Module 4 – Lesson 3

Learning ObjectivesSuccess Criteria  Learn about diseases that affect plants.  Understand how diseases spread between plants.  Understand how plants defend themselves against pathogens.  Give examples of plant diseases and the pathogens that cause them.  Describe how pathogens spread between plants, and the role of vectors and spores.  Describe and explain the passive and active mechanisms plants use to defend against pathogenic attack.

Starter  Every animal on Earth depends on plants as a food source in some way (even carnivores).  Humans and their pets depend on agriculture to keep well- fed. What major crops can you think of? Rice; Wheat; Maize; Potatoes; Bananas etc.  These species all suffer from major diseases. What can be done to prevent or manage such diseases? Are there any techniques scientists could use to help?

Task – Plants Diseases  Read about the plant diseases detailed in the table on the hand-out. Questions: 1. Which plant diseases have no cure at the moment? 2. Suggest why tobacco mosaic virus has no binomial name. 3. Suggest why potatoes cannot be grown in an area infected by ring rot for another 2 years. 4. What feature does the potato blight pathogen have in common with fungi, even thought it is a protoctist? 5. Explain why banana plants infected with black sigatoka have a reduced yield. All of them. Viruses aren’t classified as living. Only living things are given a binomial name. The bacteria is still present in the soil and may infect new tubers. It has hyphae, which penetrate the cells of the plant. The disease affects the leaves, turning them black. These areas cannot photosynthesise and therefore produce less sugars.

Transmission of Plant Diseases Direct Transmission  Pathogens are present in the soil and may simply enter the roots.  Fungal pathogens produce spores for reproduction, which may be carried by the wind.  Pathogens in leaves and fruit may be transported elsewhere by wind or animals. Indirect Transmission  Vectors affect plants too.  Burrowing insects such as beetles transport pathogens from tree to tree. The beetle that carries Dutch elm disease.

Physical Defences  Plants have evolved defence mechanisms against pathogens.  Some are quite simple and passive, such as:  Plants can also actively reinforce their cells by producing extra physical barriers on demand. Physical barriers: Waxy cuticle of leaves Bark on trees Cellulose cell walls Callose Enzyme Beta-Glucose  A polysaccharide called callose is synthesised within minutes of attack.  The callose acts like polyfilla – plugging holes and damaged areas around the infected cells.

Callose  Study the diagram and follow the sequence of events.  Start at the top, where the pathogen first attacks.  The callose seals the infected cell off from surrounding healthy cells.  Lignin is also added, making the barrier thicker and stronger.  Callose also seals off sieve plates in the phloem, preventing the spread of pathogens throughout the plant.

Chemical Defences  Plants produce powerful chemicals when attacked by pathogens or their vectors.  Some of these are useful to us (caffeine, antiseptics etc.). Examples: Insect Repellents Repel insect vectors such as beetles Insecticides Kill insect vectors Antibacterials Break down bacterial cell walls Antifungals Eg: Chintinases break down cell walls Defensive Proteins Disrupt ion channels General Toxins Chemicals such as cyanide and ricin

Questions 1. Explain the difference between passive and active defence mechanisms in plants. 2. Callose is a large polysaccharide composed of beta-glucose units linked by 1,3 and 1,6 linkages. What does this mean? 3. Describe how callose production acts to protect against attack by pathogens. 4. Suggest why plant defences such as chemicals are only produced once a pathogen attacks. 5. In some plants, infected cells intentionally commit “suicide”. How might this save the whole plant? Passive mechanisms prevent entry of pathogens. Active mechanisms prevent further damage and spread of pathogens. The beta-glucose monomers are linked to each other by glycosidic bonds between the 1 and 3 carbons, and the 1 and 6 carbons. Detection of the pathogen leads to the cells synthesising callose. The callose is deposited into the cell walls, strengthening them and preventing pathogen spread to nearby cells. The detection of the pathogen acts as a stimulus, which causes defence mechanisms to kick in. Requires transcription and translation of certain genes for enzymes. The death of the plant’s cells prevents the pathogens from reproducing. The pathogens usually use the cellular machinery/nutrients to grow/reproduce.

Plenary  Bananas are grown in over 130 countries where they are important as a food crop and economically as a cash crop.  They are the 4 th most important crop in the developing world after rice, wheat and maize.  In East Africa, bananas (known as plantains), are the staple food for around 50% of the population. People eat around 400kg of bananas per year.  Black sigatoka has resulted in a 40% reduction in yield. Question: Geneticists are working on producing resistant strains of banana plants. What plant defence mechanisms can you think of that would be useful to engineer into these new varieties? How exactly would they prevent loss of yield?

Learning ObjectivesSuccess Criteria  Learn about diseases that affect plants.  Understand how diseases spread between plants.  Understand how plants defend themselves against pathogens.  Give examples of plant diseases and the pathogens that cause them.  Describe how pathogens spread between plants, and the role of vectors and spores.  Describe and explain the passive and active mechanisms plants use to defend against pathogenic attack.