An introduction to modelling flower primordium initiation

International audienceIn this chapter we present models of processes involved in the initiation and development of a flower. In the first section, we briefly present models of hormonal transport. We focus on two key aspects of flower development, namely the initiation, due to the periodic local accumulation of auxin (a plant hormone) near at the plant apex, and the genetic regulation of its development. In the first section, we described the main assumptions about auxin transport that have been proposed and tested in the literature. We show how the use of models make it possible to test assumptions expressed in terms of local cell-to-cell interaction rules and to check if they lead to patterning in the growing tissue consistent with observation.Then, we investigated gene regulatory networks that controls the initial steps of flower development and differentiation. In a simplified form, this network contains a dozen of actors interacting with each other in space and time. The understanding of such a complex system here also requires a modeling approach in order to quantify these interactions and analyze their properties. We briefly present the two main formalisms that are used to model GRN: the Boolean and the ODE formalisms. We illustrate on a sub-module of the flower GRN both types of models and discuss their main advantages and drawbacks. We show how manipulations of the network models can be used to make predictions corresponding to possible biological manipulations of the GRN (e.g. loss-of-function mutants).Throughout the chapter, we highlight specific mathematical topics of particular interest to the development of the ideas developed in the different sections in separated boxes (called Math-boxes). The reading of these boxes is relatively independent of the main text

Le processus de thermo-érosion du pergélisol dans la zone de pergélisol continu

Les polygones à coin de glace sont très répandus dans la zone du pergélisol continu. Lorsque le ruissellement d’eau de fonte nivale s’infiltre de façon concentrée dans une cavité, il peut initier le processus de thermo-érosion du pergélisol (notamment des coins de glace) pouvant mener à la formation de ravins. Dans la vallée de Qalikturvik sur l’Ile Bylot (NU, Canada), le développement de ravins de thermo-érosion dans un milieu de polygones à coins de glace entraîne comme impact : i. la réorganisation des réseaux de drainage impliquant un assèchement des milieux humides en marge des chenaux d’érosion, ii. des variations dans le régime thermique et de l’humidité de proche-surface et iii. la prise en charge et le déplacement des sédiments vers l’extérieur du bassin-versant. L’objectif de cette thèse vise à approfondir les connaissances géomorphologiques propres au ravinement par thermo-érosion, d’examiner, caractériser et quantifier les impacts du ravinement (tel que sus-mentionné en i. ii. iii.) et le rôle de celui-ci dans une optique d’évolution du paysage périglaciaire à l’échelle temporelle de l’année à la décennie. Les ravins sont dynamiques : un ravin en particulier déclenché en 1999 et étudié depuis s’érodait à une vitesse de 38 à 50 m/a durant sa première décennie d’existence, pour atteindre une longueur totale de ~750 m et une surface érodée de ~25 000 m² en 2009. Des puits sont localisés près des zones de ravinement actives ; des levées alluviale, mares et polygones effondrés dans les zones stabilisées post-perturbation. Sur la terrasse de polygones recouvrant le plancher de la vallée au site à l’étude, 35 ravins furent identifiés et 1401 polygones furent perturbés avec 200 000 m³ de sols transportés. Une amélioration du drainage, une dégradation de la capacité de rétention de l’humidité, une transition d’un écoulement de ruissellement vers un écoulement canalisé caractérise les aires ravinées et leurs environs. Les polygones intacts sont homogènes d’un à l’autre et dans leurs centres ; les polygones perturbés ont une réponse hétérogène (flore, humidité et régime thermique). Les milieux érodés hétérogènes succèdent aux milieux homogènes et deviennent le nouvel état d’équilibre pour plusieurs décennies.Ice wedges polygons are very common features characterizing continuous permafrost environments. When concentrated snowmelt runoff recurrently flows into opened frost cracks in ice wedges (or nearby depressions), thermo-erosion of permafrost leading to gullies can occur. In the valley of Qalikturvik on Bylot Island (NU, Canada), rapid development of gullies due to thermo-erosion processes in wet polygons exert the following: i. forced rerouting of drainage networks, changing moisture input in wetlands adjacent to a gullied channel; ii. variation in near-surface thermal regime and moisture; iii. erosion and displacement of thawed soils outside the watershed. The objectives of this thesis are to quantify and widen the understanding of the thermo-erosion gullying geomorphology, the related impacts (as aforementioned in i. ii. iii.) and the role of such erosion in the context of the periglacial landscape evolution on a short to medium timescale. In this study, gullies are active and dynamic: one gully triggered in 1999 (and monitored since then) elongated at a rate varying between 38 and 50 m/y during the first decade following its initiation. Its total length at the end of that decade (1999-2009) was ~750 m for an eroded surface of ~25 000 m². Sinkholes were exclusive to the active zone of a gully where alluvial levees and collapsed polygons were common in the stable zone. Thirty-five gullies were identified over the polygon terrace featuring the valley floor. 1401 polygons were consequently breached due to thermo-erosion resulting in 200 000 m³ of displaced soils and thawed permafrost. Gullied areas were characterized by improved drainage conditions, a reduced capacity to retain moisture and a transition from dominantly surface runoff toward channelized flow. Intact wet polygons were homogeneous between each other and in their center; disturbed polygons were heterogeneous (for moisture, thermal regime and vegetation) following a breach of at least one side. Wetland polygons, when disturbed due to gullying, evolve toward an heterogeneous landscape which will become the new equilibrium for that terrain for decades to come

