Presentation on plant innate immunity.pptx

Plants use an innate immune system to combat potential disease and employ a battery of immune mechanisms. The first mode of plant immunity is triggered upon the perception of microbe-associated or pathogen-associated molecular patterns (MAMPs or PAMPs) through pattern recognition receptors (PRRs)

1. WELCOME

2. UNIVERSITY OF AGRICULTURAL SCIENCES, BANGALORE COLLEGE OF AGRICULTURE V. C. FARM MANDYA Presentation on: Plant innate immunity: PAMP and DAMP’s Reddy Kumar A V PAMM3005 Dept. of Plant Pathology

3. Plants, like animals, have evolved sophisticated defense mechanisms to protect themselves from various pathogens, including bacteria, fungi, viruses, and insects. Unlike animals, plants lack an adaptive immune system and rely solely on innate immunity. Plant innate immunity is a complex network of preformed and inducible defense responses that allow them to recognize and respond to potential threats in the environment. INTRODUCTION

4. The plant immune system is generally categorized into two layers: Pattern-Triggered Immunity (PTI) : This is the first line of defense, activated when plants detect conserved microbial signatures known as pathogen-associated molecular patterns (PAMPs), such as flagellin or chitin. PAMP recognition is mediated by cell surface pattern recognition receptors (PRRs), triggering a defense response that includes production of reactive oxygen species (ROS), cell wall fortification, and activation of defense-related genes. 2.Effector-Triggered Immunity (ETI) : Some pathogens bypass PTI by delivering effector molecules into plant cells. To counter this, plants have evolved resistance (R) proteins that can recognize these effectors, often resulting in a stronger and more localized response, including hypersensitive cell death at the infection site. This layer of immunity is often referred to as a "gene-for-gene" interaction between plant R genes and pathogen effectors.

5. Fig. 1: Plant immune response: PTI and ETI

6. Definition Pathogen-Associated Molecular Patterns (PAMPs) are conserved molecular motifs found on pathogens that are recognized by the host immune system to initiate defense mechanisms. Types of PAMPs Bacterial PAMPs Lipopolysaccharides (LPS) : Found on the outer membrane of Gram-negative bacteria. Flagellin : A protein that makes up the flagella of motile bacteria. Peptidoglycan (PGN) : A component of bacterial cell walls, particularly in Gram-positive bacteria. Lipoteichoic Acid (LTA) : Found in the cell wall of Gram-positive bacteria. Fungal PAMPs Chitin : A component of fungal cell walls. β- Glucans : Polysaccharides in the fungal cell wall. PAMP’s

7. 3.Viral PAMPs Viral RNA : Double-stranded RNA (dsRNA) produced during viral replication. Unmethylated CpG DNA : Unmethylated regions in viral DNA. 4. Other Microbial PAMPs Zymosan : A component of fungal cell walls. Mannans : Polysaccharides present on the surface of fungi and some bacteria.

8. PAMP-Triggered Immunity (PTI) is the first line of defense in plants against microbial pathogens. It is initiated when plants recognize conserved pathogen-associated molecular patterns (PAMPs) using specialized pattern recognition receptors (PRRs) located on the cell surface. This recognition triggers a cascade of signaling events that lead to a broad-spectrum immune response aimed at stopping the pathogen's infection. PAMP-Triggered Immunity (PTI)

9. Recognition of PAMPs: PAMPs are conserved molecular signatures found in various classes of pathogens, including bacteria, fungi, and viruses. Some common PAMPs include: Flagellin (flg22): A conserved peptide in bacterial flagella. Chitin: A component of fungal cell walls. Lipopolysaccharides (LPS): Found in bacterial outer membranes. 2. PRRs on the plant cell membrane recognize these PAMPs. For example: FLS2 (Flagellin-Sensing 2) recognizes bacterial flagellin. CERK1 (Chitin Elicitor Receptor Kinase 1) recognizes chitin. Mechanisms of PTI

