Effects of Abscisic Acid on the Synthesis of Cell-Wall Polysaccharides in Segments of Etiolated Squash Hypocotyl II. Levels of UDP-Neutral Sugars

The effects of abscisic acid (ABA) on the formation of UDP-sugars in segments of squash (Cucurbita maxima Duch.) hypocotyls were examined with [3H]glucose. After a 3-h incubation, 74 % of total radioactivity incorporated into neutral sugar residues of UDP-sugars was found in glucose residues, 19 % in galactose residues and less than 7 % in other sugar residues. Exogenous-ly applied ABA increased the extent of incorporation of [3H]glucose into UDP-glucose, UDP-galactose and UDP-xylose by 30%, 30%, and 20%, respectively. However, ABA neither affected the total incorporation of radioactivity into segments nor the incorporation into free glu-cose and sucrose in the segments. Since ABA inhibits the synthesis of cellulose and hemicellulosic polysaccharides in cell walls of segments of squash hypocotyls prior to the suppres-sion of elongation of segments (Wakabayashi et al. 1989a, b), the present results indicate that ABA inhibits cell-wall synthesis by affecting some step(s) beyond the formation of UDP-sugars and not by decreasing the synthesis of UDP-sugars in the hypocotyl segments. Key words: ABA — Cell-wall synthesis — [3H]Glucose — Squash (Cucurbita maxima) — UDP-sugar. The inhibitory effect of ABA on the growth of roots, shoots and leaves has been reported in many plant systems (Davies et al. 1980, Bornman 1983). Our previous results (Wakabayashi et al. 1989a, b) demonstrated that exoge-nously applied ABA inhibits both elongation and cell-wall synthesis (in particular, the synthesis of cellulose and hemicellulosic polysaccharides) in segments of squash hypo-cotyls, and that ABA-mediated inhibition of cell-wall syn-thesis occurs prior to the suppression of growth of segments. Furthermore, ABA was found not to affect the osmotic potentials in the hypocotyl segments, and in-hibitors of cell-wall synthesis, such as 2,6-dichloro-benzonitrile and monensin, suppressed the growth of segments to the same extent as ABA (Wakabayashi et al