Abstract
Indole-3-acetic acid (IAA) is a major plant hormone that affects many cellular processes in plants, bacteria, yeast, and human cells through still unknown mechanisms. In this study, we demonstrated that the IAA-treatment of two unrelated bacteria, the Ensifer meliloti 1021 and Escherichia coli, harboring two different host range plasmids, influences the supercoiled state of the two plasmid DNAs in vivo. Results obtained from in vitro assays show that IAA interacts with DNA, leading to DNA conformational changes commonly induced by intercalating agents. We provide evidence that IAA inhibits the activity of the type IA topoisomerase, which regulates the DNA topological state in bacteria, through the relaxation of the negative supercoiled DNA. In addition, we demonstrate that the treatment of E. meliloti cells with IAA induces the expression of some genes, including the ones related to nitrogen fixation. In contrast, these genes were significantly repressed by the treatment with novobiocin, which reduces the DNA supercoiling in bacterial cells. Taking into account the overall results reported, we hypothesize that the IAA action and the DNA structure/function might be correlated and involved in the regulation of gene expression. This work points out that checking whether IAA influences the DNA topology under physiological conditions could be a useful strategy to clarify the mechanism of action of this hormone, not only in plants but also in other unrelated organisms.
Highlights
Auxin biology is among the oldest fields of experimental plant research
To verify whether indole-3-acetic acid (IAA) affected the DNA topology in vivo, the level of DNA supercoiling was evaluated in two unrelated bacteria: The soil bacterium and poor IAA-producer E. meliloti 1021 and the enteric bacterium E. coli, which does not synthesize IAA and for which we have already demonstrated that IAA plays a role in gene expression changes [7]
Our study demonstrated that IAA is able to influence the DNA topology and expression of genes sensitive to changes in DNA supercoiling
Summary
Auxin biology is among the oldest fields of experimental plant research. Back in 1880, it was proposed [1], for the first time, that a diffusible molecule, sensitive to light, was transported downward and bent coleoptiles towards light. Eight decades after the structural elucidation of IAA, many aspects of auxin metabolism, transport, and signaling have been well established; more than a few fundamental questions and innumerable details remain unsolved. In the last two decades, it has been reported that IAA induces major changes in developmental and metabolic functions in quite diverse systems, such as cancer cells [3], plants [4], fungi [5], and bacteria [6,7,8], through still unknown mechanisms.
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