Nitrate modulates stem cell dynamics in Arabidopsis shoot meristems through cytokinins

The shoot apical meristem (SAM) is responsible for the generation of all the aerial parts of plants. Given its critical role, dynamical changes in SAM activity should play a central role in the adaptation of plant architecture to the environment. Using quantitative microscopy, grafting experiments, and genetic perturbations, we connect the plant environment to the SAM by describing the molecular mechanism by which cytokinins signal the level of nutrient availability to the SAM. We show that a systemic signal of cytokinin precursors mediates the adaptation of SAM size and organogenesis rate to the availability of mineral nutrients by modulating the expression of WUSCHEL, a key regulator of stem cell homeostasis. In time-lapse experiments, we further show that this mechanism allows meristems to adapt to rapid changes in nitrate concentration, and thereby modulate their rate of organ production to the availability of mineral nutrients within a few days. Our work sheds light on the role of the stem cell regulatory network by showing that it not only maintains meristem homeostasis but also allows plants to adapt to rapid changes in the environment

Million-year-scale alternation of warm–humid and semi-arid periods as a mid-latitude climate mode in the Early Jurassic (late Sinemurian, Laurasian Seaway)

Clay mineral and stable isotope (C, O) data are reported from the upper Sinemurian (Lower Jurassic) of the Cardigan Bay Basin (Llanbedr–Mochras Farm borehole, northwestern Wales) and the Paris Basin (Montcornet borehole, northern France) to highlight the prevailing environmental and climatic conditions. In both basins, located at similar palaeolatitudes of 30–35∘ N, the clay mineral assemblages comprise chlorite, illite, illite–smectite mixed layers (R1 I-S), smectite, and kaolinite in various proportions. Because the influence of burial diagenesis and authigenesis is negligible in both boreholes, the clay minerals are interpreted to be derived from the erosion of the Caledonian and Variscan massifs, including their basement and pedogenic cover. In the Cardigan Bay Basin, the variations in the proportions of smectite and kaolinite are inversely related to each other through the entire upper Sinemurian. As in the succeeding Pliensbachian, the upper Sinemurian stratigraphic distribution reveals an alternation of kaolinite-rich intervals reflecting strong hydrolysing conditions and smectite-rich intervals indicating a semi-arid climate. Kaolinite is particularly abundant in the upper part of the obtusum zone and in the oxynotum zone, suggesting more intense hydrolysing conditions likely coeval with warm conditions responsible for an acceleration of the hydrological cycle. In the north of the Paris Basin, the succession is less continuous compared to the Cardigan Bay Basin site, as the oxynotum zone and the upper raricostatum zone are either absent or highly condensed. The clay assemblages are dominantly composed of illite and kaolinite without significant stratigraphic trends, but a smectite-rich interval identified in the obtusum zone is interpreted as a consequence of the emersion of the London–Brabant Massif following a lowering of sea level. Following a slight negative carbon isotope excursion at the obtusum–oxynotum zone transition, a long-term decrease in δ13Corg from the late oxynotum–early raricostatum zones is recorded in the two sites and may precede or partly include the negative carbon isotope excursion of the Sinemurian–Pliensbachian Boundary Event, which is recognised in most basins worldwide and interpreted to signify a late pulse of the Central Atlantic Magmatic Province volcanism

A MAP6-Related Protein Is Present in Protozoa and Is Involved in Flagellum Motility

In vertebrates the microtubule-associated proteins MAP6 and MAP6d1 stabilize cold-resistant microtubules. Cilia and flagella have cold-stable microtubules but MAP6 proteins have not been identified in these organelles. Here, we describe TbSAXO as the first MAP6-related protein to be identified in a protozoan, Trypanosoma brucei. Using a heterologous expression system, we show that TbSAXO is a microtubule stabilizing protein. Furthermore we identify the domains of the protein responsible for microtubule binding and stabilizing and show that they share homologies with the microtubule-stabilizing Mn domains of the MAP6 proteins. We demonstrate, in the flagellated parasite, that TbSAXO is an axonemal protein that plays a role in flagellum motility. Lastly we provide evidence that TbSAXO belongs to a group of MAP6-related proteins (SAXO proteins) present only in ciliated or flagellated organisms ranging from protozoa to mammals. We discuss the potential roles of the SAXO proteins in cilia and flagella function