10. 2. Activation of Signaling Pathways: Upon PAMP recognition, the PRRs activate downstream signaling pathways. Key molecules involved in these pathways include: Reactive Oxygen Species (ROS): A burst of ROS is one of the early responses to PAMP recognition, causing direct damage to pathogens and reinforcing cell walls. Calcium (Ca2+) Influx: Increased cytosolic calcium levels act as secondary messengers in activating defense -related gene expression. Mitogen-Activated Protein Kinase (MAPK) Cascades: MAPK cascades are triggered, leading to phosphorylation of various transcription factors that regulate defense genes. Phytohormones: Such as salicylic acid (SA), jasmonic acid (JA), and ethylene (ET), which modulate the defense response.

11. 3. Defense Gene Expression: Activation of defense -related genes leads to the production of antimicrobial proteins and compounds, such as: Pathogenesis-related (PR) proteins: These proteins have antimicrobial activity and degrade pathogen components. Phytoalexins: Small antimicrobial molecules synthesized in response to infection. Callose deposition: Callose is deposited at the site of infection to reinforce the cell wall and prevent pathogen penetration.

12. Key Signal Molecules in PTI: Pattern Recognition Receptors (PRRs): Example: FLS2 (recognizes flagellin), CERK1 (recognizes chitin). ROS (Reactive Oxygen Species): Key for oxidative burst, leading to direct pathogen damage and strengthening of cell walls. Ca2+ Influx: Functions as a second messenger, transmitting the signal downstream to activate defense responses.

13. 4. MAPKs (Mitogen-Activated Protein Kinases): Involved in amplifying and transducing the signal leading to activation of transcription factors and defense genes. 5. Phytohormones: Salicylic acid (SA): Primarily associated with defense against biotrophic pathogens. Jasmonic acid (JA): Involved in defense against necrotrophic pathogens and herbivorous insects. Ethylene (ET): Often works in concert with JA to modulate the defense response.

14. Outcome of PTI: Enhanced Cell Wall Reinforcement: Deposition of callose and lignin to prevent pathogen entry. Production of Antimicrobial Compounds: Synthesis of pathogenesis-related proteins, phytoalexins, and defensins that inhibit pathogen growth. Localized Cell Death: In some cases, PTI can lead to programmed cell death (hypersensitive response) to restrict pathogen spread. Systemic Acquired Resistance (SAR): A long-lasting, broad-spectrum immune response that is activated throughout the plant in response to localized pathogen attack, often mediated by salicylic acid.

15. Example of PTI in Action: Bacterial flagellin (flg22) is recognized by the PRR FLS2 . This triggers a ROS burst, MAPK activation, and defense gene expression leading to increased production of antimicrobial compounds and cell wall reinforcement. The outcome is that the bacterium is either prevented from entering the plant cell or its growth is slowed down, preventing infection.

16. Damage-Associated Molecular Patterns (DAMPs) are molecules that are released from damaged or stressed cells. Unlike pathogen-associated molecular patterns (PAMPs), which are derived from infectious agents, DAMPs originate from the host's own cells and trigger immune responses. DAMPs signal the immune system to the presence of cellular damage or danger, promoting inflammatory responses and tissue repair. Definition: DAMPs are endogenous molecules released from damaged or dying cells that can initiate and perpetuate an immune response. They act as "danger signals," alerting the body to injury or stress. DAMP’S

17. Types of DAMPs: DAMPs can be grouped based on their molecular origin or class. Below are some common types: Nuclear DAMPs: HMGB1 (High Mobility Group Box 1): Released during cell damage or necrosis. It can bind to receptors like TLR4 and RAGE to promote inflammation. Histones: When released extracellularly, they trigger inflammation and coagulation. Mitochondrial DAMPs: Mitochondrial DNA ( mtDNA ): Structurally similar to bacterial DNA, when released, it can stimulate the immune system via Toll-like receptor 9 (TLR9). ATP (Adenosine Triphosphate): Released from injured or dying cells, acting as a pro-inflammatory signal through P2 receptors.

