Plant Pathogens and Diseases Flashcards
what are 5 facts about plant pathogens ?
- Plant pathogen is an organism able to cause disease in particular plant host
- May be virus, bacteria or fungi
- May affect different stages of plant life cycle
- Of major significance in world food/crop production
- Have certain features enabling them to cause disease.
How do viruses transmit disease into plants ?
- Viruses cannot directly penetrate the plant surface
- They need a vector - which transmit or disperses pathogens
- May also be transmitted into the seed.
How does bacteria enter a plant to infect it ?
- Enters plant through natural opening or wound sites.
- They cannot directly penetrate plants, but they can cause wounds by secreting enzymes that break down the plant cell wall
- Some are mobile by flagella
How is bacteria spread ?
Spread through water, wind, farm equipment, muddy shoes, insects.
what is fungi and how is it spread and what can it effect ?
- Plant pathogenic fungi are collectively those species that derive nutrients from plants and have a negative effect on plants health
- Generally transmitted between plants as spores. Wind,water,insect-dispersed.
- Can affect harvested fruits.
Explain how fungal spores attach and germinate on plants ?
- Fungal spores are dispersed by wind, water or insect vectors
- Spores attach to various surfaces including host or non host plants – attachment prevents spores from being washed away from the host surface before penetration
- Many pathogenic fungi are triggered to germinate and penetrate plants in response to chemical and surface signals.
Explain from the fungi germinating on plant then forms an apresorium ?
- The spores germinate ( producing a germ tube )
- Fungi can invade plants by direct penetration of the plant epidermis ( also through wounds )
- To penetrate the plant surface, the hyphal/germ tube trip differentiate into a swelled structure ( appressorium)
- The appressorium tightly attaches to the host surface producing a penetration peg
- The appressorium applies a turgor-driven mechanical force onto the penetration peg that ruptures the plant surface at the base of the appressorium and grows into the underlying plant epidermal cell
- Physical pressure + enzymatic degradation of the host cuticle and cell wall enable the peg to penetrate host tissues.
What’s host colonisation ?
- Pathogens must obtain nutrients from plants to develop and reproduce
- They must also neutralise any plant defence responses
What host dependencies do plant pathogenic fungi have ?
- Fungi depends entirely on their host - obligate pathogen
- Fungi establish a close association with the plant but can complete their life cycle off the plant - facultative pathogens
How does plant pathogenic fungi get nutrition from plant ?
- BIOTROPHS - Feed on living tissue without killing the hosts, so they avoid recognition and divert plant photo assimilates for their own use. They are the ‘Conmen’
- NECROTROPHS - Kill the hosts and feed using enzymes and toxins to affect the structure integrity of plant cells. They then develop off the nutrients that are released. They are the THUGS.
- HEMIBIOTROPHS - Combine biotrophic and neurotrophic nutrition.
What does biographic fungi comprise of ?
Comprises of those species that can only feed on living host tissue to meet their nutritional requirements and complete their life cycle.
What does obligate biotrophs depend on ?
depends on their host for completion of their life cycle and match the life cycle of their host plants.
What is facultative biotrophs - ( non obligate biotrophs ?
They can survive and live without the host.
What is neurotrophic fungi and what does it do to plants ?
- Necrotrophic fungi feed on dead tissue. They attack and kill healthy plants.
- The initial contact of the pathogen is with a living tissue.
- The pathogen sneaks in and subverts the plant defences and generate a zone of dead tissue
- Infection progresses as the necrosis spreads.
What is nectrotrophic fungi and what does it do to plants ?
- Neutrophil fungi feed on dead tissue. They attack and kill healthy plants
- The initial contact of the pathogen is with a living tissue
- The pathogen sneaks in and subvert the plant defences and generate a zone of dead tissue
- Infection progresses as the necrosis spreads
What is hemibiotrophic fungi ?
Combines biographic and neutrophic lifestyles
Have a variable length of a biographic phase before switching to necrotrophy.
What is tobacco mosaic virus ?
Its a viral diseases
Symptoms on plants with viral mosaic diseases show an overall lighter colour along with mosaic patterns ( alternating light and dark green areas ) on at least some leaves, especially on the younger leaves.
Where are bacterial diseases present and what are the symptoms ?
- Tumour-like growths.
- Present in soil.
What are 4 different resistance that make plants resistant to most microbes ?
- General innate resistance
- Host-specific resistance
- Systemic resistance
- Plant-to-plant communication
What are 5 mechanisms that prevent a plant species from getting infected by a pathogen ?
ANATOMICAL FEATURES -
1. Cuticle, layers of wax and cellulose fibres prevent pathogen entry
2.Bark on trees
3. Each plant cell also surrounded by cellulose wall.
VECTOR -
Insect vector may not feed on particular plant species
INNATE IMMUNITY -
Plant has an innate defence response that allows it to recognise microbes as non-self and defend itself. Bacteria and fungi have molecules on their surface that are recognised by plants - referred to as PAMPs
SIGNALS - Plant has wrong surface structure/lack of appropriate chemical signals to stimulate pathogen
PEFORMED INHIBATORS -
Produce toxic metabolites to pathogens - phenols, alkaloids, tannins, anti-microbial proteins
What are some ways plants have developed genetically inherited ways of recognising these electors/effectors that microbes have developed to overcome plant defences ?
Plants do not produce antibodies and do not have a circulatory system.Therefore, each cell must be capable of recognising pathogen attack and then including a defence against it.
What is quantitive and vertical resistance ?
- QUANTATIVE - Numerous genes have small additive effects
- VERTICAL RESISTANCE - Resistance is either complete immunity if the resistance gene is present or complete susceptibility if it is absent
Explain the gene for gene theory between plants and pathogens ?
The pathogen has a gene encoding a product (effector), which is recognised by a (receptor ) encoded by a plant resistance gene
This recognition process triggers the (hypersensitive response )
The plant resistance gene is dominant
Pathogen gene is (avirulence gene )
What is hypersensitive response commonly considered as an effective defence strategy for ?
against biographic and hemibiotrophic plant pathogens, which require living tissue to gain nutrients.
What is systemic resistance ?
Once plant has undergone pathogen attack, it is often better able to resist subsequent attack - both in other parts of plant AND by other pathogens
What is the difference between systemic acquired resistance and induced systemic resistance ?
SAR = Is generally used to define responses generated through plant hypersensitive response )
ISR = Is the resistance induced by some neurotrophic fungi and insects.
Explain how plants communicate chemically between each other ?
- Plants possess systemic signals which induce resistance to pathogens
- There is also evidence that plants under attack from pathogens and insects emit volatile messages ( methyl salicylate and methyl jasmonate ) These are detected by neighbouring plants which respond by acquiring enhanced resistance
