On The Mechanical and Dynamic Properties of Plant Cell Membranes: Their Role in Growth, Direct Gene Transfer and Protoplast Fusion

In plants the rate of endocytotic-like and exocytotic-like membrane exchange between the plasma membrane and an intracellular membrane reservoir depends upon a mechanical parameter, the tension of the plasma membrane. In this paper we discuss the relevance of this striking property to physiological reactions such as plant cell growth and reversible volume changes, and its significance in applications such as direct gene transfer to isolated protoplasts and protoplast fusion. The initial event in plant cell growth is the turgor-driven yielding of the cell wall. We argue that wall yielding will generate a tension in the plane of the plasma membrane and that it is this tension which controls the net incorporation of membrane material into the plasma membrane during cell growth. Studies on isolated protoplasts revealed that tension-induced plasma membrane expansion was dependent upon the transmembrane electrical potential difference and upon the level of free intracellular and extracellular Ca2+. These parameters are also known to influence plant cell growth in a manner which supports the hypothesis that it involves similar mechanisms. We furthermore propose that polyethylene glycol (PEG)-mediated DNA transformation of protoplasts relies on the uptake of DNA via tension-dependent endocytotic vesiculation. Finally, we argue that such vesiculation is also involved in protoplast fusion.