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Module 7: Infectious Disease
Quick questions on Plant responses to pathogens, physical and chemical defences: HSC Biology Module 7
14short 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 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 are 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 are 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 are 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 are defensive enzymes?Show answer
Plants produce chitinases (degrade fungal cell walls), glucanases and protease inhibitors that disable pathogen enzymes.
What are 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.
What is q1?Show answer
Identify two physical defences and two chemical defences used by an Australian eucalypt against a pathogen, naming the pathogen. [4 marks]
What is q2?Show answer
A study measures callose deposition in barley leaves 6, 12 and 24 hours after powdery mildew inoculation, finding 5, 38 and 62 callose papillae per mm squared respectively. Describe the trend and explain its role in resistance. [3 marks]
What is q3?Show answer
Compare physical and chemical defences in Eucalyptus marginata against Phytophthora cinnamomi. (a) Identify one physical defence. (b) Identify one chemical defence.