For the first half of this lecture, I concentrated on the oat (Avena) shoot gravitropic response as elucidated by Peter Kaufman. This was a slide show, mostly recapping the previous lecture.
The additional information in this lecture dealt mostly with the details unique to oats. The important additional information had to do with cell wall structure leading toward a mechanism of how cells grow.
It is critical to remember that plant cells have a cell wall. This wall is a basket-weave of cellulose microfibrils. These are cross-linked with other polysaccharides. Peter Albersheim spent his career studying the composition of sycamore cell walls.
The cross-linking polymers include hemicelluloses (xyloglucans and rhamnoglacturonans) and pectins galacturonic acid polymers. Together, the microfibrils and cross-links make the wall very ridgid. It can withstand the turgor pressure induced by osmosis in hypotonic environments. Most plant cells exist in that kind of setting.
If the cell is surrounded by the equivalent of rebar (microfibrils) and concrete (hemicelluloses and pectins) how can the cell possibly expand in growth?
The cellulose fibrils cannot be cut, or the turgor would cause the cell to burst in the area where the fibrils are cut. The turgor pressure causes the membrane to open. Not good.
Instead, for the cell to grow, the hemicellulose and pectin cross links must be severed. This will allow the microfibrils to remain intact (holding back turgor, perhaps less-well) but to slide past one another. This permits the cell to expand.
If cellulose microfibrils are oriented mostly in the transverse plane (around the cell like a belt), the removing of cross links will allow turgor to drive the cell to elongate. In another cell, if the microfibrils are oriented mostly in the axial plane (up-over-and-down the cell), then the cell will expand laterally. If the microfibrils are oriented randomly, then the cells will expand in all directions, remaining isodiametric. Thus the orientation of a subcellular structure determines the direction of cell growth. It explains the developmental difference between a conducting cell of xylem or phloem and an adjacent cortical or pith cell.
Obviously, then, for a cell to expand, the cell must produce an enzyme to cleave the cross-links between cellulose microfibrils. These would be xyloglucanases or rhamnogalacturonases. To get these enzymes in the cell wall to accomplish the cross-link clipping, the enzymes translated in the cytoplasm must be secreted out across the cell membrane. This will not be diffusion, facilitated transport, or active transport. These enzymes are proteins. They must be secreted by exocytosis...the dictyosome and vesicle mechanism.
I also showed a few slides about heliotropism which operates in a pattern not unlike that of phototropism.
Go back to the Course Schedule.