MtEFD, un facteur de transcription impliqué dans les interactions symbiotique et pathogène ainsi que dans le développement racinaire chez Medicago truncatula

La rhizosphère est peuplée de nombreux microorganismes pouvant être bénéfiques ou au contraire néfastes pour les plantes. La légumineuse modèle M. truncatula permet d'étudier les associations symbiotiques avec la bactérie fixatrice d'azote Sinorhizobium meliloti, mais aussi les interactions avec la bactérie pathogène racinaire Ralstonia solanacearum. Les interactions symbiotiques fixatrices d'azote nécessitent la formation d'un nouvel organe racinaire, appelé nodosité, au sein duquel les bactéries différenciées fixent l'azote atmosphérique au bénéfice de la plante hôte. L'organogenèse nodulaire s'accompagne d'une phase de différenciation des cellules végétales, dont le contrôle est mal connu, au cours de laquelle le facteur de transcription MtEFD (M. truncatula ethylene response factor required for nodule differentiation) joue un rôle important. Durant ma thèse, nous avons montré que MtEFD pourrait contrôler la différenciation nodulaire via la régulation de gènes hautement spécifiques des nodosités, et qu'une mutation dans MtEFD impacte sur un processus clé de la différenciation, l'endoréduplication, plus précisément le nombre d'endocycles (cycle cellulaire sans mitose) des cellules végétales et bactériennes. De plus, nous avons observé une similitude dans le profil d'expression de MtEFD au cours du développement nodulaire et racinaire et montré que MtEFD a un effet à la fois sur la formation des racines latérales et la synthèse d'ADN dans les extrémités racinaires. Enfin, nous avons démontré que MtEFD joue également un rôle positif dans le développement de la maladie causée par R. solanacearum. Dans la recherche de mécanismes communs à ces différents processus, nous avons porté une attention particulière aux cytokinines (CK), phytohormones dont MtEFD contrôle un régulateur négatif, le gène MtRR4 (M. truncatula response regulator 4).The rhizosphere is composed of microorganisms which are beneficial or pathogenic for the plants. The model legume M. truncatula is used to study the symbiotic interaction with the nitrogen-fixing bacteria Sinorhizobium meliloti, but also the pathogenic interaction with the telluric bacteria Ralstonia solanacearum. The nitrogen-fixing symbiotic associations involve the formation of a new root organ, called nodule, in which differentiated bacteria are able to fix atmospheric nitrogen to benefit the host plant. The nodule organogenesis is associated with a stage of plant cell differentiation, whose control is poorly understood, and during which the transcription factor MtEFD (M. truncatula ethylene response factor required for nodule differentiation) plays an important role. During my thesis, we showed that MtEFD could control the nodule differentiation via the regulation of highly nodule specific genes, and a mutation in MtEFD impacts a key process of the differentiation, the endoreduplication, more precisely the number of endocycles (cell cycle without mitosis) of plant cells and bacteria. Moreover, we observed a similarity in the MtEFD expression pattern during nodule and root development, and we showed that MtEFD has an impact in both the lateral root formation and the DNA synthesis in the root tips. Finally, we also proved that MtEFD was positively involved in the disease development induced by R. solanacearum. In the search of common mechanism in these different processes, we paid particular attention to cytokinins (CK), as MtEFD controls a CK negative regulator, the MtRR4 (M. truncatula response regulator 4) gene

Unraveling new molecular players involved in the autoregulation of nodulation in Medicago truncatula

The number of legume root nodules resulting from a symbiosis with rhizobia is tightly controlled by the plant. Certain members of the CLAVATA3/Embryo Surrounding Region (CLE) peptide family, specifically MtCLE12 and MtCLE13 in Medicago truncatula, act in the systemic autoregulation of nodulation (AON) pathway that negatively regulates the number of nodules. Little is known about the molecular pathways that operate downstream of the AON-related CLE peptides. Here, by means of a transcriptome analysis, we show that roots ectopically expressing MtCLE13 deregulate only a limited number of genes, including three down-regulated genes encoding lysin motif receptor-like kinases (LysM-RLKs), among which are the nodulation factor (NF) receptor NF Perception gene (NFP) and two up-regulated genes, MtTML1 and MtTML2, encoding Too Much Love (TML)-related Kelch-repeat containing F-box proteins. The observed deregulation was specific for the ectopic expression of nodulation-related MtCLE genes and depended on the Super Numeric Nodules (SUNN) AON RLK. Moreover, overexpression and silencing of these two MtTML genes demonstrated that they play a role in the negative regulation of nodule numbers. Hence, the identified MtTML genes are the functional counterpart of the Lotus japonicus TML gene shown to be central in the AON pathway. Additionally, we propose that the down-regulation of a subset of LysM-RLK-encoding genes, among which is NFP, might contribute to the restriction of further nodulation once the first nodules have been formed

