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Module 7: Infectious Disease
Quick questions on Plant responses to pathogens, physical and chemical defences: HSC Biology Module 7
15short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is physical defences (passive and structural)?Show answer
Cuticle and bark. The outer surfaces of leaves and stems are covered by a waxy cuticle (cutin and waxes) that resists water loss and pathogen entry. Woody stems have lignified bark, a tough physical barrier that few pathogens can penetrate.
What is induced physical defences?Show answer
Callose deposition. When a pathogen attempts to enter, the plant deposits callose (beta-1,3-glucan) into the cell wall at the site of attack, forming a localised plug.
What is chemical defences?Show answer
Phytoalexins. Small antimicrobial molecules (often terpenes, alkaloids or phenolics) synthesised in response to infection. Examples include camalexin in Arabidopsis and the terpene-based oils in Eucalyptus species.
What is the hypersensitive response?Show answer
The most dramatic plant defence. On detecting pathogen effector proteins, infected cells trigger programmed cell death, killing themselves and the pathogen at the infection site. The result is a small lesion of dead tissue that isolates the pathogen.
What is cuticle and bark?Show answer
The outer surfaces of leaves and stems are covered by a waxy cuticle (cutin and waxes) that resists water loss and pathogen entry. Woody stems have lignified bark, a tough physical barrier that few pathogens can penetrate.
What is cell walls?Show answer
Each plant cell is enclosed in a rigid cellulose cell wall. Pathogens must produce wall-degrading enzymes (cellulases, pectinases) to enter.
What is trichomes and thorns?Show answer
Hair-like trichomes and physical spines deter macroparasites and reduce pathogen contact.
What is stomatal closure?Show answer
Stomata are the main entry point for airborne pathogens. Guard cells detect pathogen-associated molecular patterns (PAMPs) such as flagellin and close the stomatal pore.
What is callose deposition?Show answer
When a pathogen attempts to enter, the plant deposits callose (beta-1,3-glucan) into the cell wall at the site of attack, forming a localised plug.
What is tylose formation?Show answer
In xylem vessels, neighbouring cells extrude into the vessel lumen, forming tyloses that block fungal spread through the vascular system.
What is phytoalexins?Show answer
Small antimicrobial molecules (often terpenes, alkaloids or phenolics) synthesised in response to infection. Examples include camalexin in Arabidopsis and the terpene-based oils in Eucalyptus species.
What is reactive oxygen species?Show answer
Plants produce hydrogen peroxide and superoxide at the infection site, damaging pathogen membranes and triggering further defence signalling.
What is defensive enzymes?Show answer
Plants produce chitinases (degrade fungal cell walls), glucanases and protease inhibitors that disable pathogen enzymes.
What is pre-formed antimicrobials?Show answer
Many plants store compounds in vacuoles or specialised cells that are released on wounding. Eucalyptus essential oils (cineole, pinene) and tea tree oil (terpinen-4-ol) are antimicrobial constituents of native Australian plants.
What is pathogen?Show answer
P. cinnamomi is an oomycete (water mould). It produces motile zoospores in moist soil that swim toward root exudates and infect fine roots.