Indole-3-acetic acid biosynthesis in isolated axes from germinating bean seeds: The effect of wounding on the biosynthetic pathway

A facile radiotracer method for discriminating between the activities of tryptophan-dependent and tryptophan-independent pathways for IAA biosynthesis is described. This method utilized the simultaneous exposure of plants to [¹⁴C]anthranilate in the presence or absence of excess unlabeled tryptophan in order to determine if tryptophan feeding can affect the relative enrichment of the IAA pool. Using this radiotracer method, the activities of the two biosynthetic pathways were analyzed in isolated axes of germinating bean seedlings at various times after cotyledon excision. Unlabeled tryptophan suppressed [¹⁴C]anthranilate conversion into IAA in isolated axes of different ages immediately following cotyledon excision. On the other hand, tryptophan feeding did not inhibit [¹⁴C]IAA accumulation in isolated axes 36 or 120 h after cotyledon excision. Thus, this method was able to resolve time-dependent differences following cotyledon excision in the biosynthetic activities of the two pathways. Moreover, the present results lend further support to the emerging consensus that the tryptophan-dependent pathway acts to maintain very high IAA levels required for mediating rapid cell proliferation in wounded tissues and, as previously shown, young embryos.