Auxin Stimulates the Synthesis but not the Loosening of Cell Walls in Isolated Outer Tissue of Dark-Grown Squash ( Cucurbita maxima Duch.) Hypocotyls

Abstract

Indole-3-acetic acid (IAA) promoted elongation and cell wall loosening of outer tissue of intact segments from squash ( Cucurbita maxima Duch.) hypocotyls. On the other hand, when the segment was separated into the outer and inner tissues and then incubated, the isolated outer tissue slightly shrank and did not elongate, while the isolated inner one rapidly elongated. IAA did not affect either growth or the mechanical properties of cell walls of isolated outer and inner tissues. In intact segments, IAA significantly promoted the incorporation of radioactivity from [ 14 C]glucose into cell wall fractions of outer tissue without affecting uptake into the intact segments but it did not promote that of inner tissue. In isolated outer tissue, IAA substantially inhibited the uptake but significantly promoted the distribution of the incorporated radioactivity into cell wall fractions. IAA, however, barely affected the uptake and incorporation of radioactivity into cell wall fractions in isolated inner tissue. These results suggest that IAA primarily stimulates the synthesis of certain cell wall polysaccharides in outer tissue, irrespective of being intact or isolated, but the stimulation to that extent is not directly involved in the processes of cell wall loosening in outer tissue.