i) Histological defense structures :
a) Cork layer:
- Infection by fungi, bacteria, some viruses and nematodes induce plants to form several layers of cork cells.
- It then blocks the spread of toxin substances secreted by the pathogen.
- Furthermore, cork layers stop the flow of nutrients and water from the healthy to the infected area and deprive the pathogen of nourishment.
- Ex: Potato tubers infected by Rhizoctonia; Prunus domestica leaves attacked by Coccomyces pruniphorae.
Fig : Cork layer
b) Abscission layers :
- An abscission layer consists of a gap formed between infected and healthy cells of a leaf surrounding the locus of infection due to the disintegration of the middle lamella of parenchymatous tissue.
- Gradually, infected area shrivels, dies, and sloughs off, carrying with it the pathogen.
- Ex: Xanthomonas pruni, and Closterosporium carpophylum on peach leaves
Fig : Abscission layer.
c) Tyloses :
- Tyloses are the overgrowths of the protoplast of adjacent living parenchymatous cells, which protrude into xylem vessels through pits.
- In susceptible varieties, few or no tyloses are formed ahead of pathogen invasion.
- Ex: Tyloses form in xylem vessels of most plants under invasion by most of the vascular wilt pathogens.
Fig : Tyloses
ii) Cellular defense structures:
a) Hyphal sheathing:
- The hyphae penetrating the cell wall and growing into the cell lumen are enveloped by a cellulosic sheath (callose) formed by extension of cell wall, which become infused with phenolic substances and prevents further spread of the pathogen.
- Ex: Hyphal sheathing is observed in flax infected with Fusarium oxysporum f.sp. Lini.
Fig : Hyphal sheathing
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