L2 - plant diseases and defences

• Recognising an attack:

  -plants don’t have an immune system but do use cell signalling to initiate defence measures and communicate to other cells

  -receptors in the cell membrane respond to pathogens or toxins

  -signalling molecules are released and switch on genes in the nucleus

  -this leads to certain responses (e.g synthesis of defensive chemicals)
• Recognising an attack:

  
• Physical barriers to prevent the entry of disease:

  -waxy cuticle on leaves

  -bark on trees

  -cellulose cell walls

  -lignificad cell walls

  -callus formation (like scars)
• Chemical plant defences:

  -preventing insects

  -prevent herbivore feeding

  -dealing with pathogens

  -dealing with fungi

  -hydrolytic enzymes
• Production of callose:

  -when a pathogen is detected the plant cells rapidly produce high levels of a callose

  -callose is a polysaccharide, made of beta glucose monomers
• Role of callose in plant defence:

  -callose is deposited between cell walls and cell membranes next to the infected cells. this barrier prevents further spread

  -lignin is added to the callose to strengthen the barrier and increase the effectiveness of the mechanical barrier

  -callose also blocks the sieve plates of the phloem sealing off the infected cells and preventing spread

  -callose is also deposited in the plasmodesmata of connecting cells
• Preventing insects:

  -insects can act as vector organisms for the spread of disease

  -insect repellents (e.g pine resin and citronella from lemon grass)

  -insecticides (e.g caffeine which is toxic to insects and fungi)

  -pyrethrins act as a neurotoxin to insects
• Preventing herbivore feeding:

  -these give a bigger taste to inhibit herbivore feeding

  -alkaloids (e.g caffeine, nicotine, and morphine)

  -cyanide, usually bound to sugar molecules. defend the plant against herbivores. found in substantial amounts in certain seeds and fruit stones (e.g apple seeds)
• Dealing with pathogens:

  -antibacterial compounds containing antibiotics

  -phenols (antiseptic produced in many plants)

  -defensins (cysteine rich proteins that disrupt membranes of bacteria and fungi)
• Dealing with fungi:

  -antifungal compounds

  -gossypol (a terpenoid produced by cotton, toxic to insects and fungi)

  -caffeine (toxic to fungi and insects)

  -saponins (chemicals in plant cell membranes that interfere with fungal cell membranes)
• Hydrolytic enzymes:

  -these enzymes are found in the spaces between cells

  -chitinases (break down chitin in fungal cell walls)

  -glucanase (an enzyme to hydrolyse glycosidic binds in glucans found in cell walls of fungi
• Decreasing the spread of black Sigatoka:

  -providing more space between banana plants

  -removing all plant traces after harvesting

  -rotating crops

  -strict hygiene practises (e.g handwashing and sterilisation)

  -controlling insect vectors
• Plant disease can be transmitted by:

  -direct transmission

  -fungal spores in wind and water

  -indirect transmission

  -animal vectors
• Indirect transmission:

  -via the soil

  -machinery

  -hand equipment

  -storage sheds

  -compost

  -seeds

  -logs
• Animal vectors:

  -elm bark

  -beetles

  -aphids

  -birds

  -humans
• Ring rot is caused by the gram positive bacteria (clavibacter michiganesis) that damages leaves, tubers and fruit. it can destroy up to 80% of the plant with no cure and the field remains infected for at least 2 years
• Tobacco mosaic virus is a virus that infects tobacco plants and around 150 other species. it damages leaves, flowers, and fruit, stunting growth and reducing yields leading to almost total crop loss with no cure
• Potato blight is caused by fungus like protocist oomycete (phytophthora infestans). the hyphae penetrate host cells and destroy leaves, tubers, and fruit, causing millions of pounds worth of crop damage annually with no cure
• Black Sigatoka is a banana disease caused by the fungus (mycosphaerella fifiensis) which attacks and penetrates leaves, digesting cells and turning leaves black. if plants are infected it can cause a 50% reduction in yield with no cure
• Factors affecting the transmission of plant diseases:

  -choice of plant species for farming and horticulture

  -overcrowding and monoculture

  -poor availability of minerals

  -damp, warm conditions

  -farming practices checked

  -poor control of imports and exports

  -climate change
• Choice of plant species:

  -choose crop varieties resistant to disease
• Overcrowding and monoculture:

  -make transmission between plants more likely
• Poor availability of minerals:

  -reduces health and disease tolerance
• Damp, warm conditions:

  -increase the spread and survival of pathogens and spores
• Check farming practises:

  -disinfect equipment

  -practise crop rotation

  -wash hands
• Poor control of imports and exports:

  -of plant products and plants
• Climate change:

  -increased wind and rainfall promotes the spread of disease

  -also allows animal/insect vectors to spread to new areas