Creation of zebrafish knock‐in reporter lines in the nefma gene by Cas9‐mediated homologous recombination

International audienceCRISPR/Cas9-based strategies are widely used for genome editing in many organisms, including zebrafish. Although most applications consist in introducing double strand break (DSB)-induced mutations, it is also possible to use CRISPR/Cas9 to enhance homology directed repair (HDR) at a chosen genomic location to create knock-ins with optimally controlled precision. Here, we describe the use of CRISPR/Cas9-targeted DSB followed by HDR to generate zebrafish transgenic lines where exogenous coding sequences are added in the nefma gene, in frame with the endogenous coding sequence. The resulting knock-in embryos express the added gene (fluorescent reporter or KalTA4 transactivator) specifically in the populations of neurons that express nefma, making them convenient tools for research on these populations

Growth Conditions Determine the DNF2 Requirement for Symbiosis

International audienceRhizobia and legumes are able to interact in a symbiotic way leading to the development of root nodules. Within nodules, rhizobia fix nitrogen for the benefit of the plant. These interactions are efficient because spectacularly high densities of nitrogen fixing rhizobia are maintained in the plant cells. DNF2, a Medicago truncatula gene has been described as required for nitrogen fixation, bacteroid's persistence and to prevent defense-like reactions in the nodules. This manuscript shows that a Rhizobium mutant unable to differentiate is not sufficient to trigger defense-like reactions in this organ. Furthermore, we show that the requirement of DNF2 for effective symbiosis can be overcome by permissive growth conditions. The dnf2 knockout mutants grown in vitro on agarose or Phytagel as gelling agents are able to produce nodules fixing nitrogen with the same efficiency as the wild-type. However, when agarose medium is supplemented with the plant defense elicitor ulvan, the dnf2 mutant recovers the fix- phenotype. Together, our data show that plant growth conditions impact the gene requirement for symbiotic nitrogen fixation and suggest that they influence the symbiotic suppression of defense reactions in nodules

A Conserved Role for the NAM/miR164 Developmental Module Reveals a Common Mechanism Underlying Carpel Margin Fusion in Monocarpous and Syncarpous Eurosids

International audienceThe majority of angiosperms are syncarpous- their gynoecium is composed of two or more fused carpels. In Arabidopsis thaliana, this fusion is regulated through the balance of expression between CUP SHAPED COTYLEDON (CUC) genes, which are orthologs of the Petunia hybrida transcription factor NO APICAL MERISTEM (NAM), and their post transcriptional regulator miR164. Accordingly, the expression of a miR164-insensitive form of A. thallana CUC2 causes a radical breakdown of carpel fusion. Here, we investigate the role of the NAM/miR164 genetic module in carpel closure in monocarpous plants. We show that the disruption of this module in monocarpous flowers of A. thaliana aux1-22 mutants causes a failure of carpel closure, similar to the failure of carpel fusion observed in the wild-type genetic background. This observation suggested that closely related mechanisms may bring about carpel closure and carpel fusion, at least in A. thaliana. We therefore tested whether these mechanisms were conserved in a eurosid species that is monocarpous in its wild type form. We observed that expression of MtNAM, the NAM ortholog in the monocarpous eurosid Medicago truncatula, decreases during carpel margin fusion, suggesting a role for the NAM/miR164 module in this process. We transformed M. truncatula with a miR164-resistant form of MtNAM and observed, among other phenotypes, incomplete carpel closure in the resulting transformants. These data confirm the underlying mechanistic similarity between carpel closure and carpel fusion which we observed in A. thaliana, Our observations suggest that the role of the NAM/miR164 module in the fusion of carpel margins has been conserved at least since the most recent common ancestor of the eurosid clade, and open the possibility that a similar mechanism may have been responsible for carpel closure at much earlier stages of angiosperm evolution. We combine our results with studies of early diverging angiosperms to speculate on the role of the NAM/miR164 module in the origin and further evolution of the angiosperm carpel

Arabidopsis thaliana SHOOT MERISTEMLESS Substitutes for Medicago truncatula SINGLE LEAFLET1 to Form Complex Leaves and Petals

International audienceLEAFY plant-specific transcription factors, which are key regulators of flower meristem identity and floral patterning, also contribute to meristem activity. Notably, in some legumes, LFY orthologs such as Medicago truncatula SINGLE LEAFLET (SGL1) are essential in maintaining an undifferentiated and proliferating fate required for leaflet formation. This function contrasts with most other species, in which leaf dissection depends on the reactivation of KNOTTED-like class I homeobox genes (KNOXI). KNOXI and SGL1 genes appear to induce leaf complexity through conserved downstream genes such as the meristematic and boundary CUP-SHAPED COTYLEDON genes. Here, we compare in M. truncatula the function of SGL1 with that of the Arabidopsis thaliana KNOXI gene, SHOOT MERISTEMLESS (AtSTM). Our data show that AtSTM can substitute for SGL1 to form complex leaves when ectopically expressed in M. truncatula. The shared function between AtSTM and SGL1 extended to the major contribution of SGL1 during floral development as ectopic AtSTM expression could promote floral organ identity gene expression in sgl1 flowers and restore sepal shape and petal formation. Together, our work reveals a function for AtSTM in floral organ identity and a higher level of interchangeability between meristematic and floral identity functions for the AtSTM and SGL1 transcription factors than previously thought

