Regulation of Glucose Metabolism and Cell Wall Synthesis in Avena Stem Segments by Gibberellic Acid

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Abstract
GA stimulated both the elongation of A. sativa stem segments and increased synthesis of cell wall material. The effects of GA on glucose metabolism, as related to cell wall synthesis, were investigated in order to find specific events regulated by GA. GA caused a decline in the levels of glucose, G6P and fructose 6-phosphate if exogenous sugar was not supplied to the segments, whereas the hormone caused no change in the levels of G6P, fructose 6-phosphate, UDP-glucose or the adenylate energy charge if the segments were incubated in 0.1 M glucose. No GA-induced change was demonstrated in the activities of hexokinase, phosphoglucomutase, UDP-glucose pyrophosphorylase or polysaccharide synthetases using UDP-glucose, UDP-galactose, UDP-xylose and UDP-arabinose as substrates. GA stimulated the activity of GDP-glucose-dependent .beta.-glucan synthetase by 2- to 4-fold over the control. When glucan synthetase was assayed using UDP-glucose as substrate, only .beta.-1,3-linked glucan was synthesized in vitro, whereas with GDP-glucose, only .beta.-1,4-linked glucan was synthesized. These results suggest that one part of the mechanism by which GA stimulates cell wall synthesis concurrently with elongation in Avena stem segments may be through a stimulation of cell wall polysaccharide synthetase activity.