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Basal plant immunity
Lecture overview
• How bacteria and filamentous plant pathogens penetrate and colonize the plant tissue
• How cell polarization/focal immunity contributes to elimination of pathogens
• Describe the components of plant innate immunity
• General features of pathogen associated molecular patterns
• Explain the plant mechanisms for sensing microbial invasion at plant surface
Part 1 – Cell and Molecular Biology of plant-microbe interactions
Global hunger
• Food crisis, hunger is on the rise
• 1bil people at the limit of starvation
• Not an issue in developed countries
• Rise in food prices
• Rise in population (world population of 9 billion by 2020), need in increase food production to
meet this
• Lots of technologies in agriculture but not meeting the global need for food production –
another approach is to prevent crop losses
Plants are attacked by a diverse range of organisms as they produce food
• Bacteria
• Filamentous plant pathogens (fungi and oomycetes) – eukaryotic parasites
• Viruses
• Nematodes
• Insects
• Parasitic plants
Filamentous plant pathogens
• Filamentous plant pathogens (fungi and oomycetes) cause destructive crop diseases
• Five of the top seven most dangerous plant pathogens are filamentous plant pathogens
• Rice blast, huge impact on rice production globally
• Black sigatoka disease on bananas
• Asian soyabean rust – soybean production
• Wheat stem rust
• Potato blight – oomycete pathogen, triggered great Irish famine in the 1850s
• Panama disease – destruction of bananas, huge threat for banana production
o 70% of banana production is one variety, resistance carried by this variety overcome by
panama disease (fungal pathogen)
o Happened in 1965 – there was a better banana variety but fully susceptible to this fungi
o Evolved over time and fungi can infect new variety
Global trade routes boost distributions of plant and animal pathogens
• Global pathogens spread by transport (shipping routes, roads, air travel)
• Food security problems, harm to environment, forests being destroyed by pathogens
introduced from different parts of the world

Infection strategies of filamentous plant pathogens
• Filamentous plant pathogens can enter plants in several ways
• Plants have cuticles on leaves to prevent entry of bacteria, virus and other pathogens, but
filamentous plant pathogens can breach this by physical means
• Fungi can enter through stroma (natural openings) and live in the extracellular space, leaching
nutrients from hosts
• Can directly penetrate in between
two cells, breach cuticle, can
grow in extracellular space and
produce extra penetrations to go
further through plants
(haustoria)
• Intracellular entry, direct
penetration of epidermal cell
layer, appressorium (specialised
cell that produced high turbor
pressure to breach cuticle) and
secretes enzymes to degrade the
cell wall, directly penetrate cell
Waxy epidermal cuticle and cell wall
• Prevents water loss and microbes from entering the epidermal later
• Filamentous plant pathogens can break this cuticle and cell wall
• Spore germinates and forms appressorium and can breach the cell wall
• Can genetically modify with fluorescent proteins to watch
• Intracellular region of plants is highly dynamic
o Lots of intracellular vesicular movements – good weapon to defend
o Can reprogram trafficking routes to overcome external changes (eg
...
) – bacteria produce the
extracellular polysaccharide xanthan to protect from environmental factors
• Form type III secretion machinery (essential pathogenicity factor) which spans the bacterial
membrane and is associated with an extracellular pilus
• The pilus is connected to a channel-like translocon that is inserted into the plant plasma
membrane
• Penetrates cell membrane and injects virulence factors (approximately 30 different effector
proteins) into the plant cell, refold and have different compartments to target
• Bacterial effector proteins are transported to different cellular compartments and manipulate
host cellular pathways
• Can alter plant gene expression in nucleus or vesicular trafficking in golgi
• Effector proteins suppress plant defence responses and promote multiplication at the infection
site
• Microbe can successfully take control of the plant cell, immune system shut down, take sugars
from the leaves
• Multiplication leads to necrotic spots on the plants and fruit
Xanthomonas infection in resistant plant
• Bacteria can also invade resistant plants and inject effector proteins (virulence factors)
• In resistant plants individual effector proteins are mostly recognised by cognate resistance
receptors (intracellular) which sense virulence factors (directly or indirectly) and activate
defence reactions
• Effector protein triggered plant defence leads to death of the plant cells at the site of infection
and bacterial starvation and arrest of growth
• Resistant plant stays healthy – few cells sacrificed at point of infection but remaining plant
stays healthy
Part I summary
• Plant pathogens use various different routes to enter host tissue:
o Through wounds and natural openings (stomata)
o By breaching cuticle and penetrating in between two cells
o Directly penetrating epidermal cells
• Cell polarization plays a major role in non-host resistance by preventing pathogen penetration,
whereas it also contributes to defense against adapted pathogens to an extent

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Focal immunity includes cytoplasmic streaming of endomembrane system and nucleus
towards pathogen ingress sites, followed by targeted cell wall fortifications and defense
related secretion
Focal immunity is suppressed during susceptibility
Pathogens penetrating the plant cells are enveloped by host derived membranes with
unknown origin

Part II – Surface immune recognition in plants
Most plants are actively resistant to most pathogens
• Disease is the exception, not the rule
• Disease occurs certain environment conditions favouring the pathogen and specialised
pathogens that are able to infect the plant
• Plants have an innate ability to recognise potential invade pathogens and to mount effective
defences
o Plants can produce their own food so simply pro

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