Plant disease and plant defences questions

Question 1

A grower has ring rot in one field of potatoes and mosaic mottling in a glasshouse of tomatoes nearby. Suggest what should be done in each case, and suggest why no treatment can be offered to the infected plants themselves.

Accepted Answer

B1 ring rot is a notifiable disease, so the infected stock is destroyed, the machinery and washing water that touched it are disinfected, and the land grows something other than potatoes for several years B1 the tubers are the route, and infected ones can look healthy, so future crops must be grown from certified disease-free seed potatoes B1 the tomatoes have tobacco mosaic virus, so infected plants are removed and destroyed rather than composted, since the virus survives for years in dried plant material B1 spread is by contact through wounds, so hands and tools are washed and disinfected between plants, tobacco is kept away from the glasshouse, and resistant varieties are grown in future seasons B1 neither disease can be cured: there is no chemical that clears a bacterium from the vascular tissue of every plant in a field, and no antibiotic acts on a virus, so all the useful action is containment before and around infection rather than treatment of it

Question 2

Describe how a plant contains an infection once a pathogen has entered a leaf, naming the two materials it deposits.

Accepted Answer

B1 callose, a polysaccharide of beta-1,3 linked glucose, is laid down between the cell membrane and the wall within minutes of the infection being detected B1 it plugs the plasmodesmata, the channels connecting cell to cell, so the pathogen cannot spread from the infected cells to their neighbours B1 it also seals the sieve plates of the phloem, which would otherwise carry a virus the length of the plant in hours in the flowing sap B1 over the following days lignin, the tough waterproof polymer of woody tissue, is deposited in and around the walls of the infected region, making the barricade permanent with the pathogen walled in

Question 3

Outline the defences a plant uses against herbivores, referring to alkaloids, tannins and volatile signals in your answer.

Accepted Answer

B1 alkaloids are bitter, nitrogen-containing compounds, caffeine, nicotine, morphine and quinine among them, which are toxic to herbivores or deter them from feeding B1 tannins are phenolic compounds that bind proteins, inactivating the digestive enzymes of whatever is chewing and making the tissue bitter, so an insect eating tannin-rich leaf grows slowly or gives up B1 a plant being chewed releases volatile signals that drift downwind, and parasitic wasps follow the plume to lay their eggs in exactly those caterpillars, so the plant summons its herbivore's own enemy B1 neighbouring plants detect the same volatiles and raise their tannin and toxin levels before they are bitten; some plants add movement, as Mimosa pudica does when a touched leaf folds and dislodges insects

Question 4

Explain why a Venus flytrap closes only after two touches within about thirty seconds, and explain why about five action potentials are needed before it begins to digest.

Accepted Answer

B1 bending a trigger hair opens stretch-sensitive ion channels at its base, and the depolarisation fires an action potential across the trap B1 one action potential is held as a raised calcium ion concentration that decays over tens of seconds, so only a second touch within that window takes the calcium concentration past threshold and commits the trap B1 a raindrop or a blown seed touches once whereas prey walks, and walking touches twice, so the count is a filter against false alarms, which would cost the plant energy and hold its photosynthetic surface folded shut for a day B1 a caged insect struggles and bends the hairs again, so about five action potentials in total confirm live prey before the glands are switched on and the enzymes are spent; a trap closed on debris receives no further touches and simply reopens

Question 5

Describe how touching a leaf of Mimosa pudica causes its leaflets to fold together within a second or two.

Accepted Answer

B1 touching the leaf generates an electrical signal, an action potential, that travels away from the point touched B1 at each leaflet's hinge, a pad of cells called the pulvinus, potassium ions leave the cells on one side B1 water follows the potassium ions out of those cells by osmosis, so turgor in them falls A1 the loss of turgor on one side of the hinge makes it fold, closing the leaflets together

Question 6

Compare the defences a plant has against a pathogen with the defences a mammal has.

Accepted Answer

B1 both detect the invader chemically as something that should not be there: a plant recognises molecules characteristic of pathogens and fragments of its own damaged walls, whereas a mammal recognises non-self antigens B1 a mammal brings phagocytes and lymphocytes to the site in the blood, whereas a plant has no circulating cells at all and each cell defends where it stands, sealing with callose and lignin and poisoning with its own chemistry B1 a mammal makes antibodies and keeps memory cells, so it acquires immunity, whereas a plant makes neither, so it learns nothing from an infection and has no immunity to the same pathogen next season

Question 7

Explain why a plant can secrete a chitinase without digesting its own tissues, and name one plant compound that acts against bacteria.

