Wall-Yielding Properties of Cell Walls from Elongating Cucumber Hypocotyls in Relation to the Action of Expansin

The wall-yielding properties of cell walls were examined using frozen–thawed and pressed segments (FTPs) obtained from the elongation zones of cucumber hypocotyls with a newly developed programmable creep meter. The rate of wall extension characteristically changed depending on both tension and pH. By treatment of the FTPs with acid, the yield tension (y) was shifted downward and the extensibility (f) was increased. However, the downward shift of y was greatly suppressed and the increase in f was partly inhibited in boiled FTPs. The boiled FTPs reconstituted with expansin fully recovered the acid-induced downward y shift as well as the increase in f. Even under the tension below y, wall extension took place pH dependently. Such extension was markedly slower (low-rate extension) than that under the tension above y (high-rate extension). At a higher concentration (8 M), urea markedly inhibited the creep ascribable to the inhibition of the acid-induced downward y shift and increase in f. Moderate concentrations (2 M) of urea promoted wall creep pH dependently. The promotion was equivalent to a 0.5 decrease in pH. The promotion of creep by 2 M urea was observed in boiled FTPs reconstituted with expansin but not in boiled FTPs. These findings indicated that the acidfacilitated creep was controlled by y as well as f in cucumber cell walls. However, y and f might be inseparable and mutually related parameters because the curve of the stress extension rate (SER) showed a gradual change from the low-rate extension to the high-rate extension. Expansin played a role in pH-dependent regulation of both y and f. The physiological meaning of the pH-dependent regulation of wall creep under different creep tensions is also discussed with reference to a performance chart obtained from the SER curves.