Plant cells are remarkably tough&emdash;their walls can withstand internal pressures five times greater than the air pressure in a car tire. But that very strength has been a long-standing enigma to plant scientists. How does such an unyielding structure "give" as a plant cell grows&emdash;a root cell, say, which may grow up to 100 times its original size before it pushes the root through the soil? Two biologists think they've figured out the mechanism that makes such growth possible. They discovered a unique protein that temporarily relaxes the rigid scaffolding that encases plant cells.
A plant cell wall consists of cellulose fibers embedded in a gel of carbohydrates, pectin, and protein. This mix of threadlike fibers in a gel matrix is remarkably strong, so strong that it would seem to allow no room for the cell to grow. But Simon McQueen-Mason of York University in England and his colleague Daniel Cosgrove of Penn State have found a protein in cucumbers and oat seedling that loosens these cellular straitjackets.
Expansin, as the researchers call the protein, attacks the chemical bonds that hold the cellulose fibers of the cell wall within their gel matrix. When the protein breaks these bonds-which are formed between molecules in the fiber and matrix that share a hydrogen atom&emdash;the cell can expand. As the cell expands, it makes new, longer fibers that bond with the gel matrix.
McQueen-Mason and Cosgrove tested the expansin protein on dead cucumber stems stretched taut between two clamps to mimic the stress on the cell wall created by pressure in the cell. They applied expansin to the stems and observed a dramatic response: the stems doubled in size within six hours. "This is the first protein shown to make cell walls expand significantly," says Cosgrove. "We now have a molecular entry into the mystery of plant-cell growth."