Accepted Answer

B1 a chitinase hydrolyses chitin, which is the polymer of fungal cell walls and of insect exoskeletons B1 no plant cell contains chitin, since a plant cell wall is made of cellulose, so the enzyme has no substrate in the plant and the weapon can be deployed freely B1 defensins, small proteins that damage bacterial membranes, or phenols, act against bacteria

Question 8

Explain why the leaves of a tobacco plant infected with tobacco mosaic virus show a mottled loss of colour, and explain the effect this has on the plant's growth.

Accepted Answer

B1 the virus damages chloroplasts in the infected patches of leaf, so those patches lose their chlorophyll, which produces the mottled (mosaic) pattern B1 chlorophyll is needed to absorb light for photosynthesis, so the rate of photosynthesis in the affected leaf tissue falls A1 with less photosynthesis taking place, less sugar is available for growth, so the plant's growth is stunted

Question 9

Sealing the sieve plates of the phloem with callose helps to contain a viral infection, but it also has a cost to the plant. Explain what this cost is.

Accepted Answer

B1 callose deposited at the sieve plates blocks the flow of sap through the phloem in that region, which is what stops the virus travelling with it B1 the phloem, however, is also how the plant normally transports sugars from where they are made to where they are needed A1 sealing the sieve plates therefore also blocks translocation of sugars through that same region, so containment comes at a cost to the plant's own transport

Question 10

A tree can lose a whole branch to lignified, walled-off infected tissue and survive, whereas similar damage to an animal's one heart would be fatal. Suggest why a plant can survive this kind of local tissue loss more easily than an animal could.

Accepted Answer

B1 a plant's structure is modular, built of many repeated parts such as leaves and branches, so one part can be lost without the whole organism failing B1 most plant organs are not unique, so growth continues from meristems elsewhere on the plant even after a branch is walled off and sacrificed A1 an animal's organs, such as the heart, are not modular in this way; each animal usually has only one, so damage to it cannot simply be sacrificed and grown around

Question 11

Compare an insect repellent produced by a plant with an insecticide produced by a plant, giving a named example of each and stating when each acts.

Accepted Answer

B1 a repellent, such as citronella from lemon grass, acts before an insect bites, deterring it from feeding on or landing on the plant at all B1 an insecticide, such as the pyrethrins made by chrysanthemums, acts on an insect that has already begun to feed, poisoning or killing it A1 so a repellent reduces the number of insects that attack the plant in the first place, whereas an insecticide reduces the damage done by insects that attack anyway

Question 12

Many human medicines, including quinine and morphine, were first isolated from chemical defences that plants use against herbivores or pathogens. Suggest one argument this provides for conserving plant diversity.

Accepted Answer

B1 different plant species have evolved different defensive compounds against different herbivores and pathogens, so the range of chemicals available across all plant species is very large B1 some of these compounds, such as quinine and morphine, have turned out to be useful as human medicines once isolated and tested A1 if a plant species becomes extinct before its defensive compounds have been investigated, any medically useful chemistry it made is lost permanently and cannot be recovered

Question 13

The Venus flytrap grows in bogs where the soil is very poor in nitrogen compounds. Explain why the plant catches and digests insects, and explain why this is not classed as a defence against a pathogen or a herbivore.

Accepted Answer

B1 digesting an insect releases nitrogen-containing compounds, such as amino acids, which the plant absorbs through the trap wall B1 this supplements the nitrogen the plant cannot obtain in sufficient quantity from the nitrogen-poor, waterlogged soil it grows in A1 it is a nutritional adaptation rather than a defence, because its purpose is to obtain a nutrient the plant needs, not to protect the plant from being eaten or infected

Question 14

State the type of pathogen that causes ring rot of potatoes, and state how tobacco mosaic virus travels from plant to plant.

Accepted Answer

B1 ring rot is caused by a bacterium, Clavibacter sepedonicus, which infects the vascular tissue B1 tobacco mosaic virus needs no vector and spreads by contact, from leaf to broken leaf and on tools and hands

Question 15

State what action is legally required when an outbreak of ring rot is confirmed on a UK farm, and state why symptomless tubers make this disease difficult to control.

Accepted Answer

B1 an outbreak leads to statutory action: the infected stock is destroyed, equipment is disinfected, and restrictions are placed on growing potatoes on the affected land B1 symptomless tubers can still carry the bacterium, so infected seed tubers, and the machinery or washing water that touched them, can be moved to a new field before anyone knows the crop is infected