18. 3. Cytosolic DAMPs: Heat Shock Proteins (HSPs): When outside the cell, HSPs can activate the immune system by interacting with Toll-like receptors. Uric Acid: Released from dying cells, it forms crystals that trigger inflammation (commonly seen in gout). 4. Extracellular Matrix DAMPs: Hyaluronic Acid Fragments: Breakdown products of the extracellular matrix that can activate immune cells and promote inflammation. Biglycan : Released from the extracellular matrix upon tissue injury, it activates TLR2 and TLR4 pathways to promote inflammatory responses. 5. Plasma Membrane-Derived DAMPs: S100 Proteins: These are calcium-binding proteins that, when released outside the cell during stress or damage, act as inflammatory mediators. Phosphatidylserine: Normally found on the inner leaflet of the cell membrane, it flips to the outer membrane during apoptosis and acts as a signal for phagocytosis.

19. DAMP-Triggered Immunity (DTI) is a critical part of a plant's immune response. DAMPs, or Damage-Associated Molecular Patterns , are molecules released by damaged or stressed plant cells. They signal to the plant that its own tissues are under attack, which in turn activates defense mechanisms. This response is complementary to PAMP-triggered immunity (PTI), which is activated by pathogen-associated molecular patterns. DAMP-Triggered Immunity (DTI)

20. 1. Release of DAMPs Cell Damage: During pathogen attack or physical injury, plant cells are often damaged. This damage can lead to the release of intracellular molecules, which act as DAMPs. Common DAMPs: Examples include oligogalacturonides (OGs) released from the cell wall, ATP (from damaged mitochondria), peptides (like systemin), and other intracellular molecules like DNA, RNA, or secondary metabolites. 2. Recognition of DAMPs Pattern Recognition Receptors (PRRs): The plant has receptors on the cell surface, primarily in the plasma membrane, that recognize these DAMPs. These receptors are called PRRs , typically being receptor-like kinases (RLKs) or receptor-like proteins (RLPs). Example of DAMP Receptors: PEPR1/PEPR2 (PEP receptors): These recognize endogenous peptide DAMPs like AtPEP1 in Arabidopsis. P2K1 receptor: Recognizes extracellular ATP as a DAMP. Mechanism of DTI

21. 3. Signal Transduction Cascade Once a PRR binds a DAMP, it triggers a complex signaling pathway: Activation of MAPK Cascade (Mitogen-Activated Protein Kinases): Once DAMP is recognized by a PRR, a MAPK cascade is activated. This cascade involves multiple steps of phosphorylation and ultimately activates transcription factors (TFs) in the nucleus. Calcium Influx: Recognition of DAMPs often results in a rapid influx of calcium ions (Ca²⁺) into the cytoplasm. Calcium ions act as secondary messengers in numerous signaling pathways. Reactive Oxygen Species (ROS) Production: The binding of DAMPs to their receptors often leads to the generation of ROS via NADPH oxidases, which act as both signaling molecules and antimicrobial agents. Hormonal Signals: DAMP recognition often leads to the production or modulation of phytohormones, such as jasmonic acid (JA), salicylic acid (SA), and ethylene, which play roles in further refining and amplifying the immune response.

22. 4. Defense Response Activation Gene Expression: Transcription factors activated by the signaling cascade will induce the expression of defense-related genes. These include: Pathogenesis-Related (PR) proteins: These proteins have antimicrobial properties. Cell Wall Reinforcement: The cell wall becomes reinforced through the deposition of callose and lignin to prevent pathogen entry. Localized Cell Death: In some cases, plants activate the hypersensitive response (HR) , a form of programmed cell death at the infection site to prevent the spread of the pathogen. Secondary Metabolites: The production of antimicrobial compounds, such as phytoalexins, is induced to limit pathogen growth.

23. 5. Amplification and Systemic Response Systemic Acquired Resistance (SAR): DAMP recognition can also contribute to a long-lasting defense state known as SAR, which primes the plant for enhanced resistance to future attacks across the entire plant. Cross-talk with PTI and Effector-Triggered Immunity (ETI): DAMP-triggered immunity can enhance the plant's overall immune response by working in conjunction with PTI and ETI.

