Lecture 2.6: Biotic Stress and Plant Defence against Pathogens Study Guide

Classification of Plant Defence Mechanisms

• Passive Defences (Constitutive): These are pre-existing barriers that the plant possesses regardless of the presence of a pathogen. They are subdivided into physical and chemical categories.

  • Physical Barriers:

    • Wax cuticle.

    • Cell wall.

    • Stomata (shape and timing of opening).

    • Lenticels.

  • Chemical Barriers:

    • Nutrient deprivation.

    • $pH$ levels.

    • Phytoanticipins.

    • Plant defensins.

• Active Defences (Induced): These are responses triggered upon the recognition of a pathogen. They are categorized based on the speed of the response.

  • Rapid Responses:

    • Changes in membrane function.

    • Oxidative burst (generation of reactive oxygen species).

    • Cell wall reinforcement (e.g., papillae formation).

    • Hypersensitive cell death (HR).

  • Delayed Responses:

    • Phytoalexin accumulation.

    • RNA silencing (specifically against viruses).

    • Pathogenesis-related (PR) proteins.

    • Systemic Acquired Resistance (SAR).

    • Pathogen containment.

Types of Passive Defence: Avoidance

Avoidance involves multiple heterogeneous mechanisms often dependent on the physical structure of the plant as a whole or its specific tissues.

• Flowering Structure (Hordeum vulgare): Barley cultivars with flowers that remain closed are resistant to loose smut (Ustilago nuda) because the pathogen cannot access the reproductive organs.

• Plant Height (Triticum aestivum): Tall wheat cultivars exhibit resistance because their height impedes the physical spread of glume blotch (Septoria nodorum) to the upper, crucial parts of the plant.

• Morphology of Emerging Parts (Saccharum officinale): Sugarcane resistance to smut (U. scitaminea) depends on the specific morphology of the emerging ratoon buds.

Passive Physical Barriers

Physical barriers serve as the first line of defense, preventing the initial entry or establishment of a pathogen.

• Cuticle Thickness and Development:

  • In rubber trees (Hevea brasiliensis), clone LCB870 is resistant to the mildew Oidium heveae because the period during which the young leaf has a thin (susceptible) cuticle is reduced.

  • LCB870 reaches normal cuticle thickness in $10 - 11\,\text{days}$, which is $7\,\text{days}$ faster than other clones.

• Enzymatic Degradation:

  • Many pathogens produce cutin-degrading enzymes to bypass the cuticle and access the cellulose beneath.

  • The activity of cutinolytic enzymes is directly correlated to the aggressiveness of Fusarium solani pv pisi on pea stems.

• Surface Morphology:

  • Waxy cuticles and vertical surfaces prevent the formation of a water film.

  • Water films are essential for spore germination and the motility of certain pathogens.

  • Note: While vertical surfaces prevent water films, they are more prone to spore impaction.

• Stomatal Defence:

  • Stomata of the "wrong" size or shape, or those that close during the time of day when spores typically germinate, can exclude pathogens.

  • Example: Phytophthora palmivora enters cocoa pods via stomata. Cultivars with smaller and fewer stomata produce fewer black pod lesions.

Passive Chemical Barriers: Phytoanticipins and Saponins

Chemical barriers involve exudates on the surface or compounds within cells that inhibit pathogen development.

• Host Secretions as Triggers: Interestingly, some resting spores and nematode eggs actually require the secretion of specific compounds from the host before they will germinate, representing a specialized interaction.

• Phytoanticipins:

  • These are compounds produced during normal growth (pre-formed) that inhibit pathogen growth.

  • They may be excreted into the rhizosphere (around roots) or phyllosphere (around leaves), or stored in vacuoles in an inactive form, or accumulate in dead cells.

  • Case Study (Onions):

    • Dead cells of brown onion skins contain quinones which inhibit the germination of smudge pathogen (Colletotrichum circinans) and neck rot pathogen (Botrytis cinerea).

    • White onions do not produce these quinones and are susceptible.

