tree defense against pathogensfrst307.sites.olt.ubc.ca/files/2017/09/tree_defence2.pdf ·...
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Tree defense against pathogens
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Pathogens penetrate and ‘feed’ on trees differently than insects
• Insects ingest tree foliage or stem tissue and digest it internally; they usually don’t invade host cells
• Pathogens generally digest their food externally by secreting enzymes and then importing the broken down products (sugars, nutrients)
• Pathogens can penetrate host cells
Garnica DP, et.al (2014) PLoS Pathog 10(9): e1004329.
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Biotrophic pathogens can enter their host by using natural openings • Biotrophic pathogens
require a living host • They can enter the
host by using the natural openings in the host (the ‘sneaky’ mode of entry)
• Stomata, lenticels are natural openings that are used for gas exchanges
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Necrotrophic pathogens enter their host by producing chemical compounds
• Necrotrophic pathogens produce many chemical compounds that can cause cell death
• These are secondary metabolites called phytotoxins
• Once the cells are dead, the pathogen can colonize them and feed on the substrate
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Vectored pathogens enter their host by hitchhiking on an insect
• Insects are very efficient vectors of pathogens
• The relationship can be direct and mutualistic: Dutch elm disease
• The relationship can also be loose or indirect: e.g. beech bark disease
Photo University of Maryland
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Opportunistic pathogens can enter their host by using natural injuries
• Opportunistic pathogens or saprophytes can enter the host by using natural injuries
• The se can be part of the natural tree development, such as branch stubs or wounds caused by other pathogens (e.g. Dwarf mistletoe brooms)
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Trees have similar defenses against pathogens and insects
Tree defenses
Constitutive
Physical Stomata
Chemical Defensin
Induced
Chemical Phytoalexins
Genetic Immunity
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Trees possess constitutive barriers against pathogens
• Constitutive physical barriers – bark, wax layers, cell
walls, stomata, lenticels
• Constitutive chemical barriers – pH, nutrient
deprivation – suberin, defensin
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Trees have developed unique induced defenses against pathogens • Induced defenses:
chemical – phytoalexins: plant
toxins that specific inhibit a pathogen
• Induced defenses: genetic – plant immune
response – hypersensitive reaction
White pine blister rust
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Constitutive physical barriers: controlling stomata
• Stomata are natural openings that allow gas exchange
• The number and density of stomata can vary, directly affecting the likelihood of entry by a pathogen
• Plants can actively close their stomata in response to environmental stimuli and to pathogen attacks
Drawing biology champs
Photo Kyoto University
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Constitutive chemical barriers: defensins
• Defensins are peptides (short proteins) produced by plants that inhibit the growth of fungi, bacteria and viruses
• They are not induced but instead are produced constitutively
• Interfere with pathogen nutrition, delay their development, or cause structural changes to cell walls
Untreated spore
Spores treated with defensin
Acridine orange staining of cytoplasm
Gremmniella abietina
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Active chemical defense: phytoalexins
• Phytoalexins are antimicrobial compounds produced by plants
• They are synthesized de novo and accumulate rapidly at areas of pathogen infection
• Phytoalexins may puncture cell walls, delay maturation, disrupt metabolism or prevent pathogen reproduction
Photo Dr. Art Ayers
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Active genetic defense: the plant immune system
• The plant immune system carries two types of receptors: one type senses molecules outside and the other one senses molecules inside the cell
• Both systems detect intruders and respond by activating antimicrobial defenses in the infected cell and neighboring cells
Peter N. Dodds* and John P. Rathjen, Nature Genetics:, 11:539
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The plant immune system: recognition of pathogen pattern
• First tier of the plant immune system based on the recognition of common pathogen components (it’s called Pathogen Associated Molecular Patterns; PAMP)
• Usually essential to the pathogen and do not exist in host; e.g. chitin, the major component of the fungal cell wall, is a PAMP
Corné M J Pieterse, Antonio Leon-Reyes, Sjoerd Van der Ent & Saskia C M Van Wees Nature Chemical Biology 5, 308 - 316 (2009) Published online: 17 April 2009
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The plant immune system: the pathogen fights back with effectors
• Pathogen effectors are proteins that are delivered inside the host
• Effectors manipulate the host to facilitate development of disease
• Effectors can block the host response to PAMPs and make it possible for the pathogen to invade host cells
Pathogen effector
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The plant immune system: the host responds with R-genes
• Plants have developed receptors that are activated by effectors (commonly called resistance genes or R-genes)
• R-gene products are proteins that recognize specific pathogen effectors, can directly bind to the effector and activate the immune response
Host R-gene receptor
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The plant immune system: hypersensitive response
• Following recognition of a pathogen and/or of effector, the plant triggers a hypersensitive response (HR)
• HR causes the rapid death of the host cells in the local region surrounding an infection
• The HR restricts the growth and spread of pathogens to other parts of the plant
No hypersensitive response Hypersensitive response
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Recapitulation of tree defense
Host penetration: openings, injuries, vectors
PAMP triggered immunity
Bark, stomata closing, defensins
PAMP: e.g. chitin
Secrete effectors
Effector receptor leading to immunity, HR
Constitutive defense
Induced defense