Quick questions on Plant responses to pathogens, physical and chemical defences: HSC Biology Module 7

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.

Each plant cell is enclosed in a rigid cellulose cell wall. Pathogens must produce wall-degrading enzymes (cellulases, pectinases) to enter.

Hair-like trichomes and physical spines deter macroparasites and reduce pathogen contact.

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.

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.

In xylem vessels, neighbouring cells extrude into the vessel lumen, forming tyloses that block fungal spread through the vascular system.

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.

Plants produce hydrogen peroxide and superoxide at the infection site, damaging pathogen membranes and triggering further defence signalling.

Plants produce chitinases (degrade fungal cell walls), glucanases and protease inhibitors that disable pathogen enzymes.

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.

P. cinnamomi is an oomycete (water mould). It produces motile zoospores in moist soil that swim toward root exudates and infect fine roots.

Mature Eucalyptus camaldulensis (river red gum) trees along the Murray-Darling produce essential oils dominated by 1,8-cineole (eucalyptol) and alpha-pinene, stored in pellucid oil glands visible in fresh leaves. These terpenes are constitutive chemical defences with broad antimicrobial activity against fungi such as Armillaria luteobubalina, a wood-decay basidiomycete common after fire. When fungal hyphae penetrate the bark, they encounter terpenes that disrupt fungal cell membranes.

Wheat (Triticum aestivum) infected by Puccinia graminis stem rust triggers a localised induced response: within hours of fungal penetration, plant cells around the infection site deposit callose (beta-1,3-glucan) at the penetration peg, physically blocking further hyphal entry. If the plant carries a resistance gene matching the fungal effector, it also mounts the hypersensitive response - rapid death of the infected and surrounding cells - depriving the biotrophic fungus of living host tissue. This illustrates how induced physical (callose) and the hypersensitive response work together; for a native Australian named pair in the exam, keep to jarrah and P.

Jarrah (Eucalyptus marginata) and Phytophthora cinnamomi is the safest native Australian named example. Drop it in early - the application marks depend on it.

"Outline" wants brief points; "describe" wants the steps; "compare" wants the same features for both defence types; "assess" or "evaluate" wants a justified judgement weighing strengths and limitations (e.g. why jarrah's defences still fail against P. cinnamomi).