16.2F: Plant Pathogens
- Break down the types and modes of plant pathogenicity
Most bacteria that are associated with plants are actually saprophytic, and do no harm to the plant itself. However, a small number, around 100 species, are able to cause disease. Bacterial diseases are much more prevalent in sub-tropical and tropical regions of the world. Most plant pathogenic bacteria are rod shaped (bacilli). In order to be able to colonise the plant they have specific pathogenicity factors. There are 4 main bacterial pathogenicity factors:
- Cell wall -degrading enzymes: These are used to break down the plant cell wall in order to release the nutrients inside.
- Toxins: These can be non- host -specific, which damage all plants, or host-specific, which cause damage only on a host plant.
- Phytohormones: example Agrobacterium changes the level of Auxin to cause tumours.
- Effector proteins: These can be secreted into the extracellular environment or directly into the host cell, often via the Type three secretion system. Some effectors are known to suppress host defense processes. This can include: reducing the plants internal signaling mechanisms or reduction of phytochemicals production. Bacteria, fungus and oomycetes are known for this function.
Significant bacterial plant pathogens:
- Burkholderia
- Proteobacteria
- Xanthomonas spp.
- Pseudomonas spp.
- Pseudomonas syringae pv. tomato causes tomato plants to produce less fruit, and it “continues to adapt to the tomato by minimizing its recognition by the tomato immune system. “
The majority of phytopathogenic fungi belong to the Ascomycetes and the Basidiomycetes. The fungi reproduce both sexually and asexually via the production of spores and other structures. Spores may be spread long distances by air or water, or they may be soil borne. Many soil inhabiting fungi are capable of living saprotrophically, carrying out the part of their lifecycle in the soil. These are known as facultative saprotrophs. Fungal diseases may be controlled through the use of fungicides and other agriculture practices, however new races of fungi often evolve that are resistant to various fungicides. · Biotrophic fungal pathogens colonize living plant tissue and obtain nutrients from living host cells. Necrotrophic fungal pathogens infect and kill host tissue and extract nutrients from the dead host cells.
Significant fungal plant pathogens include:
- Fusarium spp. (causal agents of Fusarium wilt disease)
- Thielaviopsis spp. (causal agents of: canker rot, black root rot, Thielaviopsis root rot)
- Verticillium spp.
- Magnaporthe grisea (causal agent of blast of rice and gray leaf spot in turfgrasse
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Key Points
- The majority of phytopathogenic fungi belong to the Ascomycetes and the Basidiomycetes.
- Many soil inhabiting fungi are capable of living saprotrophically, carrying out the part of their lifecycle in the soil.
- Bacterial plant pathogens are much more prevalent in sub-tropical and tropical regions of the world.
Key Terms
- Type three secretion system : Type three secretion system (often written Type III secretion system and abbreviated TTSS or T3SS, also called Injectisome or Injectosome) is a protein appendage found in several Gram-negative bacteria. In pathogenic bacteria, the needle-like structure is used as a sensory probe to detect the presence of eukaryotic organisms and secrete proteins that help the bacteria infect them. The proteins are secreted directly from the bacterial cell into the eukaryotic cell, also known as “the host” cell.