain. Thus, the cfa-like biosynthetic cluster is a newly discovered locus in S. scabies that contributes to host-pathogen interactions.
Pub.: 13 Jan '10, Pinned: 06 Jun '17
Abstract: Coronafacoyl phytotoxins are secondary metabolites that are produced by various phytopathogenic bacteria, including several pathovars of the Gram‐negative bacterium Pseudomonas syringae as well as the Gram‐positive potato scab pathogen Streptomyces scabies. The phytotoxins are composed of the polyketide coronafacic acid (CFA) linked via an amide bond to amino acids or amino acid derivatives, and their biosynthesis involves the cfa and cfa‐like gene clusters that are found in P. syringae and S. scabies, respectively. The S. scabies cfa‐like gene cluster was previously reported to contain several genes that are absent from the P. syringae cfa gene cluster, including one (oxr) encoding a putative F420—dependent oxidoreductase, and another (sdr) encoding a predicted short‐chain dehydrogenase/reductase. Using gene deletion analysis, we demonstrated that both oxr and sdr are required for normal production of the S. scabies coronafacoyl phytotoxins, and structural analysis of metabolites that accumulated in the Δsdr mutant cultures revealed that Sdr is directly involved in the biosynthesis of the CFA moiety. Our results suggest that S. scabies and P. syringae use distinct biosynthetic pathways for producing coronafacoyl phytotoxins, which are important mediators of host‐pathogen interactions in various plant pathosystems.
Pub.: 20 Apr '16, Pinned: 06 Jun '17