Development and Defence at the Cell Wall Flashcards
(26 cards)
What are the main roles of cell walls in plants?
- Define and limit crop growth
- Needed for pathogen defence
- Crucial for environmental stress responses
- Can be made into plastics/biomaterials/chemicals
What are the two main types of cell walls and their characteristics?
- Primary walls: thin, flexible, found in growth-associated tissues
- Secondary walls: thick, lignified, have structural roles (e.g., in xylem)
What are the major components of cell walls and their roles?
- Cellulose - provides mechanical strength
- Hemicellulose - provides flexibility and crosslinking
- Pectin - controls porosity and crosslinking
- Lignin - provides rigidity, strength and water resistance
Explain the acid growth hypothesis
- Plant tissue extends rapidly when Auxin is added
- Auxin activates proton pumps
- H+ ions are pumped out of the cell, acidifying the cell wall
- This enables expansion of cell wall for growth
What are Expansins and their role in cell wall growth?
- Small cell wall localized proteins that induce cell growth
- Are pH dependent and activated by auxin-induced acidification
- Disrupt and temporarily sever cellulose-xyloglucan interactions
- Lead to wall loosening and cell elongation
What are some desirable crop traits influenced by cell walls?
- Improved biomass for more productive biofuels
- Enhanced digestibility for livestock/human nutrition
- Mechanical resilience (lodging) for better yield
- Water retention for drought resistance
- Pathogen defence for improved yield
How do cell walls respond to drought conditions?
- Cell walls undergo active remodeling during drought
- Changes in pectin components (both increases and decreases)
- Reduction in hemicellulose content
- Example: Sorghum mutants with reduced lignin show improved drought tolerance
How do cell walls function in plant defence?
- Act as first line of defence
- Provide ready-made barrier with no additional cost
- Composition determines effectiveness against pathogens
What are the two main pathogen attack strategies on cell walls?
- Physical structures:
- Formation of appressorium
- Building up turgor
- Creating infection peg
- Biochemical attack:
- Secretion of wall degrading enzymes (cellulases, xylanases, pectinases, xyloglucanases)
- These are among first substances secreted upon contact
What proportion of the cell wall is callose?
1-3%
Is callose synthesis slow or rapid?
Can be extremely rapid
It is synthesised by a protein complex that sits at the membrane until needed
Made from sucrose
When is callose deposited?
Callose is rapidly deposited at cell wall upon pathogen detection
(A number of hours after detection)
It reinforces the cell wall in the short term, preventing pathogen spread
Why is callose not a long term solution for protection?
Callose lacks structural strength
It is replaced with more permanent structures
Callose remodelling only happens at the site of pathogen attack
What are peroxidases?
They interact with ROS and cross-link lignin with other cell wall components by condensation reaction
How do plants know they are being attacked?
Pathogen associated molecular markers (PAMPs) - On the pathogen surface, secreted by pathogens
Damage associated molecular markers (DAMPs) - Released by plant cells upon damage, cell wall degrading enzymes released, fragments of cell wall act as DAMPs. Activated the plant immune system (pattern recognition receptor-triggered immunity (PTI)
What are wall associated kinases (WAKs)?
- Detect damage of cell wall integrity, activate the immune system
- Proteins that span the membrane
- Three domains: intracellular, extracellular and cell wall
- The extracellular domain binds pectin, involved in cell expansion
What happens if WAKs are removed?
More intense infection
What is Homogalacturonan (HG) composed of, what does it do??
A linear chain of galacturonic acid (GalA) residues
Major component of pectin
Modifications of it allow for complex signalling at the cell wall
What pectic polysaccharides can be derived from modified HG?
Acetylated HG
Xylogalacturonan (XG)
Rhamnogalacturonan-II (RG-II)
What is the structure of Rhamnogalacturonan-I (RG-I)?
An alternating backbone of rhamnose (Rha) and galacturonic acid (GalA) with side chains
What percentage of Rha residues in RG-I are substituted with side chains?
20-80%
Can the GalA in RG-I be modified?
It can be methyl-esterified or acetylated, similar to HG
What was found when studying the abundance of cell wall components in plants under fungal infection or drought?
Responses are different
Not a universal stress response
Infection causes huge callose increase, but drought does not
Infection causes decrease in simple pectin but drought does not
-> Drought and infection induce different cell wall responses
However, complex pectin increased under both stresses - could this overlapping response improve stress tolerance?
Can cell walls be primed for tolerance?
Mild drought stress primed plants for greater resistance to biotic stress
Complex pectin, more fragments = potentially more DAMPs
