Plant Pathogens and Diseases

Question 1

what are 5 facts about plant pathogens ?

Answer 1

1. Plant pathogen is an organism able to cause disease in particular plant host
2. May be virus, bacteria or fungi
3. May affect different stages of plant life cycle
4. Of major significance in world food/crop production
5. Have certain features enabling them to cause disease.

Question 2

How do viruses transmit disease into plants ?

Answer 2

1. Viruses cannot directly penetrate the plant surface
2. They need a vector - which transmit or disperses pathogens
3. May also be transmitted into the seed.

Question 3

How does bacteria enter a plant to infect it ?

Answer 3

1. Enters plant through natural opening or wound sites.
2. They cannot directly penetrate plants, but they can cause wounds by secreting enzymes that break down the plant cell wall
3. Some are mobile by flagella

Question 4

How is bacteria spread ?

Answer 4

Spread through water, wind, farm equipment, muddy shoes, insects.

Question 5

what is fungi and how is it spread and what can it effect ?

Answer 5

1. Plant pathogenic fungi are collectively those species that derive nutrients from plants and have a negative effect on plants health
2. Generally transmitted between plants as spores. Wind,water,insect-dispersed.
3. Can affect harvested fruits.

Question 6

Explain how fungal spores attach and germinate on plants ?

Answer 6

1. Fungal spores are dispersed by wind, water or insect vectors
2. Spores attach to various surfaces including host or non host plants – attachment prevents spores from being washed away from the host surface before penetration
3. Many pathogenic fungi are triggered to germinate and penetrate plants in response to chemical and surface signals.

Question 7

Explain from the fungi germinating on plant then forms an apresorium ?

Answer 7

1. The spores germinate ( producing a germ tube )
2. Fungi can invade plants by direct penetration of the plant epidermis ( also through wounds )
3. To penetrate the plant surface, the hyphal/germ tube trip differentiate into a swelled structure ( appressorium)
4. The appressorium tightly attaches to the host surface producing a penetration peg
5. 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
6. Physical pressure + enzymatic degradation of the host cuticle and cell wall enable the peg to penetrate host tissues.

Question 8

What’s host colonisation ?

Answer 8

1. Pathogens must obtain nutrients from plants to develop and reproduce
2. They must also neutralise any plant defence responses

Question 9

What host dependencies do plant pathogenic fungi have ?

Answer 9

1. Fungi depends entirely on their host - obligate pathogen
2. Fungi establish a close association with the plant but can complete their life cycle off the plant - facultative pathogens

Question 10

How does plant pathogenic fungi get nutrition from plant ?

Answer 10

1. 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’
2. 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.
3. HEMIBIOTROPHS - Combine biotrophic and neurotrophic nutrition.

Question 11

What does biographic fungi comprise of ?

Answer 11

Comprises of those species that can only feed on living host tissue to meet their nutritional requirements and complete their life cycle.

Question 12

What does obligate biotrophs depend on ?

Answer 12

depends on their host for completion of their life cycle and match the life cycle of their host plants.

Question 13

What is facultative biotrophs - ( non obligate biotrophs ?

Answer 13

They can survive and live without the host.

Question 14

What is neurotrophic fungi and what does it do to plants ?

Answer 14

1. Necrotrophic fungi feed on dead tissue. They attack and kill healthy plants.
2. The initial contact of the pathogen is with a living tissue.
3. The pathogen sneaks in and subverts the plant defences and generate a zone of dead tissue
4. Infection progresses as the necrosis spreads.

Question 15

What is nectrotrophic fungi and what does it do to plants ?

Answer 15

1. Neutrophil fungi feed on dead tissue. They attack and kill healthy plants
2. The initial contact of the pathogen is with a living tissue
3. The pathogen sneaks in and subvert the plant defences and generate a zone of dead tissue
4. Infection progresses as the necrosis spreads

Question 16

What is hemibiotrophic fungi ?

Answer 16

Combines biographic and neutrophic lifestyles
Have a variable length of a biographic phase before switching to necrotrophy.

Question 17

What is tobacco mosaic virus ?

Answer 17

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.

Question 18

Where are bacterial diseases present and what are the symptoms ?

Answer 18

1. Tumour-like growths.
2. Present in soil.

Question 19

What are 4 different resistance that make plants resistant to most microbes ?

Answer 19

1. General innate resistance
2. Host-specific resistance
3. Systemic resistance
4. Plant-to-plant communication

Question 20

What are 5 mechanisms that prevent a plant species from getting infected by a pathogen ?

Answer 20

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

Question 21

What are some ways plants have developed genetically inherited ways of recognising these electors/effectors that microbes have developed to overcome plant defences ?

Answer 21

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.

Question 22

What is quantitive and vertical resistance ?

Answer 22

1. QUANTATIVE - Numerous genes have small additive effects
2. VERTICAL RESISTANCE - Resistance is either complete immunity if the resistance gene is present or complete susceptibility if it is absent

Question 23

Explain the gene for gene theory between plants and pathogens ?

Answer 23

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 )

Question 24

What is hypersensitive response commonly considered as an effective defence strategy for ?

Answer 24

against biographic and hemibiotrophic plant pathogens, which require living tissue to gain nutrients.

Question 25

What is systemic resistance ?

Answer 25

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

Question 26

What is the difference between systemic acquired resistance and induced systemic resistance ?

Answer 26

SAR = Is generally used to define responses generated through plant hypersensitive response )
ISR = Is the resistance induced by some neurotrophic fungi and insects.

Question 27

Explain how plants communicate chemically between each other ?

Answer 27

1. Plants possess systemic signals which induce resistance to pathogens
2. 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