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. Exogenously 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 glucose 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 suppression 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.
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Wakabayashi et al. (1991) studied this question.