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How do pathogens attack plants?

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Introduction

Clare Johnson Queen's College 24th January 2004 How do pathogens attack plants? Pathogens have evolved the ability to exploit substances produced by plants and have therefore also developed ways to attack plants in order to utilise these substances. However, plants have many structral and biochemical defense mechanisms which the pathogen must breach. In order for a pathogen to successfully infect a plant it must firstly arrive at the plant, attach to the plant, penetrate, obtain the plant's nutrients, establish and disperse while at the same time neutralising the defense actions of the plant. These activities are achieved by mechanical force or by secretions of chemicals that affect the metabolic host processes. Plant pathogens have one of three modes of nutrition which determine the way the pathogen attacks the plant. Necrotrophs such as Botrytis cinera attack with the intention of killing the host. They are highly opportunistic, have a wide host range, release many enzymes and feed as they grow. Biotrophs do not want to kill their host as they require a living host to complete their life cycle. Enzyme secretion is much lower and more controlled and they only begin to feed once inside the host. Examples include wheat stem rust and powdery mildew fungus. Hemibiotrophic nutrition is where pathogens begin being biotrophic but as they begin to run out of food they adopt nectrophy. ...read more.

Middle

0.5?m is found to be the height of the guard cells that surround the stomata of the fungus' host plant Phaseolus vulgaris. Pathogen attack on plants is achieved by mechanical force, chemical activity or by a combination of both. Only some fungi, nematodes and parasitic plants use mechanical force. In the case of parasitic plants and fungi, adhesion to the plant is achieved firstly by the formation of intermolecular forces between the plant and pathogen. Sometimes, spores, which land on moist surfaces, adhere by the formation of an adhesion pad that is helped by enzymatic action. In addition, adhesive substances which pre exist on the spore will allow adhesion once hydrated. Once an appropriate site is found, adhesion is followed by the radical or fungal hyphal tip increasing in diameter to form a flattened, bulb-like structure known as an appressorium that increases the degree of adhesion. A penetration peg then extends from the appressorium, and penetrates the cuticle and cell wall. Once the penetration peg has passed through the cuticle it increases in diameter. The best example of a pathogen that forms appressoria is the rice blast fungus, Magnaporthe grisea. The M. grisea appressoria forms at the end of germ tubes in response to a hard, hydrophobic surface, requires water and responds to chemical signals of waxes and lipids from the plants surface. ...read more.

Conclusion

- specific toxins Toxin Organism Disease Mechanism of action and symptoms Victorin Fungus - Cochliobolus victoriae Leaf blight Toxin is produced and carried to the leaves and kills entire plant T toxin Cochliobolus heterostrophus Southern corn leaf blight Acts on mitochondria of susceptible cells and inhibits ATP sythesis HC toxin Cochliobolus carbonum Leaf spot disease of maize Mechanism not known Growth regulators are plant hormones and include auxins, gibberellins, cytokinins and ethylene. They work in small concentrations, are usually synthesised at a distance from the site of action and promote the syntheses of m-RNA molecules which leads to the formation of specific enzymes. Plant pathogens can produce more of the same growth regulators, inhibitors of the growth regulators or new growth regulators which causes imbalances in the hormonal system of the plant. Growth Regulator Organism Disease Mechanism of action Auxin Pseudomonas solanacearum Bacterial wilt of solanaceous plants Increases plasticity of cell wall and inhibits ligninfication Gibberellins Gibberella fujkuroi Foolish seedling disease of rice Rapid growth Cytokinin Rhodococcus fascians Crown galls Prevents genes from being switched off and activating previously switched off genes Ethylene Pseudomonas solanacearum Yellowing of fruit Chlorsis, leaf abscission, increased cell membrane permeability Polysaccarides are of the least important but are significant in wilt diseases of plants. There are also thought to be suppressors that suppress the defense mechanisms of the plant. In conclusion, pathogens attack on plants is achieved through many mechanisms that vary depending on the pathogen and plant. ...read more.

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