How do pathogens attack plants?

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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. The rice blast fungus is an example.

Pathogen attack begins with the pathogen arriving at the plant, which can occur in a number of ways including wind, water, vectors and diseased stock. The force of the wind can blow spores such as rust spores to a plant; water can transport bacteria which can arise through the incorrect management of irrigation systems, vectors such as bumble bees can carry the fire blight disease and diseased stock will cause other stock to become diseased. With pathogens in the soil, plant roots may grow so that they arrive at the pathogen instead of the other way around. In addition chemotaxis which is where pathogens are attracted to plants due to chemicals produced by the plant can also help the pathogen to arrive at the plant. This is particularly associated with pathogens that have motile stages, which means that they can move towards the plant in response to stimuli. For example, border cells that originate from the root cap meristem have been shown to affect bacterial and fungal plant pathogens; Acetetosyringone, which is a phenolic compound secreted from wounded plants such as tobacco chemo-attracts Agrobacterium tumefaciens bacteria to attack them which causes crown gall.

Once the pathogen has arrived at the plant it must attach to it so that it is not washed or blown off. This can occur either passively for example by becoming trapped in the surface hairs of the plant or actively by the pathogen secreting an extra-cellular matrix and sticking to the plant. Adhesion to the plant may also be necessary for germination of the pathogen. As the majority of substances that the pathogen requires are in the protoplast of the plant cell, pathogens need to enter the plant. They can either enter through natural openings, wounds or by using enzymatic and/ or mechanical force to penetrate the host's barriers which there are, unfortunately for the pathogen, many.
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Plants have a number of natural openings that mean that the plant's exterior barriers do not have to be breached. These include stomata, nectaries, hydathodes and lenticels. Stomata and nectaries are utilised by fungi and bacteria especially when they arrive by wind or rain. This is illustrated by the bacterial pathogen that causes wide fire of tobacco entering via the stomata and the pathogen that causes Fire Blight that commonly enters through nectaries. Hydathodes are structures that have guttering. When there is high humidity, droplets are exuded which can become contaminated by pathogens. Wounding is generally needed by ...

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