24. 1. Extracellular ATP ( eATP ): Released from damaged cells and recognized by purinergic receptors like P2K1 in plants. Triggers immune responses such as reactive oxygen species (ROS) production, calcium influx, and activation of mitogen-activated protein kinase (MAPK) cascades. 2. Cell Wall Fragments (Oligosaccharides): Fragments of plant cell wall components such as oligogalacturonides (OGs) released during pathogen attack. OGs are perceived by receptors like wall-associated kinases (WAKs) and trigger immune responses, including ROS production and transcription of defense -related genes. 3. Reactive Oxygen Species (ROS): Generated rapidly after damage or pathogen detection, ROS act both as signaling molecules and antimicrobial agents. ROS burst is an early event in DTI, which also helps in amplifying defense responses. Key signal molecules involved in DTI

25. 4. Hydrogen Peroxide (H₂O₂): A type of ROS that plays a dual role: signaling for cell wall reinforcement (e.g., lignification) and activating downstream defense pathways. 5. Calcium Ions (Ca²⁺): Calcium influx is a common early signal in DTI, essential for activating various defense responses. Ca²⁺ sensors like calmodulin and CBL-CIPK complexes (calcineurin B-like proteins) mediate downstream signaling . 6. DNA Fragments (Nucleotides): Released from damaged or dying cells, DNA fragments act as DAMPs, triggering immune responses similar to eATP . 7. HMGB Proteins (High Mobility Group Box Proteins): These proteins are typically nuclear but can be released upon cell damage. In mammals, HMGB1 is a well-known DAMP. They help in amplifying immune responses when released extracellularly.

26. 8. Heat Shock Proteins (HSPs): Released under stress or cell damage, these proteins are recognized by immune receptors and contribute to immune activation. 9. Lipid Mediators (Oxylipins): Compounds such as oxylipins (derived from polyunsaturated fatty acids) serve as DAMPs. They play roles in local and systemic defense signaling in response to cell damage. 10. Nitric Oxide (NO): A signaling molecule involved in coordinating immune responses, NO can modulate ROS production and activate defense-related genes.

27. Activation of Defense Genes : DAMPs bind to pattern recognition receptors (PRRs) on the plant cell surface, leading to the activation of transcription factors that induce the expression of defense-related genes. These genes help produce antimicrobial compounds, enzymes, and proteins involved in plant defense. Production of Reactive Oxygen Species (ROS) : ROS production is a rapid and localized response that acts as a signaling molecule to trigger further immune responses. It can also directly inhibit pathogen growth. Callose Deposition : Callose, a polysaccharide, is deposited at the site of infection or damage. It strengthens the cell wall, preventing pathogen invasion. Hormonal Signaling : DTI often leads to the modulation of plant hormones like salicylic acid (SA), jasmonic acid (JA), and ethylene (ET), which coordinate long-term defense responses, particularly systemic acquired resistance (SAR) and induced systemic resistance (ISR). OUTCOME

28. Ion Flux Changes : DTI causes an influx of calcium ions (Ca²⁺) and other ion fluxes across the plasma membrane. This is important for downstream signaling cascades that regulate immune responses. Programmed Cell Death (PCD) : In some cases, DAMPs can trigger localized programmed cell death, called the hypersensitive response (HR), which restricts the spread of pathogens to other tissues. Systemic Signals : DAMPs can activate systemic signaling pathways, leading to a broad, systemic immune response that protects the plant beyond the immediate site of infection.

29. In the case of Botrytis cinerea (a necrotrophic fungal pathogen that causes gray mold disease), the pathogen secretes enzymes like polygalacturonases to degrade the plant cell wall, releasing OGs. These oligogalacturonides are recognized by plant receptors, triggering an immune response. This response leads to the production of reactive oxygen species (ROS), activation of defense-related genes, and reinforcement of the cell wall, which helps restrict pathogen progression. Example of DTI in Action

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