Beneficial plant microbe interactions Flashcards
What are some examples of good plant microbes?
Plant growth-promoting (rhizo)bacteria (PGPR/PGPB)
Arbuscular mycorrhizal fungi (AMF)
Other fungi (Trichoderma)
Where do PGPB colonise?
The rhizosphere, the rhizoplane (root surfaces), the root itself (within radicular surfaces), and the pylosphere
What percentage of plant colonising bacteria promote growth?
1-2%
What type of bacteria does PGPB tend to be?
Mostly Gram-negative bacteria
What di biofertilising PGPB do?
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
What are the 5 major types of phytohormones?
Auxins (AUX) Gibberellins (GA) Cytokines (CK) Abscisic acid (ABA) Ethylene (ET)
Where are auxins produced?
Biosynthesis is found in some fungi, protozoa and extensively in plant-associated bacteria
What do auxins do?
Have a central role in coordinating plant growth and development and are quantitatively the most abundant phytohormone synthesised by bacteria
How are auxins produced?
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
What does auxin production rely on?
[IAA]
IAA diffusion
Bacterial competitiveness and survival
What is the main precursor of auxins?
L-Try
What do the effects of auxins vary because of?
The exogenous concentration
What does a low concentration of auxin give?
Promotion of primary root growth
What does a medium concentration of auxin give?
Promotion of lateral and adventitious root emergence
What does a high concentration of auxin give?
Inhibition of root system development
What does ethylene do in a plant?
It plays an important role in root initiation and elongation, nodulation, senescence, abscission, ripening, and stress signaling
What does ethylene do when produced endogenously?
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
What happens when ethylene is applied exogenously?
Causes adventitious root formation and root hair initiation and also begins the process of fruit ripening, flower wilting, and leaf senescence
Where is ACC deaminase ( 1-aminocyclopropane-1-carboxylate deaminase) found?
In PGPB, fungi, and yeast but not in plants
What does ACC deaminase do?
It cleaves the plant ethylene precursor ACC and thus lowers ethylene levels in a developing or stressed plant
What happens to plants inoculated with PGPB containing ACC deaminase?
They are more resistant to environmental stress (temp/salinity/water deficit etc)
How do some PGPB solubilise phosphate?
They produce metabolites that lead to the lowering of the pH and release of phosphate
Why are siderophore producing PGPB important?
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)
What are siderophores?
Low molecular weight compounds (500-1500 Da) that possess a high affinity and selectivity for iron (III)
What are diazotrophs?
N2 fixing bacteria
What is biocontrol?
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
Where can biocontrol PGPB be found?
They can be isolated from roots and other parts of plants
What modes of action do biocontrol PGPB have?
Competition for nutrients
Niche exclusion
Production of metabolites
Induced systemic resistance
How can PGPB biocontrol through competition for nutrients and niche exclusion?
They can prevent the proliferation of pathogens by producing siderophores and can also colonise roots and exclude other microorganisms from that ecological niche
How can PGPB biocontrol through metabolite production?
They can produce antibiotics, fungicides, and enzymes such as; broad-spectrum antibiotics Lactic acid (lactobacilli) Lysozymes Exotoxins Bacteriocins
How can PGPB biocontrol through induced systemic resistance?
Specific recognition between the plant and the ISR inducing PGPB Potential inducers; Lipopolysaccharides Siderophores Secreted metabolites Quorum sensing molecules Flagella
What are PGPF?
Mycorrhizal fungi that can promote plant growth by root colonisation
