Lecture 5: Biotic Interactions Flashcards
Symbiosis (Beneficial Interactions)
-Many plants form root association with soil bacteria/fungi
-Mutual cooperation: microorganisms increase the supply of essential nutrients to the plant
-Plants give sugars produced by photosynthesis.
-Mutual recognition and development of symbiotic structures
e.g.: nitrogen-fixing bacteria, mychorrizal fungi
Mycorrhizae (symbiosis)
Symbiotic relationship with phosphorus
Nitrogen-fixing bacteria (symbiosis)
Symbiotic associations between nitrogen-fixing bacteria (diazotrophs), and plants
Triggering of nodule formation (3 steps)
- Recognition
- Nod genes and factors
- Nodule development is triggered
Triggering of Nodule formation: Recognition
-Host plant and rhizobia recognize one another
-Plant roots secrete flavonoid compounds into the rhizosphere chemotactic response
-Each rhizobia species recognises specific flavonoids
Triggering of Nodule formation: Nod genes and factors
-Bacterial nod genes are expressed (common/host-specific)
Lip-chito-oligosaccharides, Nod factors:
-Nod factors are released by bacterium
-Initial recognition of Nod factors by plant roots is mediated by two RLKs
-present on epidermal cell plasma membranes.
Triggering of Nodule formation: Nodule development is triggered
Early response to the presence of Nod factors a transient, rapid increase in nuclear Ca2+ concentration in the root hair cells (calcium spiking)
Nodule formation
- Rhizobia enters the root hairs
- Nodule primordium:
-Cells in the root cortex and pericycle divide to form a nodule primordium
-The end result is a structure in which groups of the modified rhizobia, known as bacteroids, enclosed within modified root cells.
Nitrogenase
Enzyme which catalyses the conversion of atmospheric nitrogen to ammonia:
-Oxygen sensitive: Nitrogenase must be protected from oxygen in order to function
-Nodule interior: physical barrier
-Diffusion barrier: Suberin in
-Endodermis+periderm
-Chemical barrier: leghemoglobin
Leghemoglobin
-Chemical barrier which regulates oxygen availability
-delivers oxygen to the bacteroids only the amount needed to support respiration
-structure is similar to haemoglobin
-red-pink color
Harmful interaction response when crop production lost to pests: First line of defence
Mechanical barriers (thorns, spines, prickles, trichomes)
Second line of defence
Constitutive chemical defences: Primary & Secondary metabolites
-Phenolics: flavonoids, lignin
-Alkaloids
-Terpenoids
-Direct inducible defences, indirect inducible defences
Why the inducible defence response?
-Consitutive are expensive
-Pests and pathogens can adapt
-Inducible is more flexible
Herbivores
-Phloem feeders stylet into the sieve-tube (aphids)
-Cell-content feeders: mites and thrips
-Chewing insects: caterpillars, grasshoppers, beetles
General response
-Elicitors
-Damage: mechanical cause or biotic attack?
-Compounds in insects’ saliva
-Trigger defence responses in plants
-HAMPS: herbivore-associated molecular patterns
(act locally, spread systemically)
