Phytostimulatory effect of Azospirillum brasilense wild type and mutant strains altered in IAA production on wheat

Auxin production by Azospirillum is believed to play a major role in the ob served plant growth promoting effect. By using different genetically modifi ed strains, the contribution of auxin biosynthesis by A. brasilense in alte ring root morphology was evaluated in a plate assay. Inoculation with the w ild type strains A. brasilense Sp245 and Sp7 resulted in a strong decrease in root length and increase in root hair formation. This effect was abolish ed when inoculating with an ipdC mutant of A. brasilense. The ipdC gene enc odes a key enzyme in the IPyA pathway of IAA synthesis by A. brasilense. On the other hand, the observed auxin effect was further enhanced by adding t ryptophan, a precursor of IAA, to the plates and could be mimicked by repla cing the Azospirillum cells by a particular concentration of IAA. Furthermo re, particular mutants (rpoN, scrp) and transconjugants (extra copy of ipdC ) of A. brasilense were tested in the plate assay. Together, these results confirm the important role of IAA produced by Azospirillum in altering root morphology and illustrate the power of combining genetic tools and bioassa ys to elucidate the mechanism of a beneficial Azospirillum-plant interactio n.