Effects of ABA on Synthesis of Cell-Wall Polysaccharides in Segments of Etiolated Squash Hypocotyl I. Changes in Incorporation of Glucose and myo-Inositol into Cell-Wall Components

Externally supplied ³H]myo-inositol and ¹⁴C]glucose were incorporatedin cell-wall fractions of segments of etiolated squash hypocotyl.The extent of incorporation of ¹⁴C]glucose into cell-wall fractionswas very much greater than that of ³H]myo-inositol. Radioactivityfrom ¹⁴C]-glucose was effectively incorporated into hemicelluloseB and cellulose fractions and was incorporated uniformly intohexose, pentose and uronic acid residues, but radioactivityfrom ³H]myo-inositol was incorporated predominantly into uronicacid and pentose residues in the pectin and hemicellulose Bfractions. Exogenously applied ABA significantly suppressed the elongationof segments of squash hypocotyl and the incorporation of radioactivityfrom ^l4C]glucose and ³H]myo-inositol into the segments. Furthermore,ABA significantly inhibited the distribution of incorporatedradioactivity from ¹⁴C]glucose into the cellulose fraction,but did not affect distribution into the pectic fraction. Bycontrast, ABA only slightly inhibited the distribution of theincorporated radioactivity from ³H]myo-inositol into the pecticfraction. These results suggest that most of the cell-wall polysaccharidesin segments of squash hypocotyl are synthesized via the UDP-sugarpathway, and that ABA significantly inhibits the synthesis ofcellulose but not the synthesis of pectic polysaccharides whenABA suppresses the elongation of the segments. (Received March 25, 1988; Accepted November 15, 1988)