Laboratory of Plant Systematics, K.U.Leuven, Leuven, Belgium.
PLoS One. 2011;6(9):e24430. doi: 10.1371/journal.pone.0024430. Epub 2011 Sep 7.
Bacterial leaf symbiosis is a unique and intimate interaction between bacteria and flowering plants, in which endosymbionts are organized in specialized leaf structures. Previously, bacterial leaf symbiosis has been described as a cyclic and obligate interaction in which the endosymbionts are vertically transmitted between plant generations and lack autonomous growth. Theoretically this allows for co-speciation between leaf nodulated plants and their endosymbionts. We sequenced the nodulated Burkholderia endosymbionts of 54 plant species from known leaf nodulated angiosperm genera, i.e. Ardisia, Pavetta, Psychotria and Sericanthe. Phylogenetic reconstruction of bacterial leaf symbionts and closely related free-living bacteria indicates the occurrence of multiple horizontal transfers of bacteria from the environment to leaf nodulated plant species. This rejects the hypothesis of a long co-speciation process between the bacterial endosymbionts and their host plants. Our results indicate a recent evolutionary process towards a stable and host specific interaction confirming the proposed maternal transmission mode of the endosymbionts through the seeds. Divergence estimates provide evidence for a relatively recent origin of bacterial leaf symbiosis, dating back to the Miocene (5-23 Mya). This geological epoch was characterized by cool and arid conditions, which may have triggered the origin of bacterial leaf symbiosis.
细菌叶共生是细菌和开花植物之间一种独特而密切的相互作用，其中内共生体组织在专门的叶片结构中。以前，细菌叶共生被描述为一种循环的和必需的相互作用，其中内共生体在植物世代之间垂直传播，缺乏自主生长。从理论上讲，这允许与叶结节植物及其内共生体的共同进化。我们对来自已知叶结节被子植物属（即 Ardisia、Pavetta、Psychotria 和 Sericanthe）的 54 种植物物种的结节 Burkholderia 内共生体进行了测序。细菌叶共生体及其密切相关的自由生活细菌的系统发育重建表明，细菌从环境到叶结节植物物种的多次水平转移发生。这否定了细菌内共生体与其宿主植物之间长期共同进化过程的假设。我们的结果表明，最近的进化过程是朝着稳定和宿主特异性相互作用的方向发展，这证实了内共生体通过种子进行母体传递的模式。分歧估计为细菌叶共生的相对较近起源提供了证据，可追溯到中新世（5-2300 万年前）。这个地质时期的特点是凉爽和干旱的条件，这可能引发了细菌叶共生的起源。
