Pseudomonas syringae pv. tomato (Pst) is a common pathogen of tomato which causes bacterial speck disease. This disease serves as a useful model for studying the interactions of microbial pathogens and plants. Most gram-negative bacteria, including Pst, have type III secretion system (TTSS). Encoded by hrp gene clusters, the TTSS is used to deliver effector proteins into the host cytosol. The hrp genes also control the expression of the avirulence genes (avr). One Avr protein, AvrPto, functions as ligand to elicit a hypersensitive response (HR) in the tomato plant after recognition by the protein encoded by the host resistance gene, Pto. The AvrPto-Pto interaction is the most widely studied systems. It has been discovered that Pto is linked with Fen, the gene responsible for susceptibility to an organophosphate insecticide, fenthion. Functioning of Pto requires another gene called Prf, which lies embedded in Pto. Though the system is well characterized, several aspects are still not understood. With the availability of completed genome sequence of Pst and the full sequence of tomato expected in the future, we may anticipate that our understanding of the mechanisms of this host-pathogen interaction to be improved.
Table of Contents
1. Summary
2. Biological aspect of interaction
3. Evidence for Communication between Microbe and Plant or vise versa-
4. Physiological Alterations in the Participants
5. Molecular Biology and Molecular Genetics
Research Objectives and Themes
This work explores the molecular and physiological interactions between the pathogen Pseudomonas syringae pv. tomato (Pst) and its host plant, the tomato. The primary research focus lies in understanding the host-pathogen interplay, particularly how virulence factors and plant defense mechanisms, such as the Pto/Prf system, regulate disease progression and resistance.
- Mechanisms of bacterial entry and stomatal modulation via coronatine.
- Role of the Type III Secretion System (TTSS) in delivering effector proteins.
- Physiological and genetic shifts in host cells, including nitrogen metabolism and senescence-like responses.
- The molecular basis of gene-for-gene resistance and the function of the Pto/Prf complex.
- Signaling pathways involved in downstream defense induction, including MAPK and salicylic acid.
Excerpt from the Book
Biological aspect of interaction
Pseudomonas syringae pv. tomato (Pst) causes bacterial speck disease of tomato wherever tomatoes are grown (8). Primary sites of infection are stomata, the bases of leaf trichomes and wounds (2). Following infection, bacteria multiply in the leaf interior by forming microcolonies in close physical association with the cell wall of host mesophyll cells (15). Once the host intercellular spaces are filled with Pst, host cells are polymerized and degenerate (37). Disease symptoms may be evident on all aboveground plant parts, though immature tissues are the most susceptible. Symptoms on leaves are often indistinct. Leaf spots are dark and round, and often have a discrete halo. As the disease progresses, lesions may extend into the petiole and stems. On fruit, the disease initially appears as small black spots of 1/8 – 1/4 inch diameter with distinct margins. These small spots are superficial, do not rupture the epidermis and will not develop into soft rot. Lesions on fruit are sometimes surrounded by an area that is slow to ripen. When fruits are infected early, the spots may cause pit-like distortions because the host tissues within lesions grow slower than unaffected tissue. Mature fruits are resistant to Pst infection as a result of their high acidity (8,38,42). Formation of halo in speck symptoms is due to the toxin, coronatine (COR), produced by Pst (3). Serious disease outbreaks, though rare, are favored by high leaf wetness, cool temperature and cultural practices that allow bacteria to be disseminated between hosts (26). Though the economic impact of disease is minimal, it is important from the point of view of scientific study of host pathogen interactions. When Pst infects susceptible tomato plants it causes typical disease symptoms. In contrast, infection of a resistant plant is restricted by the localized death of the cells at the site of infection known as the hypersensitive response (HR). The HR is generally microscopic but when it occurs over larger areas, becomes macroscopic.
Summary of Chapters
Summary: Provides an overview of Pseudomonas syringae pv. tomato, its pathogenesis, and the critical role of the Pto-AvrPto interaction model in plant pathology.
Biological aspect of interaction: Details the physical infection process, symptom development, and the role of environmental factors in disease progression.
Evidence for Communication between Microbe and Plant or vise versa-: Discusses how the pathogen modulates stomatal aperture using coronatine and utilizes the Type III Secretion System for successful colonization.
Physiological Alterations in the Participants: Analyzes the metabolic and genetic shifts in tomato plants, such as nitrogen assimilation changes and senescence-like responses, post-infection.
Molecular Biology and Molecular Genetics: Examines the genetic structure of the hrp cluster, the regulation of effector proteins, and the gene-for-gene resistance mechanisms involving the Pto and Prf proteins.
Keywords
Pseudomonas syringae pv. tomato, Bacterial speck disease, Type III secretion system, hrp genes, AvrPto, Pto, Prf, hypersensitive response, coronatine, plant defense, host-pathogen interaction, disease resistance, stomata, gene-for-gene model, molecular genetics.
Frequently Asked Questions
What is the primary focus of this publication?
This work provides an insight into the molecular and physiological interactions between the tomato plant and the pathogen Pseudomonas syringae pv. tomato, specifically focusing on disease resistance mechanisms.
What are the central themes of the research?
The central themes include the bacterial infection process, the role of virulence factors like coronatine, the Type III Secretion System (TTSS), and the molecular basis of the Pto/Prf resistance system.
What is the ultimate goal of studying the Pst-tomato interaction?
The goal is to better understand the mechanisms of host-pathogen interactions and plant disease resistance, which serves as a valuable model system in molecular biology.
Which scientific methods are discussed?
The publication discusses genetic mapping, mutational analysis, genome sequencing, and gene expression profiling as tools to study plant defense responses and bacterial virulence.
What does the main body cover?
The main body covers the physical interaction in leaf tissue, bacterial communication via toxins, physiological host changes like nitrogen metabolism, and the detailed genetic regulation of virulence and resistance genes.
Which keywords characterize this paper?
Key terms include Pst, hrp genes, AvrPto, Pto, Prf, hypersensitive response (HR), and Type III secretion system.
How does coronatine facilitate bacterial invasion?
Coronatine promotes stomatal reopening by interfering with plant signaling pathways, allowing the bacteria to enter the leaf interior more easily.
Why is the Pto gene significant in tomato plants?
Pto encodes a kinase that recognizes specific pathogen effectors, triggering a defense response known as the hypersensitive response.
What is the role of the Prf gene in this interaction?
Prf is essential for the function of the Pto-mediated resistance; it encodes a large protein that works in tandem with Pto to activate defense mechanisms.
How does the "affinity-enhancement" model differ from the "guard hypothesis"?
Unlike the guard hypothesis, which suggested that Prf monitors Pto-AvrPto interaction, the affinity-enhancement model suggests that Pto and Prf form a complex that is stabilized by AvrPto binding, thereby increasing plant defense activity.
- Quote paper
- Pravin Gautam (Author), 2008, Bacterial Speck Disease of Tomato: An Insight into Host-Bacteria Interaction, Munich, GRIN Verlag, https://www.hausarbeiten.de/document/178581
