Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis
Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis
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DOI：
10.1073/pnas.0600912103
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发表时间：
2006-03-28
影响因子：
11.1
通讯作者：
Kondorosi, E
中科院分区：
文献类型：
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作者：
Mergaert, P;Uchiumi, T;Kondorosi, E
豆科植物和根瘤菌之间的共生导致根瘤的形成，其中受感染的植物细胞中的细菌转化为固氮类杆菌。具有宿存分生组织的根瘤是不确定的，而没有分生组织的根瘤是确定的。两种根瘤类型中的共生植物细胞都是多倍体，因为有几个循环的核内复制（没有有丝分裂和胞质分裂的基因组复制），并因此生长到极端大小。在这里，我们表明，分化的类杆菌在苜蓿和相关豆科植物的不确定结节的盔状花序分支显示出显着的相似性宿主细胞分化。在类杆菌成熟过程中，无胞质分裂的重复DNA复制导致整个细菌基因组的广泛扩增和细菌的延伸。这一发现揭示了原核生物中DNA含量与细胞大小之间的正相关性，与真核生物中的正相关性相似。这些多倍体类杆菌具有代谢功能，但表现出增加的膜渗透性，并且是不可存活的，因为它们失去了恢复生长的能力。相比之下，类细菌在确定的结节nongalegoid豆科植物莲花和豆是可比的自由生活的细菌在其基因组DNA含量，细胞大小和活力。利用重组根瘤菌菌株的两个豆科植物类型，我们表明，类杆菌分化是由宿主植物。植物因子存在于盔状瘤豆科植物的根瘤中，但不存在于非盔状瘤豆科植物的根瘤中，其阻断细菌细胞分裂并触发核内复制循环，从而迫使内共生体朝向终末分化状态。因此，苜蓿和相关的豆科植物已经进化出一种主导共生的机制。
Symbiosis between legumes and Rhizobium bacteria leads to the formation of root nodules where bacteria in the infected plant cells are converted into nitrogen-fixing bacteroids. Nodules with a persistent meristem are indeterminate, whereas nodules without meristem are determinate. The symbiotic plant cells in both nodule types are polyploid because of several cycles of endoreduplication (genome replication without mitosis and cytokinesis) and grow consequently to extreme sizes. Here we demonstrate that differentiation of bacteroids in indeterminate nodules of Medicago and related legumes from the galegoid clade shows remarkable similarity to host cell differentiation. During bacteroid maturation, repeated DNA replication without cytokinesis results in extensive amplification of the entire bacterial genome and elongation of bacteria. This finding reveals a positive correlation in prokaryotes between DNA content and cell size, similar to that in eukaryotes. These polyploid bacteroids are metabolically functional but display increased membrane permeability and are nonviable, because they lose their ability to resume growth. In contrast, bacteroids in determinate nodules of the nongalegoid legumes lotus and bean are comparable to free-living bacteria in their genomic DNA content, cell size, and viability. Using recombinant Rhizobium strains nodulating both legume types, we show that bacteroid differentiation is controlled by the host plant. Plant factors present in nodules of galegoid legumes but absent from nodules of nongalegoid legumes block bacterial cell division and trigger endoreduplication cycles, thereby forcing the endosymbionts toward a terminally differentiated state. Hence, Medicago and related legumes have evolved a mechanism to dominate the symbiosis.