Plant Pathogenic Rust Fungi

Host barrier interations

Spores which land on leaves do not necessarily infect them instantly, as there are many barriers that the spore must overcome. For a start, the physical properties of the plant make it difficult for the fungus to take hold. Virtually all plants have evolved some kind of defence mechanism against pathogenic fungi. These include:

1. Felt-like surface hairs on leaf can catch the spore and prevent it from making contact with leaf in the first place. As the spore is unable to touch the leaf surface, penetration of the leaf is made impossible.

2. Very smooth leaves can have a resistance effect on fungi, because the smoothness of the leaves allows rain and wind to flush spores off the leaf surface more easily. This means that the rust may not be able to form it’s penetration apparatus quickly enough before it is removed from the plant

3. Prevent invader from becoming active i.e. prevent spore germination. Water is necessary for spore germination, so a highly hydrophobic leaf cuticle can be useful defense against fungi of any kind. Eg. spines.

4. Prevent mechanical penetration. The cuticle is the first physical barrier if no leaf hairs are present. It is comprised of layers of cellulose fibrils, wax and cutin. There are also tough materials in the plant cell wall, such as cellulose, lignin or suberin. The penetration peg of the fungus often cannot physically break through the wall of the cell and thus cannot invade the plant.

5. Lignituber formation. If plant cell is penetrated, the plant can quickly thicken the wall of the cell in response to the present foreign body. This has the potential to reseal the cell before the fungal hyphae are able to take hold.

6. Preformed defence compounds. Hyphae can be attacked by preformed fungicidal compounds or induced phytoalexins in the plant.

7. Hypersensitivity Reaction. Plant cells can be programmed to die upon infection by fungal hyphae. This autolysis reaction happens on first contact with the fungal hyphae and leaves the fungus with only dead cells with few nutrients to take up. Dead plant material containing the fungus can fall from the plant, removing the threat of fungal infection.

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