Institut des Sciences du Végétal, Centre National de la Recherche Scientifique, Unité Propre de Recherche 2355, Gif-sur-Yvette, France.
PLoS One. 2010 Mar 4;5(3):e9519. doi: 10.1371/journal.pone.0009519.
The legume plant Medicago truncatula establishes a symbiosis with the nitrogen-fixing bacterium Sinorhizobium meliloti which takes place in root nodules. The formation of nodules employs a complex developmental program involving organogenesis, specific cellular differentiation of the host cells and the endosymbiotic bacteria, called bacteroids, as well as the specific activation of a large number of plant genes. By using a collection of plant and bacterial mutants inducing non-functional, Fix(-) nodules, we studied the differentiation processes of the symbiotic partners together with the nodule transcriptome, with the aim of unravelling links between cell differentiation and transcriptome activation. Two waves of transcriptional reprogramming involving the repression and the massive induction of hundreds of genes were observed during wild-type nodule formation. The dominant features of this "nodule-specific transcriptome" were the repression of plant defense-related genes, the transient activation of cell cycle and protein synthesis genes at the early stage of nodule development and the activation of the secretory pathway along with a large number of transmembrane and secretory proteins or peptides throughout organogenesis. The fifteen plant and bacterial mutants that were analyzed fell into four major categories. Members of the first category of mutants formed non-functional nodules although they had differentiated nodule cells and bacteroids. This group passed the two transcriptome switch-points similarly to the wild type. The second category, which formed nodules in which the plant cells were differentiated and infected but the bacteroids did not differentiate, passed the first transcriptome switch but not the second one. Nodules in the third category contained infection threads but were devoid of differentiated symbiotic cells and displayed a root-like transcriptome. Nodules in the fourth category were free of bacteria, devoid of differentiated symbiotic cells and also displayed a root-like transcriptome. A correlation thus exists between the differentiation of symbiotic nodule cells and the first wave of nodule specific gene activation and between differentiation of rhizobia to bacteroids and the second transcriptome wave in nodules. The differentiation of symbiotic cells and of bacteroids may therefore constitute signals for the execution of these transcriptome-switches.
模式豆科植物苜蓿与固氮菌根瘤农杆菌建立共生关系，发生在根瘤中。根瘤的形成采用了一个复杂的发育程序，涉及器官发生、宿主细胞和内共生细菌（称为类菌体）的特化细胞分化，以及大量植物基因的特异性激活。我们使用一系列诱导无功能、Fix(-)根瘤的植物和细菌突变体，研究了共生伙伴的分化过程以及根瘤转录组，目的是揭示细胞分化和转录组激活之间的联系。在野生型根瘤形成过程中，我们观察到涉及数百个基因的抑制和大量诱导的两个转录重编程波。这种“根瘤特异性转录组”的主要特征是植物防御相关基因的抑制、细胞周期和蛋白质合成基因在根瘤发育早期的短暂激活以及分泌途径的激活，以及大量跨膜和分泌蛋白或肽在整个器官发生过程中的激活。分析的 15 个植物和细菌突变体分为四大类。第一类突变体形成无功能的根瘤，尽管它们已经分化出根瘤细胞和类菌体。该组与野生型类似地通过了两个转录组开关点。第二类突变体形成根瘤，其中植物细胞分化并感染，但类菌体没有分化，通过了第一个转录组开关，但没有通过第二个。第三类根瘤含有感染线，但没有分化的共生细胞，显示出类似根的转录组。第四类根瘤没有细菌，缺乏分化的共生细胞，也显示出类似根的转录组。因此，共生根瘤细胞的分化与第一个根瘤特异性基因激活波之间存在相关性，类菌体的分化与根瘤中的第二个转录组波之间存在相关性。共生细胞和类菌体的分化可能构成了执行这些转录组开关的信号。
