The rhizosphere bacterium Azospirillum brasilense produces the auxin indole-3-acetic acid
(IAA) through the indole-3-pyruvate (IPA) pathway. As we previously demonstrated that
transcription of the indole-3-pyruvate decarboxylase (ipdC) gene is positively regulated by
IAA, produced by A. brasilense itself or added exogenously, we performed a microarray
analysis to study the overall effects of IAA on the transcriptome of A. brasilense. The
transcriptomes of A. brasilense wild-type and the ipdC knock-out mutant, both cultured in the
absence and presence of exogenously added IAA, were compared.
Interfering with the IAA biosynthesis/homeostasis in A. brasilense through inactivation of the
ipdC gene or IAA addition results in much broader transcriptional changes than anticipated.
Based on the multitude of changes observed by comparing the different transcriptomes, we
can conclude that IAA is a signaling molecule in A. brasilense. It appears that the bacterium,
when exposed to IAA, adapts itself to the plant rhizosphere, by changing its arsenal of
transport proteins and cell surface proteins. A striking example of adaptation to IAA
exposure, as happens in the rhizosphere, is the upregulation of a type VI secretion system
(T6SS) in the presence of IAA. The T6SS is described as specifically involved in bacterium –
eukaryotic host interactions. Additionally many transcription factors show an altered
regulation as well, indicating that the regulatory machinery of the bacterium is changing.