MtNRLK1, a CLAVATA1-like leucine-rich repeat receptor-like kinase upregulated during nodulation in Medicago truncatula

Peptides are signaling molecules regulating various aspects of plant development, including the balance between cell division and differentiation in different meristems. Among those, CLAVATA3/Embryo Surrounding Region-related (CLE-ESR) peptide activity depends on leucine-rich-repeat receptor-like-kinases (LRR-RLK) belonging to the subclass XI. In legume plants, such as the Medicago truncatula model, specific CLE peptides were shown to regulate root symbiotic nodulation depending on the LRR-RLK SUNN (Super Numeric Nodules). Amongst the ten M. truncatula LRR-RLK most closely related to SUNN, only one showed a nodule-induced expression, and was so-called MtNRLK1 (Nodule-induced Receptor-Like Kinase 1). MtNRLK1 expression is associated to root and nodule vasculature as well as to the proximal meristem and rhizobial infection zone in the nodule apex. Except for the root vasculature, the MtNRLK1 symbiotic expression pattern is different than the one of MtSUNN. Functional analyses either based on RNA interference, insertional mutagenesis, and overexpression of MtNRLK1 however failed to identify a significant nodulation phenotype, either regarding the number, size, organization or nitrogen fixation capacity of the symbiotic organs formed

The response of the root proteome to the synthetic strigolactone GR24 in Arabidopsis

Strigolactones are plant metabolites that act as phytohormones and rhizosphere signals. Whereas most research on unraveling the action mechanisms of strigolactones is focused on plant shoots, we investigated proteome adaptation during strigolactone signaling in the roots of Arabidopsis thaliana. Through large-scale, time-resolved, and quantitative proteomics, the impact of the strigolactone analog rac-GR24 was elucidated on the root proteome of the wild type and the signaling mutant more axillary growth 2 (max2). Our study revealed a clear MAX2-dependent rac-GR24 response: an increase in abundance of enzymes involved in flavonol biosynthesis, which was reduced in the max2-1 mutant. Mass spectrometry-driven metabolite profiling and thin-layer chromatography experiments demonstrated that these changes in protein expression lead to the accumulation of specific flavonols. Moreover, quantitative RT-PCR revealed that the flavonol-related protein expression profile was caused by rac-GR24-induced changes in transcript levels of the corresponding genes. This induction of flavonol production was shown to be activated by the two pure enantiomers that together make up rac-GR24. Finally, our data provide much needed clues concerning the multiple roles played by MAX2 in the roots and a comprehensive view of the rac-GR24-induced response in the root proteome

From lateral root density to nodule number, the strigolactone analogue GR24 shapes the root architecture of Medicago truncatula

In the rhizosphere, strigolactones not only act as crucial signalling molecules in the communication of plants with parasitic weeds and arbuscular mycorrhiza, but they also play a key role in regulating different aspects of the root system. Here we investigated how strigolactones influence the root architecture of Medicago truncatula. We provide evidence that addition of the synthetic strigolactone analogue GR24 has an inhibitory effect on the lateral root density. Moreover, treatment with GR24 of Sinorhizobium meliloti-inoculated M. truncatula plants affects the nodule number both positively and negatively, depending on the concentration. Plants treated with 0.1 μM GR24 had a slightly increased number of nodules, whereas concentrations of 2 and 5 μM strongly reduced it. This effect was independent of the autoregulation of nodulation mechanism that is controlled by SUPER NUMERIC NODULE. Furthermore, we demonstrate that GR24 controls the nodule number through crosstalk with SICKLE-dependent ethylene signalling. Additionally, because the expression of the nodulation marker EARLY NODULATION11 was strongly reduced in GR24-treated plants, we concluded that strigolactones influence nodulation at a very early stage of the symbiotic interaction.European Union Seventh Framework Programme (EU-FP7-PIRSES-GA-2008–230830 LEGIM).J.F. is indebted to the Institut National de la Recherche Agronomique for a postdoctoral fellowship (Contrat Jeune Scientifique). C.D.C. is a predoctoral fellow of the Research Foundation-Flanders.http://jxb.oxfordjournals.org2016-01-30hb201

Erratum to "From lateral root density to nodule number, the strigolactone analogue GR24 shapes the root architecture of medicago truncatula" (vol 66, pg 137, 2015)

Erratum to http://prodinra.inra.fr/record/279289Erratum to "From lateral root density to nodule number, the strigolactone analogue GR24 shapes the root architecture of [i]medicago truncatula[/i]" (vol 66, pg 137, 2015

The symbiotic transcription factor MtEFD and cytokinins are positively acting in the Medicago truncatula and Ralstonia solanacearum pathogenic interaction.

