Garden Soaps - intimacy, deceit and betrayal in the plant world.
2003's exhibit described on information boards, using striking images as a backdrop, how plants defend themselves against pests and diseases. Examples of plants and the diseases, fungi, flies and viruses that attack them illustrated the cases discussed in the text.
Board 1: Pests and Diseases.
 Red Spider mite. |
Plants are food for a wide range of animals – including many we identify as garden or agricultural pests. Plants are also food (in the general sense of raw materials and energy) for bacteria, fungi and viruses. Consequently, plants have evolved many protective mechanisms to reduce the damage caused by pests and diseases (from spines and thorns to toxic chemicals and even the means to switch off the genes of invading organisms). |
It is estimated that 20-40% of the world’s food production is lost to pests and diseases even after the use of chemical pesticides, without these controls it is estimated losses would approach 80%. Understanding how plants and their pests interact is crucial in breeding and managing crops to reduce both crop losses and dependence on chemical inputs.
Board 2: Raising the alarm.
Part of a plant’s defence against disease attack is an internal communication system that transmits alarm signals around the plant. The signal alerts cells in parts of the plant at a distance from the initial infection site and they switch on defence systems that make them resistant to disease infection. This prevents the disease spreading through the plant or starting new infection sites away from the original infection.
This resistance is not always present, but is acquired as a result of an initial infection and is systemic (spread throughout the plant), so it is called Systemic Acquired Resistance or SAR.
Scientists have recently identified several of the important genes and proteins involved in the SAR system and are steadily uncovering more about how this important element in a plant’s defences works. They have also seen how some diseases can subvert this system, preventing the signal being transmitted and the alarm being raised.
Board 3: Garden Soaps.
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Some plants, eg. tomatoes and oats, produce natural soaps (called saponins). These chemicals are poisonous to bacteria and fungi and so help defend the plant against disease attack. The microbes that can successfully invade saponin-containing plants often produce enzymes that break down the saponins into less toxic chemicals, thus allowing the invader to breach the plant’s first line of defence. Recently, scientists have discovered that fungi that attack tomatoes not only break down these defences but actually use them to their own advantage. The breakdown products produced as the fungi destroy the saponins, interfere with essential communication processes in the plant. Signalling pathways, which would normally set off an alarm system to activate plant defence responses, are disabled. So in overcoming one line of defence the microbe also disrupts the plant’s ability to trigger its other defence systems. |
 Pea Mildew. |
Board 4: Silencing the Opposition.
| Plant viruses are commonly transferred from plant-to-plant by stealth, often as the result of a pest feeding on a plant. Once inside plant cells the virus, which is a protein capsule protecting a small piece of genetic material, hijacks the genetic system of the cells it infects and uses the plant’s resources to produce millions of copies of itself ready to be transmitted to new hosts. It is known that plant cells have a mechanism that recognises foreign genetic material and that this also ‘mops up’ and cuts into pieces the genetic material from the virus, including any copies it tries to make. The virus genes are effectively switched off by the plant – so-called ‘gene silencing’ – and this prevents the virus from establishing an infection. Scientists are now trying to understand how some viruses are able to deceive the plant’s defence system and establish successful infections. |
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 Maize Streak Virus. |
Board 5: Attracting a killer.
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For thousands of years the Brassica family have been using contract killers to eliminate aphids. Brassica plants, especially broccoli and Brussel sprouts, give of a smell that attracts tiny predatory wasps. These wasps seek out young female aphids, which are feeding on the plant, and lay a single egg in the body of the living aphid. The developing larva eats the aphid alive, eventually killing it. The mature wasp cuts itself out of the husk, which is all that remains of the aphid, and flies off to predate more aphids. Researchers have begun to breed new varieties of broccoli with higher levels of these chemical attractants so as to attract more predatory wasps onto brassica crops and increase the level of natural biological control of aphids. |
 Peach Aphid. |
Board 6: Suicide as a way of life.
The relationships between plants and their pests and diseases can be complex and intimate. In some cases a single gene in the plant controls resistance or susceptibility to a particular disease, while a single gene in the bacteria or fungus that causes disease controls whether it is able to infect (is virulent) or unable to infect (is a-virulent) on that plant. A susceptible + virulent gene match results in successful infection, a mis-match means no infection.
Chemical, physical and genetic defences – commonest is suicide (hypersensitive response). Cells often respond to infection by dying, depriving the invading disease of access to nutrients and sometimes producing toxic compounds. But some disease organisms can prevent cells recognising them as invaders and so avoid their host dying in response to their attack.