Mass Spectrometric Quantification of Indole-3-Acetic Acid in Rhizobium Culture Supernatants: Relation to Root Hair Curling and Nodule Initiation

Indole-3-acetic acid (IAA) has been unequivocally identified in culture supernatants of Rhizobium strains by gas chromatography-mass spectrometry. A method for accurately quantitating IAA in bacterial culture supernatants, employing deuterium-labeled IAA as an internal standard, has been developed. Similar IAA concentrations were found in culture supernatants of chosen Rhizobium mutants (defective in nodule formation) and their corresponding parent strains. Since some of the mutants are known to adhere to root hairs, it can be concluded that root hair curling is not simply a consequence of IAA production by rhizobia.