Beneficial plant microbe interactions

Question 1

What are some examples of good plant microbes?

Answer 1

Plant growth-promoting (rhizo)bacteria (PGPR/PGPB)
Arbuscular mycorrhizal fungi (AMF)
Other fungi (Trichoderma)

Question 2

Where do PGPB colonise?

Answer 2

The rhizosphere, the rhizoplane (root surfaces), the root itself (within radicular surfaces), and the pylosphere

Question 3

What percentage of plant colonising bacteria promote growth?

Answer 3

1-2%

Question 4

What type of bacteria does PGPB tend to be?

Answer 4

Mostly Gram-negative bacteria

Question 5

What di biofertilising PGPB do?

Answer 5

Produce stimulatory bacterial volatiles and phytohormones
Lower the ethylene level in plats
Improve the plant nutrient status through the liberation of phosphates and micronutrients from insoluble sources and through symbiotic and non-symbiotic nitrogen fixation

Question 6

What are the 5 major types of phytohormones?

Answer 6

Auxins (AUX) 
Gibberellins (GA)
Cytokines (CK)
Abscisic acid (ABA)
Ethylene (ET)

Question 7

Where are auxins produced?

Answer 7

Biosynthesis is found in some fungi, protozoa and extensively in plant-associated bacteria

Question 8

What do auxins do?

Answer 8

Have a central role in coordinating plant growth and development and are quantitatively the most abundant phytohormone synthesised by bacteria

Question 9

How are auxins produced?

Answer 9

The indole-3-pyruvate (IPA) pathway is a major auxin pathway in plant beneficial bacteria
Bacterial biosynthesis of indole-3-acetic acid (IAA) triggers changes in endogenous levels of IAA in the plant, resulting in an upregulation that stimulates the growth and cell differentiation

Question 10

What does auxin production rely on?

Answer 10

[IAA]
IAA diffusion
Bacterial competitiveness and survival

Question 11

What is the main precursor of auxins?

Answer 11

L-Try

Question 12

What do the effects of auxins vary because of?

Answer 12

The exogenous concentration

Question 13

What does a low concentration of auxin give?

Answer 13

Promotion of primary root growth

Question 14

What does a medium concentration of auxin give?

Answer 14

Promotion of lateral and adventitious root emergence

Question 15

What does a high concentration of auxin give?

Answer 15

Inhibition of root system development

Question 16

What does ethylene do in a plant?

Answer 16

It plays an important role in root initiation and elongation, nodulation, senescence, abscission, ripening, and stress signaling

Question 17

What does ethylene do when produced endogenously?

Answer 17

Regulates xylem formation, flowering in some plants, induces fruit ripening and flower wilting and as part of a stress response it inhibits root elongation, nodulation, and auxin transport, induces hypertrophies, speeds aging, and promotes senescence and abscission

Question 18

What happens when ethylene is applied exogenously?

Answer 18

Causes adventitious root formation and root hair initiation and also begins the process of fruit ripening, flower wilting, and leaf senescence

Question 19

Where is ACC deaminase ( 1-aminocyclopropane-1-carboxylate deaminase) found?

Answer 19

In PGPB, fungi, and yeast but not in plants

Question 20

What does ACC deaminase do?

Answer 20

It cleaves the plant ethylene precursor ACC and thus lowers ethylene levels in a developing or stressed plant

Question 21

What happens to plants inoculated with PGPB containing ACC deaminase?

Answer 21

They are more resistant to environmental stress (temp/salinity/water deficit etc)

Question 22

How do some PGPB solubilise phosphate?

Answer 22

They produce metabolites that lead to the lowering of the pH and release of phosphate

Question 23

Why are siderophore producing PGPB important?

Answer 23

Iron is a vital element requires for many cellular processes and is not readily available in aerobic environments due to the low solubility of iron (III)

Question 24

What are siderophores?

Answer 24

Low molecular weight compounds (500-1500 Da) that possess a high affinity and selectivity for iron (III)

Question 25

What are diazotrophs?

Answer 25

N2 fixing bacteria

Question 26

What is biocontrol?

Answer 26

Any condition whereby survival or activity of a pathogen is reduced through the agency of another living organism (except by man) with the result that there is a reduction in the incidence of disease caused by pathogens

Question 27

Where can biocontrol PGPB be found?

Answer 27

They can be isolated from roots and other parts of plants

Question 28

What modes of action do biocontrol PGPB have?

Answer 28

Competition for nutrients
Niche exclusion
Production of metabolites
Induced systemic resistance

Question 29

How can PGPB biocontrol through competition for nutrients and niche exclusion?

Answer 29

They can prevent the proliferation of pathogens by producing siderophores and can also colonise roots and exclude other microorganisms from that ecological niche

Question 30

How can PGPB biocontrol through metabolite production?

Answer 30

They can produce antibiotics, fungicides, and enzymes such as;
broad-spectrum antibiotics 
Lactic acid (lactobacilli)
Lysozymes
Exotoxins
Bacteriocins

Question 31

How can PGPB biocontrol through induced systemic resistance?

Answer 31

Specific recognition between the plant and the ISR inducing PGPB 
Potential inducers;
Lipopolysaccharides
Siderophores
Secreted metabolites 
Quorum sensing molecules
Flagella

Question 32

What are PGPF?

Answer 32

Mycorrhizal fungi that can promote plant growth by root colonisation