    • Aspergillus niger is insensitive to these compounds and can attack both brown and white onions.

• Saponins:

  • Plant glycosides (plus terpenes) with surfactant properties.

  • They bind to sterols in pathogen cell membranes, destroying membrane integrity and function.

  • They are toxic to plants and fungi but notably not to Oomycetes.

  • Activation: Inactive precursors are stored in vacuoles; hydrolases are released upon wounding or infection to convert them into antimicrobial forms.

  • Host range may be determined by the ability of a fungus to detoxify specific saponins.

Plant Defensins (Inhibitors)

Plant defensins are inhibitors (lectins or peptides) that act as proteinase, polygalacturonase, or ribosome inhibitors.

• Mechanism: They interfere with pathogen nutrition and development.

• Distribution:

  • In ungerminated radish seeds, they constitute $0.5\%$ of proteins.

  • In germinating radish seeds, they constitute $30\%$ of the protein excreted, creating an antimicrobial environment around the emerging radicle.

  • They make up approximately $\leq 10\%$ of proteins in cereal, legume, and Solanaceae seeds.

  • They are also found in the outer cell layers of flowers, leaves, and tubers.

  • Some are constitutive (accumulate during normal development), while others are induced/enhanced by wounding.

Pathogen Recognition and Non-Specific Elicitors

• Non-Specific Elicitors: These signals induce defence responses across a wide range of cultivars and host species.

  • Abiotic Elicitors: Heavy metals, UV light, and metabolic inhibitors. These precipitate a physiological stress response that sometimes contributes to resistance.

  • Biotic Elicitors: Compounds released from fungi and bacteria.

  • Characteristics: The effect is typically transitory and non-specific. The magnitude of the response depends on the amount of elicitor present.

  • Contra-indication: Environmental stresses often increase susceptibility to necrotrophic pathogens.

• Glucan Elicitors: Hepta-$\beta$-glucan from Phytophthora megasperma pv glycinea is a potent elicitor in soybean, recognized by a receptor in the plasma membrane.

• Endogenous Fragments: Hydrolytic enzymes (from either the plant or the pathogen) release plant cell wall fragments that act as elicitors.

• Practical Application: Some fungicide formulations include elicitors of host defences alongside the chemical fungicide.

Suppressors and Compatibility Factors

Biotrophs must establish compatibility with their hosts to survive.

• Compatibility Factors: Virulent races may produce signals that avoid, delay, or negate host recognition ("under the radar").

• Co-inoculation Evidence: In some cases, co-inoculating a host with a compatible (virulent) strain and an incompatible (avirulent) strain allows the avirulent strain to infect. This suggests the virulent strain actively suppresses host resistance mechanisms.

• Timing: If a virulent strain is inoculated several hours after an avirulent strain, the host remains resistant to both. This indicates that suppressors cannot "switch off" resistance once it has already been activated.

Rapid Active Defences: Membrane Change and Oxidative Burst

Active responses are complex, with no universal model; the sequence and magnitude vary by pathogen.

• Changes in Membrane Function:

  • Membranes are central to pathogen recognition and signal transduction.

  • Initial changes include loss of cellular electrolytes ($K^+$) and the uptake of ($H^+$).

  • The influx of calcium ($Ca^{++}$) acts as an intracellular signal for enzyme activation and gene expression.

• Oxidative Burst:

  • A rapid increase in respiration leads to the generation of reactive oxygen species (ROS), mainly hydrogen peroxide ($H_2O_2$) and superoxide ($O_2^-$).

  • Plant cells are usually protected by antioxidants like superoxide dismutase, peroxidases, catalase, glutathione reductase, and carotenes.

• Function of the Burst:

  • A small burst occurs upon wounding.

  • A larger burst often precedes the hypersensitive response, causing lipid peroxidation and cell death.

  • ROS levels may be high enough to directly kill the pathogen.

  • Associated with cross-linking of the cell wall and triggering both local and systemic defensive gene expression.

