Plant disease and plant defences

Two named plant diseases

Once a crop plant is infected there is usually no treatment for it, so control depends on prevention and containment. Ring rot, caused by a bacterium, and tobacco mosaic virus show how this works in practice.

Describe the drawing

Two panels describing named plant diseases. Potato ring rot, caused by the bacterium Clavibacter sepedonicus, blocks and rots the vascular ring inside the tuber and spreads on machinery, in seed tubers and in washing water; infected stock is destroyed and the land restricted. Tobacco mosaic virus causes mottled leaves where chlorophyll is lost and spreads by contact through hands, tools and damaged leaves; control is hygiene and resistant varieties. A note says a plant has no phagocytes and no antibodies, so every cell defends itself.

Ring rot of potatoes is caused by the bacterium Clavibacter sepedonicus. It infects the vascular tissue, and the name is literal. Cut an infected tuber and the ring of xylem and phloem has turned to a pale rotting band, because the bacterium multiplies where the transport runs. A field can carry the infection for a season with little to show above ground. So symptomless seed tubers, and the machinery and washing water that touched them, carry the bacterium to the next field. There is no chemical cure.

Ring rot is a notifiable disease in the UK. An outbreak leads to statutory action covering destruction of the stock, disinfection of equipment and restrictions on growing potatoes on the affected land. Check the current plant-health notice for the measures and periods that apply.

Tobacco mosaic virus has a place in the history of biology as the first virus described, and is the standard example of a viral plant disease. It infects tobacco and over a hundred other species, tomatoes and peppers among them. The mosaic is the symptom: patches of leaf lose their chlorophyll, photosynthesis falls, and growth is stunted. The virus needs no vector; it spreads by contact, from leaf to broken leaf, on tools, and on hands, and it is stable enough to survive for years in dried plant material. Control is hygiene, washing hands and tools and removing infected plants, and, increasingly, growing varieties bred or engineered to be resistant.

Plants do not produce antibodies and do not have circulating immune cells. Infection is restricted locally. Cells deposit callose at plasmodesmata and sieve plates, reducing movement of a pathogen between cells and through the phloem. Lignification can strengthen barriers around affected tissue, while antimicrobial compounds and hydrolytic enzymes inhibit pathogens.

Containment: callose first, lignin after

Plant cells detect infection chemically, recognising molecules characteristic of pathogens and fragments of their own damaged walls. What matters at A level is the response, and its first and fastest part is the deposition of a polysaccharide.

Describe the drawing

Two panels each showing a pair of plant cells joined through three plasmodesmata drawn as channels through the shared wall. In the before-infection panel the channels are open, letting water, sugars and viruses pass from cell to cell. In the after-infection panel the left-hand cell holds virus particles and an amber callose plug sits in every channel, walling the infected cell in with its pathogen. Text beneath says callose is a beta-1,3 glucan laid down within minutes at plasmodesmata and sieve plates, and that lignin follows over days to make the seal permanent.

Callose

A polysaccharide of beta-1,3 linked glucose, deposited within minutes of infection at plasmodesmata, sieve plates and wounds, sealing the routes a pathogen would spread by.

Lignin

The tough, waterproof polymer of woody tissue, added to walls around an infection site over days to make the seal permanent.

Callose is a glucose polymer, like cellulose and starch, but built on beta-1,3 links, not the beta-1,4 of cellulose. That gives it a helical, gel-forming structure distinct from the structural polysaccharides of the wall. Within minutes of infection it is laid down between the cell membrane and the wall. It also blocks the two kinds of opening that matter: the plasmodesmata connecting cell to cell, and the sieve plates of the phloem. The first blocks the pathogen's short-range spread. The second matters because sap flowing through sieve tubes would otherwise carry a virus the length of the plant in hours. Sealing those routes also stops the transport of sugars through them, so containment has a cost in translocation.

Over the following days the seal is made permanent. Lignin is deposited in and around the walls of the infected region, the same polymer that stiffens xylem. The cells enclosed by it often die with the pathogen contained among them, and growth continues elsewhere. A plant can lose tissue in this way because its structure is modular: a single leaf is replaceable, and no irreplaceable organ is lost at a typical infection site.

Chemical defences

Plants also produce a wide range of defensive compounds, which are most easily organised by the organisms they act against.

First, the target follows from the chemistry. Chitin is the polymer of fungal walls and insect skeletons and appears nowhere in a plant cell, so a chitinase hydrolyses fungal walls without damaging the plant's own tissue. Second, several entries in the table are the origin of human products. Pyrethrins are the model for the synthetic pyrethroid insecticides, and the nicotine in a tobacco leaf functions as an insecticide in the plant. Many human medicines were first isolated from such defensive compounds. That is one argument for conserving plant diversity: the compounds made by an extinct species can no longer be investigated.

