Institute of Plant Biology, University of Zurich, CH-8008 Zurich, Switzerland.
Environ Microbiol. 2012 Oct;14(10):2757-69. doi: 10.1111/j.1462-2920.2012.02763.x. Epub 2012 May 2.
Several plant species of the genus Psychotria (Rubiaceae) harbour Burkholderia sp. bacteria within specialized leaf nodules. The bacteria are transmitted vertically between plant generations and have not yet been cultured outside of their host. This symbiosis is also generally described as obligatory because plants devoid of symbionts fail to develop into mature individuals. We sequenced for the first time the genome of the symbiont of Psychotria kirkii in order to shed some light on the nature of their symbiotic relationship. We found that the 4 Mb genome of Candidatus Burkholderia kirkii (B. kirkii) is small for a Burkholderia species and displays features consistent with ongoing genome erosion such as large proportions of pseudogenes and transposable elements. Reductive genome evolution affected a wide array of functional categories that may hinder the ability of the symbiont to be free-living. The genome does not encode functions commonly found in plant symbionts such as nitrogen fixation or plant hormone metabolism. Instead, a collection of genes for secondary metabolites' synthesis is located on the 140 kb plasmid of B. kirkii and suggests that leaf nodule symbiosis benefits the host by providing protection against herbivores or pathogens.
几种Psychotria 属植物（茜草科）在专门的叶结节中寄生 Burkholderia 属细菌。细菌在植物世代之间垂直传播，尚未在宿主之外培养。这种共生关系通常也被描述为强制性的，因为没有共生体的植物无法发育成熟。我们首次对 Psychotria kirkii 的共生体进行了基因组测序，以揭示它们共生关系的本质。我们发现，4 Mb 的 Candidatus Burkholderia kirkii（B. kirkii）基因组对于 Burkholderia 物种来说很小，并且具有与正在进行的基因组侵蚀一致的特征，例如大量的假基因和转座元件。还原基因组进化影响了广泛的功能类别，这可能会阻碍共生体的自由生活能力。该基因组不编码植物共生体中常见的功能，如固氮或植物激素代谢。相反，一组用于合成次生代谢物的基因位于 B. kirkii 的 140 kb 质粒上，这表明叶结节共生关系通过为宿主提供对草食动物或病原体的保护来使宿主受益。
