microbial interactions with plants 1

Question 1

Plant pathogens comprise virus, bacteria, and fungi being fungi the most frequent plant pathogens. Some of them cause serial economic and health issues.

Specific symptoms associated to the disease may help to identify the pathogen.

To know characteristics of pathogens and their life cycles are very important for their control and a key step in “good agricultural practices”.

Answer 1

…

Question 2

Plant microbiome

Answer 2

comprises plant-associated microbial communities able to profoundly effect on seed germination, seedling vigor, plant growth and development, nutrition, diseases, and productivity.

Question 3

Introduction

Effects on flowers –

Answer 3

mottling, reproductive parts replaced by fungus. Micellium where flowers supposed to be]

Question 4

Introduction

Effects on leaves –

Answer 4

chlorosis (yellowing), spots, wilting, etc.

Question 5

Introduction

Effects on fruits –

Answer 5

fail to ripen, premature fall, in situ rotting

Question 6

Introduction

Effects on stems –

Answer 6

cankers or lesions. If vascular tissue becomes blocked, shoots wilt or lose leaves.

Question 7

Introduction

Effects on roots –

Answer 7

galls, dieback, typically lead to wilting and rapid plant death.

Question 8

Pathogenic viruses
Symptoms:
4 points

Answer 8

2. Chlorosis: tissue yellowing [mainly on leaves]
3. Necrosis: browning [brown spots]
4. Mosaic pattern [characteristic of viruses. Selecting cells to infect]
5. Plant stunting [bottom pic]

Question 9

5 pathogenic viruses going to look at

Answer 9

Tobacco mosaic virus

Tomato yellow leaf curl virus

Cucumber mosaic virus

Cauliflower mosaic virus

Question 10

pathogenic viruses
Tobacco mosaic virus TMV
3 points

Answer 10

1. Infects tobacco, but many other plants too e. g. tomato, pepper, cucumbers, potatoes…
2. Symptoms: light green coloration between the veins of youngleaves at first, with the development of a “mosaic” or mottled pattern of light and dark green areas. Rugosity and stunting.

Question 11

pathogenic viruses
Tomato yellow leaf curl virus TYLCV
2 points

Answer 11

1. Infects tomato, eggplants, potatoes, tobacco, beans, and peppers.
2. Symptoms: severe stunting, reduction of leaf size, upward cupping/curling of leaves, chlorosis [reduction of photosynthetic pigment on leaves] on leaves and flowers, and reduction of fruit production. This virus can cause significant yield losses (90-100%).

Question 12

Pathogenic viruses

Cucumber mosaic virus CMV

Answer 12

1. Infects: First detected in cucumbers, but present the widest host range (melon, pepper, tomato, beans,…).
2. Symptoms: ”mosaic”, mottling, yellowing, ringspots, stunting, and leaf, flower and fruit distortion.

Question 13

Pathogenic viruses

Cauliflower mosaic virus CaMV

Answer 13

1. Infects mostly Brassicaceae family (cauliflower and turnips) and Solanaceae species [melon cucumber…].
2. Symptoms: ”mosaic”, necrotic lesions on leaf surfaces, stunted growth, and deformation of the overall plant structure

Question 14

Pathogenic bacteria

Problems mainly associated are:

Answer 14

1. post-harvesting rots
2. spoilage
3. degradation by extracellular enzymes. e. g. fruits.

Question 15

Pathogenic bacteria

focus on 4

Answer 15

1. Pseudomonas syringae pathovars
2. Ralstonia solanacearum
3. Agrobacterium tumefaciens
4. Xanthomonas oryzae pv. oryzae

Question 16

Pathogenic bacteria

4 Modes of action

Answer 16

1. Cell wall-degrading enzymes:
   used to break down the plant cell wall in order to release the nutrients inside. Used by pathogens such as Erwinia to cause soft rot.
2. Toxins:
   these can be non-host specific, and damage all plants, or host specific and only cause damage to a host plant. e.g. Pseudomonas syringae – phaseolitoxin (affecting lima beans).
3. Production of phytohormones:
   e. g. Agrobacterium changes the level of auxin to cause tumours.
4. Exopolysaccharides:
   these are produced by bacteria and block xylem vessels, often leading to the death of the plant e.g. bacterial leaf scorch pathogen, Xylella fastidiosa

Question 17

Pathogenic bacteria 6 symptoms

Answer 17

1. Bacterial spots
2. Cankers
3. Bacterial galls
4. Bacterial vascular wilts
5. Bacterial soft rots
6. Bacterial scabs

Question 18

Pathogenic bacteria
Pseudomonas syringae
3 points

Answer 18

1. Infects many plants (peas, soybean, wheat, barley, beets, …), exist more than 60 pathovars
2. Symptoms: necrotic leaf spots, discoloured and blackened leaf veins, spots and blisters on fruits, stem cankers.
3. Gram-negative

