Bean callus was induced to form roots (tissue differentiation) and vascular nodules (cell differentiation) by lowering the ratio of auxin to cytokinin in the growth medium. Both types of differentiation were inhibited by the addition of abscisic acid (at concentrations greater than I muM) to induction medium. Initiation of differentiation was inhibited, but its subsequent development was not, and the inhibition was not affected by the addition of gibberellic acid. Addition of gibberellic acid (GA) alone to induction medium stimulated tissue differentiation, although cell differentiation was unaffected (30 muM GA) or inhibited (45 muM GA) and its onset was delayed at both concentrations. Root initiation was also stimulated by gibberellic acid (0.I-45 muM) at an auxin-to-kinin ratio 10 times that normally optimal for cell differentiation. The phenylalanine ammonia lyase (PAL) activity of the calluses was closely correlated with the amount of cell differentiation which had occurred, and measurement of this confirmed that gibberellic acid delayed the initiation of cell differentiation. The increase and subsequent decline of PAL and betaI leads to 3 glucan synthetase activities, normally induced by transfer to induction medium, was abolished by abscisic acid. Addition of gibberellic acid did not affect the betaI leads to 3 glucan synthetase activity.

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