The Role of Wall Ca super(2+) in the Regulation of Wall Extensibility During the Acid-induced Extension of Soybean Hypocotyl Cell Walls

We examined the acid-facilitated yielding properties of cell walls of soybean hypocotyls and the effects of Ca super(2+) upon the properties by stress-strain analyses using glycerinated hollow cylinders (GHCs) from the elongating regions of the hypocotyls. Stress-extension rate curves of native GHCs showed characteristic changes with pH, all indicating the existence of yield threshold tension (y) as well as wall extensibility ({phi}), i.e. a downward shift of y and an increase in {phi} with wall acidification. The acid-induced downward shift of y was inhibited by boiling of GHCs. In contrast, a considerable increase in {phi} with acidification remained even after boiling. This indicates that {phi} consists of two components, i.e. heat-sensitive and heat-resistant, both being pH sensitive. A Ca super(2+) chelator (Quin 2) dramatically increased {phi} at a neutral pH. Subsequent addition of Ca super(2+) or ruthenium red suppressed the chelator-induced increase in {phi}. These findings suggest that wall Ca super(2+) plays an important role in the regulation of wall extensibility during the acid-induced wall extension by reacting with carboxyl groups of wall pectin.