Polar flagella

Produces bacterial effector proteins secreted by type III secretion system [manipulate plant so plant cant
recognise pathogenic bacteria until it is inside plant]

Question 19

Pathogenic bacteria
Ralstonia solanacearum
3 points

Answer 19

1. Infects more than 200 plant species of 50 families (eggplant, pepper, potato, tobacco, tomato, …).
2. Symptoms: bacterial vascular wilts, stunting, brown discoloration of infected tissues, milky-white exudates [polysaccharide placed by bacterium used to block vessels in plants]
3. Gram-negative
   Polar flagella
   Infection through roots wounds

Question 20

Pathogenic bacteria
Xanthomonas oryzae
3 points

Answer 20

1. Infects major host is rice, but also infected other grasses and sedges.
2. Symptoms: caused blight, leaf spots, leaves of young plants become pale-green to grey-green, cankers. Systemic infection results in wilting, desiccation of leaves and death.
3. Gram-negative
   Polar flagella

Question 21

Pathogenic bacteria
Agrobacterium tumefaciens
3 points

Answer 21

1. Infects more than 140 plant species of eudicots (apple, cherry or almond trees, roses, sugar beets, tomato, beans, etc).
2. Symptoms: generate a crown gall tumour, with uncontrolled growth. Usually does not kill the plant but may weaken and be more susceptible to drought and other diseases.
3. Gram-negative
   Lophotrichous flagella
   Infection through wounds, used in biotechnology.

Question 22

Pathogenic fungi and fungi-like organisms

2 points

Answer 22

1. Most abundant plant pathogens causing important economic losses.
2. Categories:
   Necrotrophs: kill tissue and live off remains. e. g apple rot.

Facultative biotrophs: can infect and survive on living tissue, but also on dead tissue. e. g. potato blight.

Obligate biotrophs: only infect living cells. e. g. powdery mildews

Question 23

Pathogenic fungi and fungi-like organisms

focus on 3

Answer 23

Botrytis cinerea

Fusarium oxysporum

Blumeria graminis

Question 24

Pathogenic fungi and fungi-like organisms
Botrytis cinerea
4 points

Answer 24

1. Necrotrophic fungus
2. Disease: grey mould.
3. Infects wine grapes but also other fruits as berries and many other crops.
4. Symptoms: burns in leaf edges, grey felting, shoots tend to be weaken and break.

Question 25

Pathogenic fungi and fungi-like organisms
Fusarium oxysporum
4 points

Answer 25

1. Necrotrophic fungus
2. Disease: Fusarium wilt.
3. Infects extremely broad host range, including animals and plants (gymnosperms and angiosperms), exemplified by banana, potato, tomato, eggplant or pepper.
4. Symptoms: older leaves first affected, clearing on the younger leaves and drooping of the older lower leaves, followed by stunting of the plant, yellowing of the lower leaves, defoliation, marginal necrosis and death of the plant.

Question 26

Pathogenic fungi and fungi-like organisms

Phytophthora infestans

Answer 26

1. Facultative biotroph fungus-like
2. Disease: late blight or potato blight.
3. Infects potatoes and other members of Solanaceae family, like the tomato.
4. Symptoms: at first, the spots are grey-green and water-soaked, but they soon enlarge and turn dark brown and firm, with a rough surface. Appearance of dark blotches on leaf tips and plant stems and white mould will appear under the leaves in humid conditions.

Question 27

Pathogenic fungi and fungi-like organisms

Haustorium:

Answer 27

specialized structure from pathogenic fungi to acquire nutrients from plants. The tip of the hyphae penetrates the plant cell wall and ingrowth of a bulbous extension without penetrate the plant cell membrane.

Question 28

Control of plant pathogens

4 points

Answer 28

1. Chemical control –

fungicides (originally based on
sulphur and copper, now organic chemicals)

could lead to environmental problems.

2. Good agricultural practices – disease forecasting
3. Use of resistant varieties
   (plant breeding – genetic engineering)
4. Biological control
   Antibiosis
   Nutrient competition
   Destructive mycoparasitism

Question 29

Control of plant pathogens

Chemical control advantages and disadvantages

Answer 29

1. cost effective
2. easy to apply
3. broad spectrum
4. environmental problems
5. human health problems
6. yearly treatments

Question 30

Control of plant pathogens

Biological control advantages and disadvantages

Answer 30

1. host specific
2. non-contaminant
3. non-toxic to human
4. may last for years
5. expensive
6. labor intensive

Question 31

good agricultural practice - Produce safe, healthy, high-quality food for consumers.

Answer 31

1. Integrated pest management (IPM)
2. Tool disinfection
3. Prevent microbial contamination
4. Water control
5. Use of specific products for the disease
6. Minimization of chemicals
7. Integrated crop management (ICM) [fertilisers, how manage crop]