Defences against herbivory

Plants are also attacked by herbivores, chiefly insects and mammals. The defences against them range from deterrent chemistry to volatile signalling.

Alkaloid

A bitter, nitrogen-containing compound made by a plant, toxic to or deterrent to herbivores; caffeine, nicotine and morphine are alkaloids.

Tannin

A phenolic compound in leaves and bark that binds proteins, inactivating digestive enzymes and making the tissue bitter and hard to digest.

Pheromone

A chemical signal released by an organism that changes the behaviour of another; used loosely in plant biology for the volatile signals plants release when attacked.

Alkaloids are bitter nitrogen-containing compounds: caffeine, nicotine, morphine, quinine and cocaine are examples. Each functions as a herbivore deterrent in the plant, and several are also used as human drugs; quinine was the first effective treatment for malaria. Tannins work more slowly: they bind proteins, including the digestive enzymes and gut lining of whatever is chewing, so an insect eating tannin-rich leaf grows slowly or gives up. The dryness of strong tea is tannin binding the proteins of your own mouth.

Volatile signalling acts at a distance. A maize plant being chewed by caterpillars releases volatile compounds that diffuse downwind, and parasitic wasps orient towards them and lay their eggs in those caterpillars. Neighbouring plants detect the same volatiles and increase their tannin and toxin concentrations before they are attacked. Strictly, a pheromone is a within-species signal, so 'a volatile signal' is the safer exam phrase for the wasp case; the specification's word is pheromone and it will be accepted.

Mimosa pudica responds to touch with rapid movement. Touching a leaf causes the leaflets to fold together in sequence within a second or two and the whole leaf to droop. An electrical signal travels from the point touched, and at each leaflet's hinge, a pad of cells called the pulvinus, potassium ions leave the cells on one side, water follows by osmosis, turgor falls and the hinge folds. The movement can dislodge insects and reduces the exposed leaf area. The ions are transported back over the following minutes and the leaf reopens, at a cost in ATP. Specifications differ in how much of this mechanism they require.

The Venus flytrap and electrical signalling

The Venus flytrap grows in bogs whose waterlogged, acidic soil is very poor in nitrogen. Its traps are modified leaves that capture insects as a source of nitrogen compounds, so they are a nutritional adaptation rather than a defence. They appear in specifications as an example of electrical signalling in plants, and are described as a sequence.

Describe the drawing

Four numbered stages with a small drawing of the trap beside each. An open trap with three trigger hairs on a lobe waits after a single touch fires one action potential. A second touch within about thirty seconds fires another, and the lobes snap shut in about a third of a second around the prey. Continued struggling fires more action potentials, and around five touches switch the closed trap into a digestive organ. In the final stage glands flood the sealed trap with hydrolytic enzymes and nitrogen compounds are absorbed over days.

Each half of the trap, each lobe, carries three stiff trigger hairs. An insect walking on the lobe bends a hair, stretch-sensitive ion channels at its base open, and the resulting depolarisation fires an action potential across the trap, carried by ion movements just as in a neurone, though more slowly. One action potential does nothing visible. The trap holds the signal, in effect a count of one, as a raised calcium ion concentration that decays over tens of seconds.

A second touch within about thirty seconds fires a second action potential, the calcium concentration crosses a threshold and the trap closes. The lobes move from convex to concave in a fraction of a second, through released elastic tension and rapid water movement between cell layers, and the marginal spines interlock. In the textbook account of that water movement, the action potentials make cells in the outer layer of each lobe pump hydrogen ions into their cell walls, using ATP. The acid loosens the walls, water enters those cells by osmosis, and as they expand the lobe flips from convex to concave.

Requiring two touches discriminates between prey and other stimuli. A raindrop or a blown seed usually touches once, while a walking insect touches repeatedly. Closing on a false stimulus would cost energy and leave the photosynthetic surface folded shut for a day.

The counting continues after closure. A caged insect struggles, and each struggle bends hairs and fires further action potentials. Around five in total switch the trap's glands on. The lobes seal at the margins, and digestive enzymes, proteases and chitinases among them, flood the cavity. Over the next days the soluble products, nitrogen compounds above all, are absorbed through the trap wall. A trap that closed on debris receives no further touches, and reopens within a day without having secreted its enzymes. The same classes of molecule that defend an ordinary leaf are used in digestion here: a chitinase hydrolyses a fungal wall in one plant and an insect exoskeleton in this one.