  • Note: Some necrotrophic pathogens exploit the oxidative burst to kill host cells before invading.

Active Cell Wall Reinforcement

• Cytoplasmic Accumulation: The first visible response is often intensified cytoplasmic streaming and the accumulation of cytoplasm under the site of attempted penetration. These aggregates contain the machinery for cell wall fortification.

• Papilla:

  • Deposits of branched $\beta\text{-}1,3$ glucan, callose, silicon, lignin, and proteins located between the host cell wall and the plasma membrane, directly beneath the penetration peg.

  • In cereals, resistant cultivars form papillae more rapidly.

• Lignitubers: Lignified callose deposits that enmesh and ensheath the invading hyphal tips.

• Hydroxyproline-rich Glycoproteins (HRGPs):

  • Structural proteins involved in organizing secondary thickening.

  • Genes encoding HRGPs are transcribed in advance of invading hyphae.

  • $H_2O_2$ from the oxidative burst causes extensive cross-linking of HRGPs with other wall components, making the wall resistant to microbial degradation.

  • Pathogen inhibition of lignin or callose biosynthesis increases their penetration efficiency.

Hypersensitive Cell Death (HR)

• Definition: A necrotic response where host cells die rapidly in the presence of a pathogen.

• Effect on Pathogen: HR can be lethal (e.g., against Rhizoctonia solani) or fungistatic (e.g., against Puccinia graminis).

• Process: Typically occurs within $12 - 24\,\text{hr}$ of attempted penetration. It involves an oxidative burst, granulation, membrane disruption, cellular decompartmentalization, and browning.

• Relation to Apoptosis: HR shares many features with programed cell death (apoptosis) in animals, suggesting it is an ancient defense mechanism.

Phytoalexins

• Definition: Low molecular weight antibiotics produced by plants specifically in response to infection.

• Chemistry: They are diverse and include isoflavonoids and sesquiterpenes.

• Properties: They are non-selective in toxicity and possess an affinity for lipids, allowing them to accumulate in cell membranes.

• Efficacy: They must accumulate to inhibitory levels at the infection court to restrict the pathogen.

• Prevalence: Over $350$ phytoalexins have been identified across more than $100$ species from $30$ families of monocots and dicots.

Genetics of Host-Parasite Relationships and Sources of Resistance

• Inheritance: Resistance and pathogenicity follow normal inheritance patterns (governed by $1$ to many genes, with dominant and recessive alleles).

• Gene-for-Gene Basis: In many systems, a specific resistance gene in the host operates in direct correspondence with a specific gene in the pathogen.

• Breeding for Resistance:

  • Germplasm: The first step is identifying germplasm with high resistance.

  • Examples of Major Genes for Resistance:

    • Dwarf beans: The dominant $I$ gene provides resistance to Bean common mosaic virus, halo-blight, and anthracnose.

    • Lettuce: The Gallega gene provides resistance to Lettuce mosaic virus and downy mildew.

  • Absence of Resistance: For some pairs, such as White clover mosaic virus in Trifolium repens, no resistance is known. Breeders must search wild relatives, which can lengthen the program.

Review Questions

• What’s the difference between passive and active defence?

• What’s the role of reactive oxygen species in biotic stress responses?

• How does all this tie in with the Optimal Defence Hypothesis?

Audience Interactions and Conduct

• Lecture Etiquette PSA: A student provided a stern reminder to the class regarding behavior during lectures:

  • Talking: Students are encouraged to stop talking/whispering during the $50\text{-minute}$ lecture as it is audible to others and distracting for those trying to focus.

  • Lateness: Late arrivals should enter through the back if possible and sit in the seat closest to the door.

  • End of Lecture: Students should avoid packing up their belongings until the lecturer has finished speaking, as it is considered rude.

  • Social Warning: The individual mentioned they had considered violence but settled for writing the note, urging peers to stay home and listen to the recording if they cannot remain quiet.

• Event Note: Pint Night has migrated to Auahi Ora (last mention).