plant pathogens

plant pathogens

importance of plants:

  • oxygen, food, environment, wildlife, carbon storage

  • also textiles, biofuels, medicines (anti-microbials), vaccines

impact of plant pathogens:

  • most are fungi others include nematodes, viruses, bacteria and protist parasites

  • cost global economy 220 billion a year

  • food security and livelihoods

  • loss of plant species and their dependent organisms

  • climate change is likely to increase spread and change distribution of plant pathogens and pests (increased rain)

  • cost of mitigation strategies

plant pathologies:

  • necrosis → death of cells, spots and rot on leaves

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  • soft rot → enzymes degrade plant tissues (rotting veg)

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  • wilt → lose turgor in leaves (affect water transport in plant)

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  • blight → discolouration, wilting and death of foliage

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  • cankers → dead sections of bark

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  • gall → tumourus growths

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Irish potato famine:

  • also affects tomatos

  • mass starvation due to successive epidemics of phytophthora infestans

    • oomycete (protist) related to brown algae and diatoms

  • synthesises cellulose cell walls → exert pressure and destroy host cells

  • 100,000 deaths

  • ireland 1845-1855

    • poor farmers gave their cereals to English landlords → diet relied on potatoes

    • 1 million people died and 2 million emigrated

    • evicted from land

  • dispersed by sporangia

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control

  • rapidly adapts to control measures

    • huge highly repetitive genome

    • effector genes important for infection show rapid expansion and turnover

  • intensive use of fungicides

    • target use when weather is more likely to spread blight (windy/stormy)

  • destroy foliage before they lift potatoes

  • breed resistant cultivars → genetic modification

    • introduce Rpi resistance genes from wild potato species

    • complicated process

    • CRISPR/Cas9 technology may help

Fusarium wilt of banana:

  • globally most traded fruit

  • provides food security and income

  • fusarium oxysporum fungus causes banana wilt (Panama disease)

  • Gros Michel banana variant was highly susceptible so switched to Cavendish variety

  • now being threatened by TR4 Fusarium (new form)

control:

  • hard to eradicate → persists in soil and infects via roots, symptoms take time to appear

  • commercial monoculture means few options to find resistant strains

  • prior exposure to avirulent strain can offer temporary protection

  • new fungicides with multiple targets → multi-site fungicides

Dutch elm disease:

  • caused by fungi Ophiostoma ulmi and O. novoulmi

  • discovered and isolated in Holland

  • produces fruiting bodies and spores

spread:

  • vector → elm bark beetle

  • breed young in bark of elm trees → sticky spores

  • root grafts → roots fuse and join

  • infected logs

pathogenesis:

  • fungus blocks xylem channels causing wilt (affects water conduction) → produces polysaccharides and glycoproteins

  • host defence response can also plug vessels

  • shepherd’s crook as they die back from tip

  • fungus has enzymes that degrade plant cell walls causing them to invade the xylem

significance:

  • kill mature trees → cant support the beetles

  • new species is highly aggressive

  • now there are very few mature elms outside cordons

  • regions where they preserve them and are highly monitored

  • young seedlings are getting big enough to support beetles so it is resurfacing

control:

  • early reporting and action

  • reduce elm bark beetle population

    • insecticides

    • remove bark/branches

    • removal of elm firewood piles

  • prevent/destroy root grafting

  • prune only when bark beetle dormant

  • use fungicides (topical or by injection)

  • can’t really eliminate once tree is infected (cut down)

  • disease-resistance elms → new varieties

    • takes time for saplings to support beetles

Ash dieback disease:

  • similar to dutch elm disease

  • wind borne fungal pathogen

  • Hymenoscyphus fraxineus (previously chalara fraxinea)

  • trees become dangerous → blow down

  • need chop down

challenges for phytopathogens:

plants as microbial habitats:

  • vary greatly in temperature (night and day)

  • very basic transport/communication system

    • inefficient transport of microbes within plant

  • surface microbes → oxygen, organic matter, light and UV radiation

  • root/soil microbes → less variable environment, high nutrient levels, less light

  • vulnerable to extreme weather

getting in:

  • waxy coatings on leaves and stems → defence mechanism

  • get in via gas/water pores or wounds

  • can multiply in intercellular spaces

  • nematodes can get in by inserting stylet into cell

  • fungi use hyphae to invade in between cells, form haustoria to enter the cells

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Plant immune response:

  1. Pathogen-associated molecular pattern (PAMP)

  • triggered immunity (PTI)

  • general immune response

  • works against many pathogens

  • fairly weak

  1. Effector-triggered immunity (ETI)

  • stronger immune response to specific pathogen

  • longer term systemic acquired resistance (SAR)

  1. in-built tolerance to some microbes eg: mycorrhizae

PAMP-triggered immunity:

  • PAMPS such as bacterial flagellin, LPS, peptidoglycan, fungal chitin

  • recognised by plant pattern recognition receptors (PRRs)

  • PRRs are transmembrane proteins with extracellular ligand-binding domain and intracellular kinase domain

  • production of host defensive molecules to impede pathogens getting any further

  • pathogens produce avirulence (Avr) effector proteins to avoid PTI

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effector-triggered immunity:

  • plant resistance (R) protein receptors recognise pathogen effectors molecules → stronger ETI response

    • NLRs → intracellular nucleotide-binding domains and leucine-rich repeat proteins

  • R proteins can trigger ETI by sensing damaged plant molecules

    • damage-associated molecular patterns (DAMPs)

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  • triggers Ca2+ signalling, reactive oxygen species (ROS) and accumulation of pathogenesis-related proteins such as phytoalexins

  • activate programmed cell death at infection site → hypersensitive response

  • changes in plant hormone levels (salicyclic acid and jasmonic acid) → SAR

results of pathogen detection:

  • cell wall modification → thickens with added defensive compounds, more resistant to turgor pressure and hydrolytic enzymes

  • closure of stomata

  • production of ROS

  • hypersensitive response

  • production of anti-parasite compounds and proteins (phytoalexins, chitinases, defensins, protease inhibitors)

management of plant pathogens:

  • avoidance → where, when, how you plant; quarantine; crop rotation

  • therapy → fungicides; antibiotics

  • eradication → heat/fumigate bulbs, seeds, soil; cut off affected part of plant; destroy diseased plant material

  • breeding resistant/genetically modified plants

  • biocontrol eg: bacteria and fungi as antagonists

  • describe a range of bacterial, protozoal, viral and fungal pathogens of animals

  • explain strategies organisms employ to avoid host defences during infection

  • consider the importance of animal husbandry

  • describe the consequences of these pathogens on the animals and the methodologies employed to control these