The rôle of indole-3-acetic acid accumulation by alpha methyl tryptophan-resistant mutants of Pseudomonas savastanoi in gall formation on oleanders

Abstract

Indole-3-acetic acid (IAA) accumulates in the culture medium of wild-type isolates of Pseudomonas savastanoi, the olive and oleander knot pathogen, and has been implicated as a factor in gall formation. Mutants of P. savastanoi were obtained by selecting for resistance to α-methyl tryptophan (MT), an analogue of tryptophan, which in other bacteria is known to be a feedback inhibitor of anthranilate synthetase, a regulatory enzyme in the tryptophan biosynthetic pathway. Thus, MT-resistant mutants may exhibit altered synthesis of tryptophan and/or IAA which is a product of tryptophan metabolism. The mutants we selected either failed to accumulate IAA or accumulated about twice the quantity accumulated by wildtype parent isolates. Mutants which did not accumulate IAA lacked tryptophan oxidative decarboxylase activity in cell-free preparations. All parent and mutant isolates exhibited similar growth patterns when infiltrated into oleander leaves. However, dissimilar symptom development was observed on oleanders inoculated with these isolates. Isolates that accumulated IAA caused formation of galls, while those which did not accumulate IAA failed to incite gall formation.