The role of wall Ca2+ 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 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 (φ), i.e. a downward shift of y and an increase in φ with wall acidification. The acid-induced downward shift of y was inhibited by boiling of GHCs. In contrast, a considerable increase in φ with acidification remained even after boiling. This indicates that φ consists of two components, i.e. heat-sensitive and heat-resistant, both being pH sensitive. A Ca 2+ chelator (Quin 2) dramatically increased φ at a neutral pH. Subsequent addition of Ca 2+ or ruthenium red suppressed the chelator-induced increase in φ. These findings suggest that wall Ca 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.