Symbiotic ß-Proteobacteria beyond Legumes: <em>Burkholderia</em> in <em>Rubiaceae</em>

<div><p>Symbiotic ß-proteobacteria not only occur in root nodules of legumes but are also found in leaves of certain <em>Rubiaceae</em>. The discovery of bacteria in plants formerly not implicated in endosymbiosis suggests a wider occurrence of plant-microbe interactions. Several ß-proteobacteria of the genus <em>Burkholderia</em> are detected in close association with tropical plants. This interaction has occurred three times independently, which suggest a recent and open plant-bacteria association. The presence or absence of <em>Burkholderia</em> endophytes is consistent on genus level and therefore implies a predictive value for the discovery of bacteria. Only a single <em>Burkholderia</em> species is found in association with a given plant species. However, the endophyte species are promiscuous and can be found in association with several plant species. Most of the endophytes are part of the plant-associated beneficial and environmental group, but others are closely related to <em>B. glathei</em>. This soil bacteria, together with related nodulating and non-nodulating endophytes, is therefore transferred to a newly defined and larger PBE group within the genus <em>Burkholderia</em>.</p>