Le burn-out des soignants travaillant en soins palliatifs

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Lithostratigraphy, hydrodynamism and thermicity in the multi-layer sedimentary system intersected by the Andra deep borehole of Montiers-sur-Saulx (Meuse, France)

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Chlorine transport processes through a 2000 m aquifer/aquitard system

International audienceIn the Paris Basin, in France, the Callovo-Oxfordian (COx) is currently studied over a 250 km2 surface area by the French national radioactive waste management agency in order to assess the feasibility of long-term underground nuclear waste repository. The COx is a 140 m thick clay-rich layer, which is part of the 2000 m aquitard/aquifer system constituting the sedimentary cover. In such sedimentary context, the transport processes of potential contaminants can be represented by both vertical diffusion and horizontal advection through the most permeable layers. Chloride is used as a natural conservative tracer, and is monitored in term of concentrations and isotopic composition (δ37Cl) for both pore and groundwater. During this study, the samples were collected from three boreholes located in the center of the studied zone, one of them (EST433) going down to 2000 m depth. The main solute transport process is shown to be vertical diffusion from the massive Keuper halite level to the rest of the sedimentary pile. This global diffusive system can be occasionally disturbed by horizontal circulation of groundwater occurring in the Oxfordian and Dogger limestone formations. Therefore, these circulations cut the global diffusive system in a succession of independent diffusive systems. In this study the data set was implemented in a simplified 2D solute transport model and scenarii reproducing known history in term of paleo circulations inside the system, were applied and allowed to obtained a good fit of the data. Model results showed that paleo circulations, occurring between −145 Ma and −110 Ma, still have an impact on current distribution of chloride in the system, especially for δ37Cl. The model highlights the need of the presence of a circulation spatially limited at the base of the Liassic formation to fit the data. The best fit obtained indicated current residence time of 500 ka in the Dogger and Oxfordian, with respective onset of the circulations at −20 Ma and −5 Ma

Evolution climatique au Jurassique inférieur (Sinémuriensupérieur) de la zone Nord-Ouest européenne, déduit de laminéralogie des argiles et des isotopes du carbone

National audienceL’étude des minéraux argileux et des isotopes stables (carbone) du Sinémuriensupérieur du bassin de Cardigan Bay (forage de Mochras, Pays de Galles) et du bassinde Paris (forage de Montcornet, France) permet de mettre en évidence les conditionsenvironnementales et climatiques dominantes à cette période. Dans les deux bassins,situés à des paléolatitudes proches (30 à 35°N), les assemblages de minéraux argileuxcomprennent des chlorites, des illites, des interstratifiés illite-smectite (IS R1), dessmectites et des kaolinites dans des proportions variables. L'influence de la diagenèsed’enfouissement et de l'authigénèse étant négligeable dans les deux forages, lesminéraux argileux sont interprétés comme provenant principalement de l'érosion desmassifs calédoniens et varisques. Dans le bassin de Cardigan Bay, les variations desproportions de smectites et de kaolinites sont opposées sur l'ensemble du Sinémuriensupérieur et révèlent des intervalles d'hydrolyse intense sous un climat chaud etrégulièrement humide (fortes proportions de kaolinites) et des intervalles semi-arides(forte proportions de smectites). Les kaolinites, particulièrement abondantes dans lapartie supérieure de la Zone à obtusum et dans la Zone à oxynotum, suggèrent unehydrolyse plus intenses probablement coïncidente avec des conditions chaudes,responsables d'une accélération du cycle hydrologique. Au nord du bassin de Paris, lasérie stratigraphique est beaucoup moins épaisse que sur le site du bassin de CardiganBay. Les assemblages argileux sont principalement composés d'illites et de kaolinitessans tendance significative, mais un intervalle riche en smectites identifié dans la Zoneà obtusum est interprété comme une conséquence de l'émersion du Massif Londres-Brabant suite à un abaissement du niveau marin. La diminution à long terme du δ13Corgà la fin de la Zone à oxynotum, enregistrée dans les deux sites, semble amorcerl’excursion négative reconnue quasi-mondialement au passageSinémurien/Pliensbachien (SPBE) probablement liée au volcanisme de la ProvinceMagmatique de l'Atlantique Central (CAMP)