Lecture 4

Cell Wall

I. Functions of the cell wall

A. Mechanical functions Figure 1 (from Cosgrove 2005)
1. acts as a skeleton, supports individual cells and ultimately the whole plant
2. it determines the shape and size of the cell
3. it determines the limit that the cell can swell
4. a uniquely plant structure, separating plants from all other organisms
5. It limits the amount of water that can go into the cell

B. Functions that indicate metabolic activity, demonstrate that it is a living part of the cell
1. receives signals such as plant growth regulators (auxins) that allow the cell to expand
2. receives signal from pathogens, a perception of herbivore attack
3. signals from the wall are transmitted to the cell membrane which in turn initiates a biochemical response in the cell, such as the production of an alkaloid or lignin deposition that may function as a barrier to the pathogen

C. Other functions of the wall
1. nourishment. The seed endosperm cell walls in Diospyros (persimmon) store mannan: β (1-4) linked mannose. The plasmodesmata and pit connections are apparent.
2. absorption or secretion - root hairs, rhizoids and transfer cells

II. Macromolecular components of the cell wall

A. Polysaccharide components of the cell wall. Nature Review: Building cell walls by L. Smith. Exploring Life @ bio.edu
 
1. Cellulose
a. β (1-4) linked D-glucose in a long, unbranched molecular chain. Recall that amylose (from starch) is α (1-4) linked glucose.
b. hydrogen bonding between the glucose molecules forms a micelle, a crystalline-like structure in a lattice arrangement. Figure
c. a number of micelles (bundles) together form a microfibril
d. microfibrils are in turn arranged in bundles to form macrofibrils.

2. Hemicellulose - a highly branched polysaccharide that forms hydrogen bonds with cellulose and so "glues" everything in place, connecting microfibrils and stabilizing the cell wall. The most common type in dicot primary cell walls is xyloglucan. At least three models for how the hemicellulose molecules binds together the cellulose microfibrils: tether, diffuse layer, stratified layer.

3. Pectins – diverse, but mainly polygalacturonic acid and rhamnogalacturonic acid. Are very hydrophilic and form a gel.
a. 30% of wall is pectin by dry weight
b. abundant in the middle lamella, region between the primary walls of adjacent cells
c. forms stable, viscous gels via water trapping
 d. calcium bridges form between pectin molecules (calcium pectate); highly acidic in pH range of cell walls, thus forming electrostatic bonds with other molecules
e. hydrophilic properties enable the cell wall to be loose and pliable, plastic. Also influences the permeability of the cell wall.
4. Callose – linear β (1-3) D-glucan, deposited between the plasma membrane and the primary cell wall.
a. most often seen associated with the sieve elements of phloem
b. also seen at site of pathogen attack and wounding; seals off plasmodesmata between adjacent cells
c. also seen in pollen tube development, secondary wall synthesis in fibers of cotton, cell plates in dividing cells.
d.  can be stained using resorcin blue or aniline blue

B. Protein components of the cell wall
 
1. enzymes (many, such as transferases, hydrolases, peroxidases, phosphataes, proteases, oxidases)
2. glycoproteins e.g. elastin, lectins
3. structural proteins - hydroxyproline rich glycoproteins (HRGP) that probably function in cross-linking the microfibrils preventing expansion of the wall.  Extensins are a family of hydroxyproline-rich glycoproteins that give mechanical strengthening by binding cellulose microfibrils together. [Don't confuse with expansins that ARE involved in cell wall expansion]

C. Lignin
1.  Are phenolic polymers of three main monomers: p-coumaryl, coniferyl, and sinapyl alcohols. Possible structure.
2.  Secondary walls are often are impregnated with lignin, which replaces pectin.
3. Lignins strengthens the wall, making it waterproof; esp. important for xylem
4.  makes cells resistant to decay and to attack by microbes and herbivores.
5. deposition of lignin on the wall is irreversible
6. can be stained with phloroglucinol in HCl (turns cells purple-red) - called the Wiesner test.

