Auxin- and Acid-Induced Changes in the Mechanical Properties of the Cell Wall

Abstract

Auxin- and acid-induced changes in the mechanical properties of the cell wall were analyzed by measuring the creep of the cell wall using pumpkin {Cucurbita moschata Duch cv. Shirakikuza) hypocotyl segments. Hypocotyl segments were treated with or without IAA and stored in 50% glycerol at — 15°C for more than 2 weeks before measurements. Creep rate increased with the increase in the load. The increase was first very slow up to the phase shift point (yield threshold, y), and after that, it was steep. The rate of the creep rate increase (creep coefficient, Cm) was larger and y was smaller at pH 4.5 than pH 6.8. This indicates cell wall loosening was facilitated under acidic conditions. IAA-pretreatment of the segments resulted in the lowering of y at pH 6.8 only. Around pH 5 and 45°C, Cm was highest and y was lowest. Boiling in distilled water almost lost the differential effect of Cm and y on pH and IAA. The differential effect of pH on Cm and y were recovered by the addition of a crude extract of cell wall-bound proteins. It is implied that some enzymatic processes are involved in the control of acid-induced cell wall extension.