Measuring cell wall loosening: the constant load extensometer

Plant growth is achieved by a combination of cell division and cell growth. The rate at which a plant cell elongates is mainly regulated by cell wall extensibility.

Cell wall loosening can be measured with constant load extensometers. The specimen is located with 2 clamps in a small chamber, so it can be bathed in a buffer. The buffer can be changed during the experiment.
One of the clamps is connected to a pulling weight, pulling on the specimen with a force relative to 5 to 30 g weight. The extension of the specimen is recorded with a Linear Variable Differential Transformer (LVDT). Positional data obtained from the LVDT can be analysed by a computer.

When pieces of plant tissue are used as cell wall specimens, the tissues are first frozen and thawed; in this manner most cell wall protein activity remains intact.The cuticle of the tissues is abraded to access the apoplast, and the tissues are pressed under a weight to sqeeze out excess fluid. The tissues are clamped in the extensometers. Our device allowes us to measure up to 10 pieces of tissue at the same time.

Various enzymes can loosen the cell wall, and expansins are considered the most important. Expansin activity is regulated by pH, it’s optimum for cell wall loosening activity is pH 4.5 and it is inactive at neutral pH.The increase in extension rate between pH 4.5 and pH 6.8 is a measure for the activity of acid-induced cell wall loosening enzymes.