III.  Cell wall layers

A. Two types of cell walls: primary (1˚) and secondary (2˚).
1. Primary – laid down while cell is expanding, starting with the cell plate.  Cellulosic microfibrils randomly arranged, lots of pectin. Found in all plant cells
2.  Secondary – produced after plant cell has ceased growing. Found in cells with thick walls (e.g. sclerenchyma). Laid down from inside, three layered, impregnated with lignin. Found in some plant cells.

B.  Region of union of adjacent cells – middle lamella which cements the two primary walls together (often best seen in intercellular spaces, at corners between cells). Composed of pectin.

C. Primary (1˚) and secondary (2˚) walls compared
1. all cells have 1˚ walls, not all cells have 2˚ walls
2. timing of deposition: 1˚ laid down first, 2˚ after
3. 1˚ walls are deposited in cells that are elongating, 2˚ walls are often deposited in cells that have ceased elongation
4. 1˚ walls are stretchable, 2˚ walls are elastic, if contorted they spring back to shape, they are elastic and resilient, not easily broken
5. 2˚ walls often are impregnated with lignin, 1˚ walls rarely have lignin.
6. Structure and composition differences. Figure Figure
a. 1˚ walls - the microfibrils are laid down randomly in a ± criss-crossing fashion. 1˚ mostly laid on top of existing microfibrils, called apposition. 2˚ sometimes interwoven, called intussusception
b. 2˚ walls - microfibrils are often deposited in bundles in the same direction (macrofibrils), more cellulose and more dense cellulose. Typically laid down in 3 layers: S1, S2, S3
1) Sl - next to l˚ wall, farthest from protoplasm. S1 microfibrils are oriented in two directions by apposition, on top of one another, not interwoven; stretchable
2) S2 - the thickest layer, microfibrils are laid down at the steepest pitch (almost right angles to S1 microfibrils)
3) S3 - adjacent to protoplasm or lumen, not as steep a pitch and not as thick as S2, in opposite direction as S2

IV.  Plasmodesmata and Pits

A. Plasmodesmata.  Holes in the cell walls of adjacent cells, with contiguous plasma membranes, thus allowing cell to cell movement of materials.  Appear as dots in surface view.  Two types distinguished: primary and secondary.
1.  Primary – occur during cell division by trapping ER in growing cell wall (see below). The ER inside the plasmodesmatal space is condensed forming the desmotubule surrounded by a cytoplasmic annulus (or sleeve).
2.  Secondary – occur between two previously formed cells.  ER lines up along plasma membrane in each cell across a thin cell wall.  Enzymes break down the wall and the ER invade the space, forming a new plasmodesma. These types are often branched on one or both sides of the middle lamella. They may interconnect at the median cavity formed in the middle lamella.
3. Plasmodesmata may occur in pits or not.

B. Pits – definition and structure
1.  Cells with 2˚ walls show depressions called pits.  When the pits of two adjacent cells line up, they form a pit pair. The thin wall in the center of the pit, formed from the two cell walls, is called the pit membrane (primary walls and middle lamella). When secondary walls are laid down, none is deposited on the pit membrane, so the pit is a discontinuity in the secondary wall.
2.  Cells with only 1˚ walls – depressions are called primary pit fields (or primordial pits, primary pits).  This area is penetrated by plasmodesmata.

C. Types of pits. Diagram.
1.  Simple pits. Secondary wall of same thickness, abruptly ends forming the pit aperture. Combination of two: simple pit-pair.
2.  Bordered pit.  Secondary wall extends over the aperture. Combination of two: bordered pit-pair.
3.  See diagram and terminology: aperture, pit membrane, torus, pit cavity, border, margo, half-bordered pit-pair.
4.  In pines, the middle portion of the membrane is thickened (torus) and the outer parts (margo) with openings formed by removal of the noncellulosic parts of the primary wall. When sapwood is converted to heartwood, the membrane moves and functions like a valve, closing the pit (aspirated state).
5. If secondary wall is thick, forms a pit cavity, outer aperture differs in shape from inner aperture (round vs. a slit).

V. Origin of primary cell wall during cell division

A.  Cytokinesis generally follows karyokinesis during late anaphase. Diagram of mitosis from the formation of the preprophase band of microtubules to appearance of phragmoplast.

B.  Unlike animal cells (that pinch in from the outside - explanation), plant cells have a cell wall that must be loosened for cell division to occur. A partition called the cell plate forms in the middle and grows to the outside, eventually connecting to the mother cell wall. Diagram from Esau.

C. But the first step is the formation of the phragmoplast, an assemblage of microtubules that are the remnants of the mitotic spindle fibers. The microtubules depolymerize where the cell plate forms and repolymerize at its growing margin.

D. The cell plate is a young cell wall (not the middle lamella as previously thought).  It arises through fusion of Golgi vesicles deposited at the equatorial plane of the phragmoplast.

E. The process of cell plate formation and maturation is complex (Figure from Samuels et al. 1995). It involves the fusion of Golgi vesicles and the formation of a network (the tubulo-vesicular network).  This network composed of cell wall polysaccharides (including cellulose and callose) traps ER which become plasmodesmata between the two daughter cells.  At later stages the cell plate becomes a fenestrated sheet (Swiss cheese-like!) and its margins connect up with the mother cell wall.  A new middle lamella grows from the existing one of the mother cell inward between the two daughter cells.

G. Video of cell plate formation during plant cell division.

VI.  Growth and development of the cell wall

A.  Cell wall expansion (or extension) requires a number of factors: respiration, polysaccharide and protein synthesis, and loosening of the existing primary (mother) cell wall. Turgor pressure inside the cell contributes pressure from the protoplast. The acid-growth hypothesis: auxin activates proton-pumping ATPases, as well as associated expansin proteins, seems the best explanation for how cell walls expand.

B.  Fusion of the vesicles to form the cell plate is followed by deposition of additional wall material on both sides. The new wall material is also deposited over the old mother cell wall so each daughter cell has a complete primary wall.

C.  As mentioned above, growth of the wall in thickness occurs by apposition (units one on top of each other) and intussusception (units interwoven). Cellulose may be laid down via both of these types whereas lignin and cutin are likely deposited by intussusception.

D. Cellulose is laid down as microfibrils whereas other polysaccharides such as hemicellulose and pectin and glycoproteins are contributed by Golgi vesicles. Figure 2 (from Cosgrove 2005)

E. Cellulose is synthesized by cellulose synthase complexes located on plasma membrane. These complexes appear as circular units called rosettes that span the plasma membrane. They are the site where cellulose microfibrils are synthesized. Figure 3 (from Cosgrove 2005).  The process is quite complex Figure (from Lerouxel 2006).

F. What determines how the microfibrils are laid down (pattern, direction) is complex and not well understood.  Some evidence suggests it is controlled internally by microtubules whereby they guide the rosettes as they move along the plasma membrane surface. See also pages by Wasteneys, Crowell et al., Emons and Mulder.

References and Links

Cosgrove, D. J. 2005. Growth of the plant cell wall. Nature Reviews Molecular Cell Biology 6: 850-861.

Crowell EF, Gonneau M, Stierhof Y-D,  Hφfte H, Vernhettes S. 2010. Regulated trafficking of cellulose synthases. Current Opinion in Plant Biology 13 (6): 700-705. HERE.

Emons AMC, Mulder BM. 2000. How the deposition of cellulose microfibrils builds cell wall architecture. Trends in Plant Science 5(1): 35-40. HERE.

Lerouxel O., Cavalier D. M., Liepman A. H., Keegstra K. 2006. Biosynthesis of plant cell wall polysaccharides — a complex process. Current Opinion in Plant Biology 9 (6): 621–630. HERE.

Samuels AL, Thomas H. Giddings TH Jr., and Staehelin LA. 1995. Cytokinesis in Tobacco BY-2 and Root Tip Cells: A New Model of Cell Plate Formation in Higher Plants. The Journal of Cell Biology 130 (6): 1345-1357.

Wasteneys G. O. 2004. Progress in understanding the role of microtubules in plant cells. Current Opinion in Plant Biology 7(6): 651-660. HERE.
Last updated: 10-Oct-22 / dln