Plant Cell Walls

What determines and predicts the position of a new plant cell wall?

The pre-prophase band of microtubules

What is pre-prophase?

Before the mitotic spindle forms but after the interphase microtubules have broken down

Where is the pre-prophase band of microtubules located?

just under plasma membrane

What organisms have the pre-prophase band structure?

Unique structure to plants and closely related green algae thought to be ancestral to Plantae lineage

When do new plant cell walls arise?

During cytokinesis (late cytokinesis)

When is the preprophase band of microtubules completely broken down?

At the end of prophase – cell "remembers" its orientation during cytokinesis

What determines the plane of cell division in animal cells?

The position of the mitotic spindle (perpendicular)

What is the term used to describe the developing cell wall?

Cell plate

When and in what direction is the cell plate formed?

Formed inside to outside in late anaphase/early telophase

What organizes the cell plate and where does it arise?

organized by the phragmoplast; which arises perpendicular to the position of the PPB of microtubules

What is the phragmoplast and what is it composed of?

microtubules, ER, vesicles

When does the phragmoplast arise?

Arises late telophase as spindle pole microtubules breaks down

How are microtubules aligned in the phragmoplast?

Microtubules align perpendicular to developing cell wall

How does the phragmoplast deliver cell wall components?

Utilize motor molecules (kinesins) to deliver vesicles carrying some of the cell wall components (proteins, polysaccharides)

What happens to some of the ER during cell plate development?

Some of the ER will become trapped in developing cell wall and extend between the two cells

What does ER trapped within the developing cell wall become?

Part of a plasmodesma

What are plasmodesmata?

Tubular channels of cytoplasm and ER that connect adjacent plant cells

Where does movement of substances occur in plasmodesmata?

Movement of substances is thru the cytoplasmic sleeve – not the ER tubule

What do plasmodesmata provide between plant cells?

Provide cytoplasmic continuity between plant cells

What types of substances communicate through plasmodesmata?

Transcription factors, gene transcripts

(mRNA's), small RNAs, hormones, proteins

What is the desmotubule?

the ocluded central rod

What is the wall collar made of?

callose (a glucose polymer)

How can the amount of callose regulate plasmodesmata?

The amount of callose can be increased to close off PD or it can be removed to increase access to PD

What proteins are localized to the spoke structures in plasmodesmata?

Actin and myosin have been localized to the spoke structures (may regulate opening/closing/restricting access to PD)

How do viral pathogens exploit plasmodesmata?

intercellular and systemic spread - often results in serious crop diseases

How are viruses able to move through plasmodesmata despite being large?

• regulate the size of the PD

• Use movement proteins (MP)

• mimic activity of endogenous plant movement proteins

What are the main components of plant cell wall composition?

Cellulose (glucose polymer), Hemicellulose (polysaccharide), Pectins (polysaccharide), Proteins (structural/enzymatic)

What additional components can some walls include?

Waxes/suberin/cutin (fatty acid derivatives), lignin (phenyl propenoids)

Where are proteins for the cell wall made?

Proteins - made in the ER and secreted into wall

Where are hemicelluloses and pectins made?

Hemicelluloses & pectins (carbohydrates) - made in Golgi and secreted into wall

Where is cellulose made?

Cellulose (carbohydrate) made in the plane of the membrane and extruded into cell wall

What processes occur in the ER for cell wall proteins?

Protein folding and N-linked glycosylation

What processes occur in the Golgi for cell wall proteins?

Protein glycosylation (O-linked) and modification of N-linked sugars, synthesis of pure polysaccharides

What is the middle lamella?

Outermost layer of plant cell wall

When is the middle lamella synthesized and secreted?

Synthesized and secreted during cell plate formation in late telophase of mitosis

What is the composition of the middle lamella?

carbohydrates called pectins

What is the function of the middle lamella?

• Glue adjacent plant cells together

• Plant cells don't/can't migrate during development

When is the primary cell wall synthesized?

Synthesized after middle lamella

What is the composition of the primary cell wall?

Carbohydrates: cellulose, hemicellulose, pectins, Proteins

What are the physical characteristics of the primary cell wall?

Thin and flexible characteristics

What two opposing properties must the primary cell wall have?

• Must be rigid enough to resist positive turgor pressure

• Must be extensible for cell expansion

What is cellulose's role in the primary cell wall?

Structural backbone

How are cellulose polymers held together?

Cellulose polymers are H+ bonded together

What higher order structure do cellulose polymers form?

cellulose microfibrils

What does H+ bonding between and within cellulose polymers produce?

