Turgor pressure and polymer contributions to plant enlargement: Chara corallina as a model system

Turgor pressure (P) provides the driving force for cell wall enlargement during plant growth, and this study reveals additional roles for P in wall deposition in internode cells of the alga, Chara corallina. Growth was inhibited in Chara cells when P was decreased with a pressure probe or by exposure to external osmotica. Reduced growth was accompanied by reduced wall deposition of labeled carbon. Uptake of inorganic carbon, exocytosis and enzyme activity were unaffected by low P, indicating that the P-responsive step was wall deposition. The in vivo delivery of new wall polysaccharides to the inner face of the growing wall was simulated in vitro by injecting solutions of dextran polymers or gold colloid into isolated walls followed by oil that pressed the solutes against the wall and created an "artificial" P. This provided one of the first measurements of cell wall pore size at full P. The wall pores were about 4.6 nm in diameter permitting entry of small solutes at any P. However, high P allowed entry of dextran with diameters larger than the pores. P created a concentration gradient and mechanical forces that moved large polymers into pores by end insertion or polymer deformation.; The P- and temperature-sensitivity of growth in living cells was retained in isolated, boiled walls under artificial P. However living cells sustained growth and wall deposition longer than isolated walls. Attempts to replace wall deposition in isolated walls showed, for the first time in a plant system, growth activity of a natural cell wall constituent (poly-galacturonic acid (PGA)). PGA removed cations from the walls thus loosening interpolymer divalent cross-links. These links were the load-bearing bonds controlling the rate of wall extension. These observations lead to a new, non-enzymatic model for in vivo growth based on wall loosening by deposition of matrix polymers and wall hardening by Ca2+ from the external medium. The model also allowed development of the first tentative mechanism for stored growth in a plant cell, due to delayed wall deposition at low P.