Differentiation of Symbiotic Cells and Endosymbionts in Medicago truncatula Nodulation Are Coupled to Two Transcriptome-Switches

Abstract

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.

Transcriptomics of Actinorhizal Symbioses Reveals Homologs of the Whole Common Symbiotic Signaling Cascade

“…During the switch from a root-specific to a nodule-specific gene expression program, important transcriptional changes would be expected to accompany bacterial recognition and invasion and plant morphological changes. This situation is extremely close to that described in different model legume transcriptomic studies (Asamizu et al, 2000;Journet et al, 2002;Colebatch et al, 2004;El Yahyaoui et al, 2004;Maunoury et al, 2010) with upregulation of DCAT, a dicarboxylate transporter for delivery of photosynthates as TCA intermediates to the symbiont and a Gln synthetase, homolog of early nodulin M. truncatula N6 (Mathis et al, 1999), for assimilation of the ammonium fixed and transferred to the plant cytosol, which is consistent with previous demonstrations that the assimilation of 13 N was through Gln synthetase synthesizing Gln (Lundberg and Lundquist, 2004). The level of expression of several defense genes (defensins, chitinases) as well as stress proteins (catalase, DnaJ, Mdr) was also modified in a similar way to what is known to occur in legume nodules (Pucciariello et al, 2009;Maunoury et al, 2010), in particular a g-expansin natriuretic peptide that belongs to a novel class of peptide signal molecules (plant natriuretic peptides) involved in the response to biotic and abiotic stresses (Gottig et al, 2008;Meier et al, 2008).…”

Medicago truncatula symbiotic peptide NCR247 contributes to bacteroid differentiation through multiple mechanisms

“…In the absence of GroEL1, Nod factor production appears to be sufficient for nodule induction but not for the infection process. Expression of the NCR genes requires cellular infection and the presence of some bacteria in the symbiotic nodule cells (21). Thus, in the infected nodules, the early NCRs are probably produced; however, the existence of nodules containing only undifferentiated bacteroids indicates that GroEL1 might be needed for NCR activities and terminal differentiation of bacteroids.…”

NODULE ROOT and COCHLEATA Maintain Nodule Development and Are Legume Orthologs of Arabidopsis BLADE-ON-PETIOLE Genes

“…For each genotype, three pools of 10 15-d-old nodules, arising from 10 independent plants, were prepared as described (Maunoury et al, 2010), except that filtered nuclei were stained with propidium iodide (50 µg/mL) and measured using a CyFlow SL3 flow cytometer. The endoreduplication index calculation was defined previously (Maunoury et al, 2010), and profiles were analyzed with Partec FloMax software.…”

“…The endoreduplication index calculation was defined previously (Maunoury et al, 2010), and profiles were analyzed with Partec FloMax software.…”