• Produces a highly ordered

  (crystalline) structure

• Produces high tensile strength (= to steel)

• Resistant to chemical attack (tight packing)

How was each particle in the rosette formed?

Each particle was synthesized in ER, processed thru the Golgi and delivered to and incorporated into plasma membrane

What is the entire structure called that synthesizes cellulose?

• Entire structure is called a particle rosette

• Ring structure with 6 particles

What is the substrate for cellulose synthase?

UDP-glucose

What does the cellulose synthase enzyme do?

• Enzyme adds Glucose to polymer

• Releases UDP to cytosol (uridine diphosphate)

How many cellulose synthase enzymes are predicted in each particle of the rosette?

3 cellulose synthase enzymes

How many cellulose polymers would a rosette with 6 particle subunits produce?

produce a cellulose microfibril with 18 cellulose polymers

Where and how are hemicelluloses and pectins synthesized?

synthesized in golgi by membrane bound enzymes

Q: How are hemicelluloses and pectins transported to the wall?

Transported to wall in vesicles and exocytosed

What do hemicelluloses and pectins form around cellulose microfibrils?

a meshwork around the cellulose microfibrils

How do hemicelluloses bind to the cell wall?

Bind very tightly to cell wall

What do the side branches in hemicelluloses prevent?

Side branches prevent aggregation with each other

What is the function of hemicelluloses regarding cellulose microfibrils?

Hemicellulose positions/spaces apart the cellulose microfibrils in cell wall-may interact with multiple cellulose microfibrils

What property helps hemicelluloses resist compression forces?

Hydrophilic/hydrated-help resist compression

forces

How easily are pectins removed from the cell wall?

Easily removed from cell wall

What type of polymers are pectins and what do they form?

Hydrated polymers: form gels (think jelly)

What three functions do pectins serve?

• Resist compression forces,

• determine wall porosity,

• provide a charged surface

  for cell-cell adhesion (glue cells together)

What group do sugar residues in pectins often have?

Sugar residues often have carboxyl group (COOH)

What form do carboxyl groups take at pH 7.0 and what does this create?

• At pH 7.0 (wall is not expanding) COO- form predominates

• pectins can form salt bridges with Ca2+ or Mg2+

• creates a stiffer gel around the other wall components

What happens to carboxyl groups at pH 5.5?

• At pH 5.5 carboxyl group is protonated-COOH

• Salt bridges are released-helps loosen wall for expansion

How are carboxyl groups synthesized and how can they be modified?

• Carboxyl group (COOH) synthesized with methyl group attached;

• Enzymes (pectin esterases) can be secreted or activated in the wall to remove methyl groups

How are proteins added to the cell wall?

• Synthesized in ER

• modified in Golgi

• Transported to wall in vesicles and exocytosed

What are hydrolytic enzymes and what pH do they require?

Hydrolytic enzymes: require low/acidic pH (5.5-5.0)

What are hydrolytic enzymes and what do they do?

• Enzymes that break down wall polymers (pectinases, cellulases, hemicellulases)

• require acidic pH (5.0-5.5);

• weaken walls to allow expansion

What are expansins?

• Proteins that break hydrogen bonds between cellulose and hemicellulose

• require acidic pH

• catalyze cell wall expansion

What are peroxidases?

• Oxidative enzymes that create covalent linkages and lignification

• strengthen and rigidify cell walls

What is the role of chitinases?

Defense proteins that weaken exoskeletons of fungi and insects

What do structural proteins do and when are they added?

• Rigidify and strengthen cell walls

• Typically secreted after growth is complete

• can be induced by wounding or pathogens

What are key properties of structural proteins?

• Hydrophilic with high lysine content

• form hydrogen bonds and ionic bonds with pectins, hemicelluloses, and other proteins

What are secondary cell walls and when do they form?

• Thick, rigid walls in specialized cells

• produced internally to primary walls after cell expansion stops

• composed of cellulose, hemicelluloses, and lignin

How do cellulose microfibrils provide strength in secondary walls?

Laid down in 3 distinct orientations to resist compression, tensile, and bending forces

How is lignin synthesized and incorporated into walls?

Synthesized from 3 phenylpropanoid precursors (H, G, S types) in cytosol; transported via ABC transporters; enzymatically linked by peroxidase; fills spaces between polymers

What are the functions of lignin?

reates hydrophobic boundary by displacing water; provides strength and rigidity; important for water transport and support

What are the main functions of secondary cell walls?

Support (fibers) and water conduction (vessels, tracheids); resist mechanical stress and negative pressures during water transport; form wood tissue

How do lignin and cellulose function in defense?

Form physical barriers against pathogens; chemically resistant to breakdown; indigestible with no nutritional value