Skip Navigation

INFECTION PROCESS: PLANT DEFENSES: PASSIVE DEFENSES

Protection from a pathogen's initial invasion is achieved via passive defences, such as physical and/or chemical barriers.

Physical barriers largely involve properties of the plant surface, that is, the cuticle, stomata and cell walls. Pathogens produce a range of cutin-degrading enzymes, which are often crucial to the successful penetration of the plant tissue. The thickness of the cuticle, the presence of secondary cell wall, and the size of stomatal pores can all affect the success with which a pathogen invades a host. Some plants invest in very thick walls and/or cuticles, and bark (where present) can also provide a physical impediment to infection. The vertical orientation of leaves can also add to plant resistance, by preventing the formation of moisture films of the leaf surfaces, inhibiting infection by pathogens reliant on water for motility.

Chemical barriers include compounds, such as "phytoanticipins", that have antimicrobial activity and compounds that affect the vectors of plant viruses. Phenols and quinones are two classes of antimicrobial compounds produced by some plants  Inhibiting compounds may be excreted into the external environment, accumulate in dead cells or be sequestered into vacuoles in an inactive form. The young fruit of numerous plants (e.g. mangoes, avocado) contain antifungal or antimicrobial compounds that are gradually metabolised during fruit ripening, making unripe fruit less susceptible to disease than ripe fruit. Lactones, cyanogenic glucosides, saponins, terpenoids, stilbenes and tannins are also plant-produced compounds associated with pathogen resistance. Saponins are a class of phytoanticipins that destroy membrane integrity in saponin-sensitive parasites, and which are stored in an inactive form in the vacuoles of the plant cell, becoming active when hydrolase enzymes are released following wounding or infection. Some pathogens are able to release enzymes that detoxify plant saponins, making them insensitive to this line of defence. Conversely, resistance of some plants to specific pathogens is the result of an insensitivity to pathogen-produced host specific toxins. Resistance genes may encode an enzyme that converts the toxin into a non-toxic derivative or the absence of a receptor to the toxin. Another group of defensive compounds are the plant defensins, which interfere with pathogen nutrition and retard their development. Secreted defensins can create an antimicrobial microenvironment for germinating seeds and accumulated defensins can provide defence against insect-transmitted viruses in flowers, leaves and tubers. There are also proteins, both constitutive and induced that play a role in plant defence.

Copyright © University of Sydney. Last updated December, 2003. Site construction and maintenance: eResources Unit. Email us here with your comments and feedback.