<i>Azoarcus</i> sp. CIB, an Anaerobic Biodegrader of Aromatic Compounds Shows an Endophytic Lifestyle

<div><p>Background</p><p>Endophytic bacteria that have plant growth promoting traits are of great interest in green biotechnology. The previous thought that the <i>Azoarcus</i> genus comprises bacteria that fit into one of two major eco-physiological groups, either free-living anaerobic biodegraders of aromatic compounds or obligate endophytes unable to degrade aromatics under anaerobic conditions, is revisited here.</p><p>Methodology/Principal Findings</p><p>Light, confocal and electron microscopy reveal that <i>Azoarcus</i> sp. CIB, a facultative anaerobe β-proteobacterium able to degrade aromatic hydrocarbons under anoxic conditions, is also able to colonize the intercellular spaces of the rice roots. In addition, the strain CIB displays plant growth promoting traits such nitrogen fixation, uptake of insoluble phosphorus and production of indoleacetic acid. Therefore, this work demonstrates by the first time that a free-living bacterium able to degrade aromatic compounds under aerobic and anoxic conditions can share also an endophytic lifestyle. The phylogenetic analyses based on the 16S rDNA and <i>nifH</i> genes confirmed that obligate endophytes of the <i>Azoarcus</i> genus and facultative endophytes, such as <i>Azoarcus</i> sp. CIB, locate into different evolutionary branches.</p><p>Conclusions/Significance</p><p>This is the first report of a bacterium, <i>Azoarcus</i> sp. CIB, able to degrade anaerobically a significant number of aromatic compounds, some of them of great environmental concern, and to colonize the rice as a facultative endophyte. Thus, <i>Azoarcus</i> sp. CIB becomes a suitable candidate for a more sustainable agricultural practice and phytoremediation technology.</p>