Selecting guide RNAs with high on-target and low off-target activity using CRISPOR

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A nonRD receptor-like kinase prevents nodule early senescence and defense-like reactions during symbiosis

International audienceRhizobia and legumes establish symbiotic interactions leading to the production of root nodules, in which bacteria fix atmospheric nitrogen for the plant's benefit. This symbiosis is efficient because of the high rhizobia population within nodules. Here, we investigated how legumes accommodate such bacterial colonization. We used a reverse genetic approach to identify a Medicago truncatula gene, SymCRK, which encodes a cysteine-rich receptor-like kinase that is required for rhizobia maintenance within the plant cells, and performed detailed phenotypic analyses of the corresponding mutant. The Medicago truncatula symCRK mutant developed nonfunctional and necrotic nodules. A nonarginine asparate (nonRD) motif, typical of receptors involved in innate immunity, is present in the SymCRK kinase domain. Similar to the dnf2 mutant, bacteroid differentiation defect, defense-like reactions and early senescence were observed in the symCRK nodules. However, the dnf2 and symCRK nodules differ by their degree of colonization, which is higher in symCRK. Furthermore, in contrast to dnf2, symCRK is not a conditional mutant. These results suggest that in M. truncatula at least two genes are involved in the symbiotic control of immunity. Furthermore, phenotype differences between the two mutants suggest that two distinct molecular mechanisms control suppression of plant immunity during nodulation

Hnf1b renal expression directed by a distal enhancer responsive to Pax8

International audienceXenopus provides a simple and efficient model system to study nephrogenesis and explore the mechanisms causing renal developmental defects in human. Hnf1b (hepatocyte nuclear factor 1 homeobox b), a gene whose mutations are the most commonly identified genetic cause of developmental kidney disease, is required for the acquisition of a proximo-intermediate nephron segment in Xenopus as well as in mouse. Genetic networks involved in Hnf1b expression during kidney development remain poorly understood. We decided to explore the transcriptional regulation of Hnf1b in the developing Xenopus pronephros and mammalian renal cells. Using phylogenetic footprinting, we identified an evolutionary conserved sequence (CNS1) located several kilobases (kb) upstream the Hnf1b transcription start and harboring epigenomic marks characteristics of a distal enhancer in embryonic and adult renal cells in mammals. By means of functional expression assays in Xenopus and mammalian renal cell lines we showed that CNS1 displays enhancer activity in renal tissue. Using CRISPR/cas9 editing in Xenopus tropicalis, we demonstrated the in vivo functional relevance of CNS1 in driving hnf1b expression in the pronephros. We further showed the importance of Pax8-CNS1 interaction for CNS1 enhancer activity allowing us to conclude that Hnf1b is a direct target of Pax8. Our work identified for the first time a Hnf1b renal specific enhancer and may open important perspectives into the diagnosis for congenital kidney anomalies in human, as well as modeling HNF1B-related diseases

Bacteroid differentiation defect is not sufficient to trigger defense-like reactions in <i>dnf2</i> nodules.

<p>Panel a: Expression of defense markers was evaluated by RT-qPCR using cDNA prepared from 14 dpi nodules. The y axis represents fold induction/WT. Panel b: 25 dpi nodules of R108 WT induced by Sm1021 WT. Panel c: 25 dpi nodules of <i>dnf2–4</i> induced by Sm1021 WT. Panel d: 25 dpi nodules of R108 WT induced by a SM1021 <i>bacA</i> derivative. Nodules in b, c and d were stained for phenolics using potassium permanganate toluidine blue (scale bars 500 µm).</p

The plant substrate triggers <i>dnf2</i> fix⁻ phenotype at distance.

<p><i>dnf2–4</i> and WT plants nodulated by <i>S. meliloti</i> Rm41 were grown on agarose-based BNM. Agar- or agarose-based plugs (1.5×1×0.5 cm) were laid onto root systems at 15 dpi and the color and numbers of nodules produced by 24 plants were monitored. The experimental set up is illustrated in (a). (b) Distribution of nodule classes 35 days after plug addition for WT and <i>dnf2–4</i> grown on agarose based medium. The experiment was repeated three times with similar results. Statistically identical distribution are attributed identical letters (Chi-Square Test of Homogeneity with Bonferroni correction, p-value = 1.32e-06).</p

Influence of the plant growth conditions on <i>dnf2</i> phenotype is transient.

<p>(a–c) Frequencies of nodule classes after transfer to agar or agarose medium. <i>M. truncatula dnf2–4</i> and WT plants (n = 24 for every conditions) inoculated with <i>S. meliloti</i> Rm41 were cultivated <i>in vitro</i> on BNM using agar or agarose as a gelling agent for 14 days and transfer to new medium with the same or different gelling agent. Pink nodules are represented by diamonds, white nodules by open squares and brownish nodules by triangles. The experiment was repeated three times with similar results. (d) analysis of the distribution of nodule classes at 35 days after transfer. Statistically identical distribution are attributed identical letters (Chi-Square Test of Homogeneity with Bonferroni correction, p-value = 2.2–16).</p