Plant cell walls have a special ability to expand without breaking or weakening, which is crucial for plant growth. New research has shown that the cellulose skeleton of the plant cell wall is what makes this possible. In a new study reported in Science, researchers have found that there are bundles of cellulose chains, which strengthen a plant cell, but these bundles can slide against one another when the cell has to stretch, giving it flexibility. This work has provided new insight into plant growth, and may help engineers create new polymers with these properties.
"For a long time, the prevailing concept of a plant cell wall has been that of a gel that is reinforced by cellulose fibers, with the stiff cellulose rods acting like steel rebar in cement," explained senior study author Daniel Cosgrove, a professor of biology at Penn State. "However, we determined that cellulose chains instead stick to each other to form a network of cellulose bundles, which provides a lot more mechanical strength than disconnected rods floating in a gel. And it's the cellulose chains, rather than other components, that limit cell wall expansion, sliding alongside each other like an extension ladder when the cell is stretched."
Unlike previous work, which has zeroed in on the very fine details of cell walls, or taken a large view of the bigger picture, this research looked at a 'just-right' model of the polymers in the cell wall. An onion cell wall was used in this model so the researchers could stretch actual onion skins to test its validity. This work revealed more about the structures that give plant cell walls their unique characteristics.
"Unlike many other models, we also accounted for the molecules' tendency to stick together by modeling the noncovalent bonding between them," noted Cosgrove. "This allowed us to investigate the consequence of interactions between the chains."
"Plant cells walls are unique because they must be very strong to help protect and support the plant and very extensible because they must expand when the plant grows," said first study author Yao Zhang, a postdoctoral researcher in biology at Penn State. "We found that the cellulose microfibers carry most of the stress and are key to the cell wall maintaining both its strength and extensibility."
This model could be useful for studying many other types of plants too. The scientists want to make the model larger so it eventually is a size that's comparable to a whole cell.
"Our technology currently can't match a plant's ability to create such a strong and extensible material," said Yao. "The design of plant cell walls may provide inspiration for the design of green materials with a variety of applications."