1. Plant Defences against disease causing organisms Part I
Resources to support GCSE and A-level curricula by the British Society for Plant Pathology

2. Wind, water, insects and chemotaxis help pathogens reach their hosts
Plants are fixed in the ground and cannot escape attack by plant pathogens. Thrips and other insects are important vectors for viruses (the disease being passed by the insect). Powdery Mildew and other fungi form spores that can be carried by the wind. Many bacteria are transferred in water droplets. Many soil-living nematodes, bacteria, fungi and oomycetes sense and respond to plant-exuded chemicals – movement in response to a chemical stimulus is known as chemotaxis.

3. Once they reach their host they need to get inside the plant to cause disease .. that’s not so easy.
Pathogens must overcome formidable plant defences: physical and chemical barriers
When the pathogen has reached a potential host, its not so easy for them to get inside the plant. Because it cannot ‘run away’ the plant has evolved many different ways to protect itself – plant defences – which may be physical barriers or chemical barriers. We will consider both in the following slides. It may help your students to imagine a plant like a castle – well designed and defended to keep out attackers (the pathogens).

4. Let’s start with the physical …
We will begin by thinking about physical barriers and the structure of the plant. Have you ever looked closely at the leaves of plants after a shower of rain? Water forms droplets and runs off, due to the hydrophobic nature of the ‘outer skin’ of the plant surface. Why is it hydrophobic and what does it look like?

5. Waxes cover many plant surfaces providing the first physical barrier to pathogen entry.
This Scanning Electron Microscope (SEM) image shows waxes around the only ‘weak spot’ in a leaf’s physical barrier – the stomate. Some pathogens head specifically for this opening.
Wax crystals cover the surface of plant leaves, providing a first barrier to pathogen attack and explaining the hydrophobic nature of the plant surface. The types of waxes vary between species. Stomata, pores in the leaf surface that allow for gaseous exchange, break up the wax layer, but these and their guard cells are also covered in waxes. Some pathogens will target this natural opening as an ‘easy way’ to enter the plant.

6. The pathogen must gain entry through wax layers, the plant cuticle and the cell wall.
The relationship between the wax layer, the plant cuticle and plant cell wall is shown schematically in this slide. All these physical barriers, made up of different substances (wax, cutin and polysaccharide), must be overcome by a pathogen before it can enter a plant cell. It’s a formidable challenge. Pathogens try to deploy an array of different enzymes and physical (turgor) pressure to overcome these defences

7. In some case, xylem vessels maybe blocked by the formation of tyloses preventing pathogen spread.
Physical barriers exist inside the plant as well. Xylem vessels used to transport water inside the plant may be blocked by the plant through the formation of tyloses preventing the spread of an infection in this way. A schematic of this process is shown next to a cross-section of the water carrying tubes, the xylem vessels, photographed using a scanning electron microscope.
SEM of xylem vessels

8. Papillae (P) may form on the inner side of cell walls
Papillae (P) may form on the inner side of cell walls. This structure can trap invading pathogens.
Papilla
Fungal Infection Peg
Fungal spore
Papillae are another physical defence against pathogens, trapping the invading pathogen structures. The slide shows a papilla, formed in response to infection by the powdery mildew fungus (stained dark blue). The papilla is formed on the inner side of the cell wall and has enveloped the infection peg of the fungus. A papilla is also the sight of many chemical reactions, defending against pathogen attack.

9. This SEM image of powdery mildew attacking barley, shows a ‘failed’ papilla (red), and the haustorium (fungal feeding structure) in a barley cell.
If the papilla ‘fails’ – red coloured structure (false colouring) - the powdery mildew fungus may be successful establishing an infection in the epidermal cell of the host (barley in this example) and forming a feeding structure (called a haustorium) through which it is able to ‘steal’ nutrients from the plant.

10. Plants actively defend themselves against pathogens
Plants resist pathogens through active processes that include recognition of the pathogen and defence responses to fight it
So we have seen that the process by which the plant defends itself is very active requiring the plant to (1) recognise the invading pathogen, and (2) put up defences against it. Even if the pathogen gets inside the plant, there are many chemicals and chemical reactions inside the plant that offer further challenges to the pathogen. Advances in imaging technology now allow scientists to visualise some of these reactions.

11. Inside the Plant the battle continues
Mint
You may be aware of many ‘scented plants’ - some produce essential oils (monoterpenoids), and humans have found some of these may be anti-microbial and used to treat diseases or act as medicines (eg. Mint and Witch Hazel), but there are many, many chemicals produced (pre-formed) or formed via internal reactions inside plants other than scented ones, providing a formidable barrier to invading pathogens. The final 2 slides demonstrate just a few of these
Witch hazel

12. Anti-microbial compounds inside roots of oat
The fluorescent compound in this cross-section of an oat root is the antimicrobial triterpene Avenacin. Produced by oat roots, it is tightly regulated within the outer cortical cells of root tips, where it protects against plant diseases.

13. Many chemical reactions occur inside plant cells in response to the invading pathogen
The interaction between barley and the powdery mildew fungus, Blumeria graminis f.sp. hordei (formerly Erysiphe graminis f.sp. hordei) visualised using immunohistochemistry.
Upper panels. Fungal structure (an appressorium) - stained green, trapped by a papilla in the plant showing the presence of arabinoxylan (red), and callose (blue). Bars, 5um. Lower panels. Different views of a papilla labelled with a cellulose probe (red). Bars, 5uM

14. Acknowledgements
The BSPP gratefully acknowledges the ASPB for granting access to its Teaching Tools in Plant Biology series. The presentation “Fighting for their lives: plants and pathogens” provided stimulus for this presentation with some slides being used directly (slides 2, 3 & 10) with the editors permission. Herman, M., and Williams, M.E. (June 27, 2012). Fighting for their lives: Plants and pathogens. Teaching Tools in Plant Biology: Lecture No tes. The Plant Cell (online), doi/ /tpc.112.tt0612.
This presentation is prepared under a Creative Commons License, Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0),
Additional images have been sourced as follows. All are within the public domain or used with the authors permission.
Slide 4 Water droplets on a leaf surface, Pixabay
Slide 5 Image courtesy Dr.Phil Smith, John Innes Centre, Norwich
Slide 6 Image courtesy of Dr. Jocelyn Rose, Cornell University, USA. Image first appeared in Plant Physiology (2013), Vol.45
Slide 7 Plant xylem vessels, image courtesy of Mrs. Kim Findlay, John Innes Centre. Schematic – drawn by Dr. Phil Smith (after Agrios, 3rd ed)
Slide 8 Image courtesy Dr.Phil Smith, John Innes Centre, Norwich.
Slide 9 Image courtesy Dr. Alan Little, ARC Centre of Excellence in Plant Cell Walls, University of Adelaide, Australia. Image first appeared in New Phytologist (2016), 212:
Slide 11 Mint: Wikipedia (Kham tran) Hamamelis & castle interior, Wikipedia commons
Slide 12 Image courtesy of Prof. Anne Osbourn, John Innes Centre, Norwich (NRP image gallery)
Slide 13 Image courtesy Dr. Alan Little, Plant Cell Wall group, University of Adelaide Australia. Image first appeared in New Phytologist (2014), 204: