Cell Wall—structure and function The three layers 1). The middle lamella (M) 2). The primary wall (1) 3) The secondary wall (2) (some cells may not have 2nd wall)

The composition M 1 2 Cellulose + + + Hemicellulose + + + Pectin + + + Protein + + ? Lignin - - +

3. Molecular structure cellulose: at least 500 glucose molecules 1,4-beta-glycoside; cellulose fibrils: intra- and inter-molecular H-bonds make the cellulose molecules bundle together into a fibril. Hemicellulose: mixture of cellulose backbones with side chains. Can interact with fibrils via backbones. Pectins: poly-galacturonic acid (from galactose) with many COO- negatively charged easily bind metals especially calcium forming a “gel”—jello. Middle lamella to “cement” cell wall of linked cells. Some cells such as mesophyll cells that need air/CO2 for photosynthesis may have cracked middle lamella (rich in air space) Proteins: cell wall proteins are very important for the function and dynamics of the wall. Often have a lot of prolines and other hydrophilin amino acids for interaction with cellulose and other carbohydrates. A typical primary wall is combination of the above molecules: fibrils embedded in a mixture/matrix of polysaccharides and proteins. Such interaction involve both covalent and non-covalent (H-bond).

4. Biosynthesis complexity: involve a number of organelles (ER/Golgi, PM) more than 100 enzymes and a dozen of monosaccharides (precursor sugars). Both linear and branched connections. Model systems to study wall synthesis: protoplast and cell division. Wall-less cell—how to make it by enzyme digestion. If protoplasts intend to survive, the first thing is to generate new wall. During cell division: the preprophase band (arrow)--microtubule bundle that marks the plane for cell division. At telophase, the polar spindles and golgi vesicles form structure called phragmoplast (arrow)

Cell plate—the phragmoplast recruit more vesicles that fuse into a membrane-like structure made of polysaccrides. The cell plate extend to connect with the cell wall of the mother cell—the new wall is born, so is the new cell. Cell plate 3). Biosynthesis of cellulose Monomer glucose (photosynthesis or starch during germination) Activation step: P-glucose+UTP = UDPG+Ppi UDPG transport to the cell wall site (not sure how) UDPG polymerization: addition of G to the chain by cellulose synthase very important enzyme (genes cloned from plants—a big family of genes) one of the genes was identified by screening for “weak cell wall” mutant. e. Deposition of cellulose and orientation of the fibrils: cellulose is rigid because It is bundled together with the rest of the fibril as soon as made. Membrane-enzyme move during the synthesis. The orientation of the fibrils are controlled by microtubule and actin filament…

5. Wall composition and cell type The primary cell wall of all cells are rather similar in composition, but the secondary wall is very different that marks the specific features of the cell type. For example, lignin is rich in supporting tissues that need a lot of strength (lignin is a polymer formed by cinnamic acids—aromatic ring structure—tough). Cutin and wax (polyester of long chain fatty acid; hydrocarbon/fatty alcohol) are found in epidermal layer (surface) for prevention of water loss because the materials are hydrophobic—think about a leaf without wax/cutin layer (cellulose is hydrophilic and loss water easily). 2) Cell wall dynamics and plant development: Cell wall is not “fixed” after its synthesis, it is constantly changing during development. The formation of the xylem vessel and sieve tubes—living cells go through cell death and wall thickening; ripening of fruits involves cell wall degradation; falling leaves is due to formation of abscission zone in the petiole (degrading cell wall); maturation of cotton fiber…

6. Cell wall function Cell support: from giant redwood trees to small seedlings of Arabidopsis. What support them? Mechanism different! 2) Cell shape: 90% water—no shape but shape of water container. Wall is also referred to as “little wooden box” but the box is elastic. What control the shape of the cells? The orientation of the fibrils. Fat seedling Thin seedling 3) Cell expansion: cell wall loosening is a critical step of cell growth. How to loosen up the wall? Enzymes and expansins—enzymes that cleave the covalent bonds; expansins are family of wall proteins that break H-bonds in the fibrils. After wall loosening, enzymes re-build larger wall by adding new materials. Discovery of expansins—a case study.

Measuring wall-loosening activity by assaying cellulose paper strength Extract proteins from cell wall---bioassay Fractionate proteins and ---bioassay Purify proteins—bioassay Sequence proteins—deduce DNA sequence and clone gene (library screening or directly get the sequence from the genome database!) Express recombinant protein and confirm activity by bioassay. 4) Cell fate: some work indicates that wall determines fate of the cell during development. The fucus experiment: A Laser surgery A B A B A B B 5) Cell adhesion and cell-cell recognition: pectin cement, proteins may serve as adhesion molecules (some evidence). In the pollen tube extension inside the pistil tissues during pollination and fertilization processes, some evidence on cell wall proteins as recognition signal. In the pollen germination and compatibility between the pollen and stigma, small proteins may serve as signals for receptor-like protein kinases—a signaling pathway (details later).

6). Cell wall and defense: “the great wall”—a physical barrier for invaders. Chemical enforcement during infection/attack by wall thickening and lignifying; wall fragments as signaling molecules to trigger a systematic defense process involving gene expression, cell death, or wall thickening etc… 7). Cell-cell communication and development: many cells communicate by “talking” to each other during development/ Small molecules such as hormones and peptides all need to go through the wall to target cells. Some of the receptor-like protein kinases are connecting cytoplasm, plasma membrane, and cell wall and play a key role in cell growth. This is WAK—next guest lecture.