Laboratory of Microbiology, Ghent University, B-9000, Ghent, Belgium.
Centre National de Recherches sur l'Environnement, BP 1739, Antananarivo 101, Madagascar.
ISME J. 2019 Jul;13(7):1831-1844. doi: 10.1038/s41396-019-0398-8. Epub 2019 Mar 15.
Various plant species establish intimate symbioses with bacteria within their aerial organs. The bacteria are contained within nodules or glands often present in distinctive patterns on the leaves in what is commonly referred to as leaf nodule symbiosis. We describe here a highly specific symbiosis between a wild yam species from Madagascar, Dioscorea sansibarensis and bacteria of the species Orrella dioscoreae. Using whole-genome sequencing of plastids and bacteria from wild-collected samples, we show phylogenetic patterns consistent with a dominant vertical mode of transmission of the symbionts. Unique so far among leaf nodule symbioses, the bacteria can be cultured and are amenable to comparative transcriptomics, revealing a potential role in complementing the host's arsenal of secondary metabolites. We propose a recent establishment of a vertical mode of transmission in this symbiosis which, together with a large effective population size explains the cultivability and apparent lack of genome reductive evolution in O. dioscoreae. We leverage these unique features to reveal pathways and functions under positive selection in these specialized endophytes, highlighting the candidate mechanisms enabling a permanent association in the phyllosphere.
各种植物物种在其气生器官中与细菌建立密切的共生关系。这些细菌被包含在结节或腺体中，这些结节或腺体通常在叶子上以独特的模式存在，通常被称为叶结节共生。我们在这里描述马达加斯加野生山药物种 Dioscorea sansibarensis 与细菌物种 Orrella dioscoreae 之间的高度特异性共生关系。使用来自野生采集样本的质体和细菌的全基因组测序，我们展示了与共生体主要垂直传播模式一致的系统发育模式。到目前为止，在叶结节共生关系中，细菌是可以培养的，并且适合进行比较转录组学研究，这揭示了它们在补充宿主次生代谢物库方面的潜在作用。我们提出了这种共生关系中最近建立的垂直传播模式，以及较大的有效种群大小，解释了 O. dioscoreae 的可培养性和明显缺乏基因组简化进化。我们利用这些独特的特征来揭示这些专门的内生菌中受正选择影响的途径和功能，突出了使它们在叶际中保持永久关联的候选机制。