• A plant-microbe dual biological system was set up involving the model legume Medicago truncatula and two bacteria, the soil-borne root pathogen Ralstonia solanacearum and the beneficial symbiont Sinorhizobium meliloti. • Comparison of transcriptomes under symbiotic and pathogenic conditions highlighted the transcription factor MtEFD (Ethylene response Factor required for nodule Differentiation) as being upregulated in both interactions, together with a set of cytokinin-related transcripts involved in metabolism, signaling and response. MtRR4 (Response Regulator), a cytokinin primary response gene negatively regulating cytokinin signaling and known as a target of MtEFD in nodulation processes, was retrieved in this set of transcripts. • Refined studies of MtEFD and MtRR4 expression during M. truncatula and R. solanacearum interaction indicated differential kinetics of induction and requirement of central regulators of bacterial pathogenicity, HrpG and HrpB. Similar to MtRR4, MtEFD upregulation during the pathogenic interaction was dependent on cytokinin perception mediated by the MtCRE1 (Cytokinin REsponse 1) receptor. • The use of M. truncatula efd-1 and cre1-1 mutants evidenced MtEFD and cytokinin perception as positive factors for bacterial wilt development. These factors therefore play an important role in both root nodulation and root disease development

MtEFD and MtEFD2: Two transcription factors with distinct neofunctionalization in symbiotic nodule development

International audienceRhizobium-legume nitrogen-fixing symbiosis involves the formation of a specific organ, the root nodule, which provides bacteria with the proper cellular environment for atmospheric nitrogen fixation. Coordinated differentiation of plant and bacterial cells is an essential step of nodule development, for which few transcriptional regulators have been characterized. Medicago truncatula ETHYLENE RESPONSE FACTOR REQUIRED FOR NODULE DIFFERENTIATION (MtEFD) encodes an APETALA2/ETHYLENE RESPONSIVE FACTOR (ERF) transcription factor, the mutation of which leads to both hypernodulation and severe defects in nodule development. MtEFD positively controls a negative regulator of cytokinin signaling, the RESPONSE REGULATOR 4 (MtRR4) gene. Here we showed that that the Mtefd-1 mutation affects both plant and bacterial endoreduplication in nodules, as well as the expression of hundreds of genes in young and mature nodules, upstream of known regulators of symbiotic differentiation. MtRR4 expressed with the MtEFD promoter complemented Mtefd-1 hypernodulation but not the nodule differentiation phenotype. Unexpectedly, a nonlegume homolog of MtEFD, AtERF003 in Arabidopsis (Arabidopsis thaliana), could efficiently complement both phenotypes of Mtefd-1, in contrast to the MtEFD paralog MtEFD2 expressed in the root and nodule meristematic zone. A domain swap experiment showed that MtEFD2 differs from MtEFD by its C-terminal fraction outside the DNA binding domain. Furthermore, clustered regularly interspaced short palindromic repeats-CRISPR associated protein 9 (CRISPR-Cas9) mutagenesis of MtEFD2 led to a reduction in the number of nodules formed in Mtefd-1, with downregulation of a set of genes, including notably NUCLEAR FACTOR-YA

Low incidence of SARS-CoV-2, risk factors of mortality and the course of illness in the French national cohort of dialysis patients

International audienceThe aim of this study was to estimate the incidence of COVID-19 disease in the French national population of dialysis patients, their course of illness and to identify the risk factors associated with mortality. Our study included all patients on dialysis recorded in the French REIN Registry in April 2020. Clinical characteristics at last follow-up and the evolution of COVID-19 illness severity over time were recorded for diagnosed cases (either suspicious clinical symptoms, characteristic signs on the chest scan or a positive reverse transcription polymerase chain reaction) for SARS-CoV-2. A total of 1,621 infected patients were reported on the REIN registry from March 16th, 2020 to May 4th, 2020. Of these, 344 died. The prevalence of COVID-19 patients varied from less than 1% to 10% between regions. The probability of being a case was higher in males, patients with diabetes, those in need of assistance for transfer or treated at a self-care unit. Dialysis at home was associated with a lower probability of being infected as was being a smoker, a former smoker, having an active malignancy, or peripheral vascular disease. Mortality in diagnosed cases (21%) was associated with the same causes as in the general population. Higher age, hypoalbuminemia and the presence of an ischemic heart disease were statistically independently associated with a higher risk of death. Being treated at a selfcare unit was associated with a lower risk. Thus, our study showed a relatively low frequency of COVID-19 among dialysis patients contrary to what might have been assumed