THE PLANT CELL WALL & CELL DYNAMICS

Plant Cell Wall definition

• Coat of polysaccharides and proteins that surrounds the plant cell, external to the plasmalemma

• major long-term sink of carbon in biosphere (cellulose & lignin)

• dynamic: sensitive to environment, development, stress

CW-Functions/ Biological Importance

• structural support for plants (via turgor and lignin)

• gives plant cells shape- tied  to cell development

• protection from pathogens, acts as stress sensors

• facilitates water movement between cells (capillary action)

•  primary contact with environment

Importance of the CW

• importance of industrial fiber (cotton, flax, hemp)

• health / dietary fiber (ß-glucans in the diet positively affect cholesterol, insulin levels)

• source of ‘solar’ energy

Primary CWs

• Can expand, found in all plant cells

• permeable to small molecules only

• larger molecules may go thru plasmodesmata

Secondary CWs

• Develop only in specialized cells

• more xylan and less xyloglucan, much
  glucomannose

• in tracheids, fibers, sclereid cells
  (mechanical support)

Cell Wall components

1. cellulose microfibrils (cellulose)

2. Matrix( Hemicellulose, pectin polysaccharide)

3. CW proteins

4. Srimary CW

5. Secondary CW

6. Ligin

CW proteins

• hydroxyproline-rich glycoproteins (HRGPs)

• glycine-rich proteins (GRPs)

• proline-rich proteins

• arabinogalactan-proteins (AGPs)

Lignin

• high MW polymer

• predominantly in xylem (wood)

• Fill spaces between fibers, making the wall rigid and resistant to compression

What pH condition causes the plant cell wall to extend?

Low (acidic) pH

What happens to the cell wall at high pH

• Expansins inactive

• No extension or growth

phytopathogenic bacteria

• cannot enter cells due to cell wall (but can get inside leaf via stomata, or wound)

• remain extracellular, and kill plant cells 'from the outside' to absorb nutrients

• secrete toxins and CW-degrading enzymes (i.e., pectate lyase) into the plant

Fungal Pathogens

• Necrotrophs 

• BIotrophs

elicitors

molecules that are recognized by the plant & stimulate defense

Pectinase

• act as an alarm signal for elicitors

• secreted to soften & separate plant cells

  • make easier to invade tissues and spread

Crux Issues: Plant CW as a source of biofuels

• Biofuels problem: need a transportable form of energy

• CW sugars can be fermented to ethanol by microbes

• Where are the bottlenecks?

  • CW evolved in part to resist microbial attack

  • the sugars are not easily accessible to microbial enzymes (crystalline cellulose)

  • Hemicelluloses are very variable in composition

Strategies and attempts to modify the CW for cellulosic biofuels

• decrease proportion of crystalline cellulose

•  reduce H-bonding of xyloglucan to cellulose, modify the structure

• less pentose (arabinose, xylose), more hexose (mannose, glucose)
  iv) reduce lignin content