Biotransformation of Tryptophan to Indole-3-acetic Acid by Xanthomonas campestris

Xanthomonas campestris pv. campestris is the etiological agent of black rot of crucifers, a disease of major economic importance which is a threat to the production of cruciferous crops in many areas of the world [1], During a recent investigation of this patho­ gen for the production of phytotoxic metabolites in liquid culture we observed the biotransformation of certain amino acid constituents of the culture medium to their corresponding nor-carboxylic acids via a deamination process [2]. Several of these carboxylic acids, noteably 3-methylthiopropanoic acid and /rans-3-methylthiopropenoic acid were previous­ ly reported as products of other xanthomonads and were described as phytotoxins [3, 4], However, the relatively weak phytotoxicity expressed by high concentrations of these compounds is apparently due to the generation of excessively high hydrogen ion activities [2]. The carboxylic acid phytohormone indole-3-acetic acid (IAA) is a compound which ex­ presses more selective and potent biological activity than those carboxylic acids previously described as products of Xanthomonas campestris [2—4], IA A is known to be produced in vitro by a wide range of microorganisms, including plant pathogenic mem­ bers of the Pseudomonadaceae [5]. Although in many cases this biochemical trait is unlikely to be involved in hyperauxiny during infection, in the case of Pseudomonas savastanoi (olive knot) IAA has been demonstrated to behave as a virulence factor in disease causation [6]. Here we report the results of a preliminary investigation of X. c. pv. campestris for the production of IAA.