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8/17/2011 How Plants Defend Themselves Against Pathogens Genetically-resistant apple showing the HR response (localized host cell death and accumulation of phenolic compounds that autofluoresce under UV light) to attempted invasion by Ventura inaequalis spores. The V. inaequalis spore can be observed at the center of the autofluorescence. (School of Biological Sciences, University of Auckland, NZ) Two Categories of Defensive Weapons • Structural Barriers Inhibit Entrance or Spread of Pathogen • Biochemical Reactions Toxins Pathogen Growth Inhibitors Plants May Possess One or More of the Weapons Combination of Defensive Weapons Depends on Various Factors • Species • Age of Plant • Organ or Tissue Attacked • Nutritional Condition of Plant • Weather Conditions First Line of Defense Is Surface of the Plant 1 8/17/2011 Wax – Water-Repellent Surface – Prevents Film of Water Needed to Germinate (Fungi) or Multiply (Bacteria) Cuticle – Thick Cuticle May Increase Resistance to Infection but Not Always Correlated with Resistance Epidermal Cell Walls – Thick, Tough Outer Walls Make Direct Penetration Difficult or Impossible – However, If Plant Invaded through Wound Site, Inner Tissues Easily Invaded • Fungal Spores Germinate at Night, then Enter Plant Stomata when They Open Early in the Morning • Some Pathogens Can Force Their Way through Closed Stomata, But Some, eg Stem Rust of Wheat, Can Enter Only When Stomata Are Open • Varieties of Wheat (Triticum aestivum) Resistant to Stem Rust Don‟t Open Stomata Until Late in Day • Stomata Regulation Some Species of Fungi and Bacteria Enter via Stomata • Some Plant Species Resistant to Certain Bacterial Pathogens Because of Stomata Structure – Very Narrow Entrance – Broad, Elevated Guard Cells caliban.mpiz-koeln.mpg.de/.../ snapdragon/anatomy/bract.html – Germinated Spores Waiting for Stomata to Open…Dry Out in Heat of Day, Die Other Anatomical Features • Sclerenchyma Bundles in Stems – Sclerenchyma fibers usually occur in bundles. The individual cells are highly elongated with overlapping end walls. They are usually lignified. Lignin makes cell walls extremely strong and inflexible. This makes sclerenchyma a good support tissue. – Sclerenchyma is usually associated with vascular tissues and may completely surround them. • Cell Wall Thickness, Toughness May Sometimes Inhibit Advance of the Pathogen • Xylem, Bundle Sheath and Sclerenchyma Cells of Leaf Veins Effectively Block Spread of Some Fungal, Bacterial and Nematode Pathogens that Cause Angular Leaf Spots – Pathogens Able to Spread Only into Areas Between, but Not Across, Veins SEM photo of a Fiber Bundle www.botany.hawaii.edu/.../CellTissOrgan/ CellTissOrgan-8.htm Angular leaf spot on pumpkin (left) and strawberry (center and right). 2 8/17/2011 Plants Release Inhibitors into Environment Fungitoxic Exudates Inhibitors in Plant Cells • Tomatoes vs. Botrytis • Sugar Beets vs. Cercospora (Fungal Leaf Spot Disease) • Red Onions vs. Onion Smudge (Colletotrichum circinans) www-plb.ucdavis.edu/.../Tomato/ Stems/epidermis1.html • Pathogens Secrete Enzymes to Break down certain Plant Substances such as Pectin – Pectin Is a Gel-Like Polysaccharide, Found in Fruit and Vegetables • Phenolics and Tannins Inhibit Enzymes that Pathogens Use to Break down Pectin – Phenolics Are Reactive Acidic Compounds – Tannins Act As Astringents, Shrinking Tissues and Contracting Structural Proteins • Saponins – Attack (Break down) Fungal Membranes – Checkmate? • Some Pathogens Produce Saponinase Enzymes to Break down the Saponin – Host Range of Fungus Depends on whether the Plant Produces Saponin or whether the Fungus Produces Saponinase • Lectins – Proteins that Bind to certain Sugars and Inhibit Fungal Growth • Enzymes – Break down Pathogen‟s Cell Walls 3 8/17/2011 Lack of Recognition Between Host and Pathogen Recognition Sites or Factors on Surface of Plant Cell Walls • Specific Molecules or Structures on or Embedded in Cell Walls – – – – Oligosaccharides Polysaccharides Proteins Glycoproteins • If Plant Cell Surface Lacks certain Molecules that the Pathogen „Recognizes,‟ the Pathogen may not Recognize the Plant as One of Its Hosts – Pathogen may not Become Attached to the Plant – May not Produce Infection Substances (Enzymes) or Structures (Appressoria, Penetration