Reversible stress softening and stress recovery of cellulose networks

The mechanical properties of the plant cell wall play an important role in communication, differentiation and growth of plant cells. In particular, the expansive growth of plant cells is only possible because the material that forms the plant cell wall yields at a specific stress level, allowing for an increase in cell wall area. This process can be defined in mechanical terms as the relaxation of the wall stress. The current understanding of plant cell growth suggests that such relaxation behaviour occurs due to the presence of wall loosening agents rather than to the properties of the cellulose network itself. It is believed that certain nonenzymatic proteins, acting as wall loosening agents, produce the yielding phenomenon that leads to wall stress relaxation. Here we suggest that cellulose networks display certain mechanical properties that might change our understanding of plant cell growth, indicating that it is probably the cellulose network itself, and not the so-called loosening agents, that is the main factor that determines wall stress relaxation. We show that cellulose networks synthesised by Gluconacetobacter bacteria exhibit reversible stress softening and stress recovery. Reversible stress softening of cellulose networks should play an important role during the wall relaxation process, while stress recovery protects the structural integrity of plant cells at higher loads. The understanding of both stress softening and stress recovery shall allow for the development of more realistic models of plant cell expansive growth.