plant responses to herbivory Flashcards
physical defences
common physical defences include thorns, barbs, spikes, spiny leaves, fibrous and inedible tissue, hairy leaves, and even stings to protect themselves and discourage herbivores from eating them.
chemical defences
Plants have also evolved a wide range of chemical responses to herbivory
- the stinging nettle manages to include both physical and chemical defences in its vicious trichomes (stinging hairs), but many other plants produce a cocktail of unpleasant chemicals too. These include:
-Tannins - part of a group of compounds called phenols produced by many plants. Tannins can make up to 50% of the dry weight of the leaves. They have a very bitter taste which puts animals off eating the leaves. They are toxic to insects - they bind to the digestive enzymes produced in the saliva and inactivate them. Tea and red wine are both rich in plant tannins.
-Alkaloids - a large group of very bitter tasting, nitrogenous compounds found in many plants. Many of them act as drugs, affecting the metabolism of animals that take them in and sometimes poisoning them. Alkaloids include caffeine, nicotine, morphine, and cocaine. Caffeine is toxic to fungi and insects, and the caffeine produced by coffee bush seedlings spreads through the soil and prevents the germination of the seeds of other plants - so caffeine protects the plant both against herbivores and against plant rivals. Nicotine is a toxin produced in the roots of tobacco plants, transported to the leaves and stored in vacuoles to be released when the leaf is eaten.
• Terpenoids - a large group of compounds produced by plants which often form essential oils but also often act as toxins to insects and fungi that might attack the plant. Pyrethrin, produced by chrysanthemums, acts as an insect neurotoxin, interfering with the nervous system. Some terpenoids act as insect repellents, for example, citronella produced by lemon grass repels insects.
pheromones
A pheromone is a chemical made by an organism which affects the social behaviour of other members of the same species. Because plants do not behave socially, they do not rely a lot on pheromones. there are a few instances where they could be regarded as using pheromones to defend themselves:
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If a maple tree is attacked by insects, it releases a pheromone which is absorbed by leaves on other branches. These leaves then make chemicals such as callose to help protect them if they are attacked.
Scientists have observed that leaves on the branches of nearby trees also prepare for attack in response to these chemical signals.
• There is some evidence that plants communicate by chemicals produced in the root systems and one plant can ‘tell’ a neighbour if it is under water stress.
However, plants do produce chemicals called volatile organic compounds (vOCs) which act rather like pheromones between themselves and other organisms, particularly insects. They diffuse through the air in and around the plant. Plants use these chemical signals to defend themselves in some amazing ways. They are usually only made when the plant detects attack by an insect pest through chemicals in the saliva of the insect. This may elicit gene switching. For example:
• When cabbages are attacked by the caterpillars of the cabbage white butterfly, they produce a chemical signal which attracts the parasitic wasp Cotesia glomerata. This insect lays its eggs in the caterpillars which are then eaten alive, protecting the plant. The signal from the plant also deters any other female cabbage white butterflies from laying their eggs. Scientists estimate up to 90% of cabbage white caterpillars are affected by the parasite. If the cabbage is attacked by the mealy cabbage greenfly, it sends out a different signal which attracts the parasitic wasp Diaretiella rapae which only attacks greenfly.
-When apple trees are attacked by spider mites, they produce VOCs which attract predatory mites that come and destroy the apple tree pests.
• Some types of wheat seedling produce VOCs when they have been attacked by aphids and these repel other aphids from the plant.
Sometimes a VOC produced by a plant that has been attacked will not only attract predators of the pest organism - it may also act as a
‘pheromone’ so that neighbouring plants begin to produce the VOC before they are actually attacked.
folding in response to touch
Most plants, with the exception of a few insectivorous plants such as the Venus fly trap, move so slowly you cannot follow the movement with the naked eye. It is revealed over hours or days, or by time-lapse photography. There are, however, some exceptions - the sensitive plant Mimosa pudica is one of a small number of plants which move at a speed you can see. This plant uses conventional defences against herbivores - it contains a toxic alkaloid and the stem has sharp prickles, but if the leaves are touched, they fold down and collapse. Scientists think this frightens off larger herbivores, and dislodges small insects which have landed on the leaves. The leaf falls in a few seconds, and recovers over 10-12 minutes as a result of potassium ion movement into specific cells, followed by osmotic water movement. The exact causes of the dramatic change in the leaves are still being researched.
folding in response to touch
Most plants, with the exception of a few insectivorous plants such as the Venus fly trap, move so slowly you cannot follow the movement with the naked eye. It is revealed over hours or days, or by time-lapse photography. There are, however, some exceptions - the sensitive plant Mimosa pudica is one of a small number of plants which move at a speed you can see. This plant uses conventional defences against herbivores - it contains a toxic alkaloid and the stem has sharp prickles, but if the leaves are touched, they fold down and collapse. Scientists think this frightens off larger herbivores, and dislodges small insects which have landed on the leaves. The leaf falls in a few seconds, and recovers over 10-12 minutes as a result of potassium ion movement into specific cells, followed by osmotic water movement. The exact causes of the dramatic change in the leaves are still being researched.