Pegs, Haustoria) Necessary for Establishment of Infection Lack of Essential Substances for Pathogen Survival • Rhizoctonia Needs a Substance from Plants to Grow Its Hyphal Cushion • In Some Host-Pathogen Combinations, Disease Develops but amount of Disease Is Reduced because certain Host Substances Are Present at lower Concentrations – Bacterial Soft Rot of Potatoes Is less Severe on Potatoes with low Reducing Sugar Content than on Potatoes High in Reducing Sugars To Trigger any Induced Change: Plant Begins to Receive Signal Molecules to Indicate Presence of Pathogen as soon as Pathogen Establishes Physical Contact with Plant 4 8/17/2011 Cytoplasmic Defense Reaction • Cytoplasm Surrounds Hyphae • Cell Nucleus Stretches, Breaks • Dense, Granular Cytoplasm Full of New Particles, Structures • May Stop Invasion of Hyphae of SlowGrowing, Weakly Pathogenic Fungi and the Mycorrhizal Fungi Cell Wall Defense Structures (Limited Effectiveness) • Plant cells are usually enclosed by a more or less rigid cell wall containing cellulose • Structure of cell walls can be compared to structure of reinforced concrete – The scaffolding substance is cellulose – The iron is embedded in an amorphous ground substance • Lignin (a phenolic compound) in wood • Suberin in cork • The cell wall has a number of functions – – – – – Cellulose Lends the cell stability Determines its shape Influences its development Protects the cell against pathogens Counterbalances the osmotic pressure Cell Wall Defense Structures Cellulose = linear chains of glucose residues Very stable chemically and extremely insoluble The primary cell wall consists a glucose polymer of roughly 6,000 glucose units Cellulose-Microfibrils in Isolated Walls of the Green Alga Oozystis solitaria http://www.biologie.unihamburg.de/b-online/e26/3.htm The secondary wall is contains 13,000 to 16,000 glucose units Cellulose chains form crystalline structures called microfibrils (diameter of 20-30 nm which contains about 2,000 molecules) 5 8/17/2011 Three Main Types: Swelling of Wall Outer Layer with Synthesis of New Amorphous Material to Surround and Trap Bacteria Phenolics Embedded in NewlyDeposited Cellulose Callose Sheaths around Hypha • Produced by Cells within Minutes after Wounding and 2-3 Hours after Inoculation by Microbes • Main Function of Callose Papillae Seems to Be Repair of Cellular Damage, Sometimes They Prevent Pathogen from Penetrating the Cell further • Sometimes Callose later Becomes Infused with Phenolic Substances Other Anatomical Defense Structures Cork Layers Abscission Layers 6 8/17/2011 Tyloses Gum Deposition in Anatomical and Physiological Responses of Bark Tissues to Mechanical Injury Alan R. Biggs, West Virginia University Xylem “…Between 8 and 12 days after wounding, the primary walls and the middle lamella in the boundary zone exhibited an increase in electron density. This is due probably to the deposition of phenolic polysaccharide material in the wall. In peach, these substances, usually referred to as gum, are produced nonspecifically in response to wounds or infections…” Vessels Hypersensitive Response • Biochemical Response with Visible Cellular Responses – aka Necrotic Defense Reaction • In Fungal Invasions, Cell Suicide Occurs • In Bacterial Invasions, Cell Membranes Destroyed, Followed By Desiccation and Necrosis of Invaded Leaf Tissues Agrios text, Fig. 5.9. Cowpea (Vigna unguiculata) Hypersensitive Response to Tobacco Ringspot Virus. Agrios text, Fig. 5.8. Stages in development of necrotic defense reaction in a cell of a very resistant potato variety infected by Phytophthora infestans (Late Blight of Potato). Hypersensitive Response (Not always Visible) 7 8/17/2011 Active Oxygen Radicals • Upon Attack, Get Rapid and Transient Generation of Superoxide (O2-), Hydrogen Peroxide (H2O2), Hydroxyl Radical (•OH) • Destroy Plant Cell Membranes • May Destroy Pathogen Membranes Reinforce Host Cell Walls with Strengthening Molecules Phenolics: Suberin and Lignin Callose Glycoproteins Mineral Elements: Silicon and Calcium • Part of Hypersensitive Response • May Oxidize Phenolics = Make them Active or more Toxic Produce Proteins Toxic to Invading Pathogens • Inhibit Spore Release, Germination • Break Down of some Fungi and Bacteria Cell Walls Detoxify Pathogen Toxins Phytoalexins (Substances Toxic to Microbes) Phenolics 8