Analyzing perturbations in phyllotaxis of Arabidopsis thaliana

International audienceVascular plants produce new organs at the tip of the stem in a very organized fashion. This patterning process occurs in small groups of stem cells, the so-called shoot apical meristems (SAM), and generates regular patterns called phyllotaxis. The phyllotaxis of the model plant Arabidopsis thaliana follows a Fibonacci spiral, the most frequent phyllotactic pattern found in nature. In this phyllotactic mode, single organs are initiated successively at a divergence angle from the previous organ close to 137.5°, the golden angle. Cytokinins, a class of plant hormones, is involved in the control of phyllotaxis but its role has remained elusive (Vernoux et al., 2010). By analyzing the expression of several cytokinin signaling regulators in the meristem, we found that the pseudo-phosphotransfer protein AHP6 is expressed specifically during early organogenesis (unpublished results). AHP6 has been demonstrated to act as an inhibitor of cytokinin signaling (Mahonen et al., 2006) and we further observed a destabilization of phyllotaxis in ahp6 null mutant. To understand how AHP6 acts in the control of Arabidopsis phyllotaxis, we analyzed sequences of divergence angles in both wild-type and ahp6 mutant plants. We thus measured the divergence angle between successive flowers on a stem from the base (older flowers) to the top (younger flowers)

Cauliflower fractal forms arise from perturbations of floral gene networks

[EN] Throughout development, plant meristems regularly produce organs in defined spiral, opposite, or whorl patterns. Cauliflowers present an unusual organ arrangement with a multitude of spirals nested over a wide range of scales. How such a fractal, self-similar organization emerges from developmental mechanisms has remained elusive. Combining experimental analyses in an Arabidopsis thaliana cauliflower-like mutant with modeling, we found that curd self-similarity arises because the meristems fail to form flowers but keep the "memory" of their transient passage in a floral state. Additional mutations affecting meristem growth can induce the production of conical structures reminiscent of the conspicuous fractal Romanesco shape. This study reveals how fractal-like forms may emerge from the combination of key, defined perturbations of floral developmental programs and growth dynamics.This work was supported by the INRAE Caulimodel project (to F.P. and C.Go.); Inria Project Lab Morphogenetics (to C.Go., E.A., and F.P.); the ANR BBSRC Flower model project (to F.P. and C.Go.); the GRAL LabEX (ANR-10-LABX-49-01) within the framework of the CBH-EUR-GS (ANR-17-EURE-0003) (to F.P., G.T., M.L.M., and J.L.); the EU H2020 773875 ROMI project (to C.Go.); and the Spanish Ministerio de Ciencia Innovacion and FEDER (grant no. PGC2018-099232-B-I00 to F.M.).Azpeitia, E.; Tichtinsky, G.; Le Masson, M.; Serrano-Mislata, A.; Lucas, J.; Gregis, V.; Gimenez, C.... (2021). Cauliflower fractal forms arise from perturbations of floral gene networks. Science. 373(6551):1-6. https://doi.org/10.1126/science.abg5999S16373655

Plexin-B2 Negatively Regulates Macrophage Motility, Rac, and Cdc42 Activation

Plexins are cell surface receptors widely studied in the nervous system, where they mediate migration and morphogenesis though the Rho family of small GTPases. More recently, plexins have been implicated in immune processes including cell-cell interaction, immune activation, migration, and cytokine production. Plexin-B2 facilitates ligand induced cell guidance and migration in the nervous system, and induces cytoskeletal changes in overexpression assays through RhoGTPase. The function of Plexin-B2 in the immune system is unknown. This report shows that Plexin-B2 is highly expressed on cells of the innate immune system in the mouse, including macrophages, conventional dendritic cells, and plasmacytoid dendritic cells. However, Plexin-B2 does not appear to regulate the production of proinflammatory cytokines, phagocytosis of a variety of targets, or directional migration towards chemoattractants or extracellular matrix in mouse macrophages. Instead, Plxnb2−/− macrophages have greater cellular motility than wild type in the unstimulated state that is accompanied by more active, GTP-bound Rac and Cdc42. Additionally, Plxnb2−/− macrophages demonstrate faster in vitro wound closure activity. Studies have shown that a closely related family member, Plexin-B1, binds to active Rac and sequesters it from downstream signaling. The interaction of Plexin-B2 with Rac has only been previously confirmed in yeast and bacterial overexpression assays. The data presented here show that Plexin-B2 functions in mouse macrophages as a negative regulator of the GTPases Rac and Cdc42 and as a negative regulator of basal cell motility and wound healing

Effects of thermo-erosion gullying on hydrologic flow networks, discharge and soil loss

Thermo-erosion gullies in continuous permafrost regions where ice-wedge polygons are widespread contribute and change the drainage of periglacial landscapes. Gullying processes are causing long-term impacts to the Arctic landscape such as drainage network restructuring, permafrost erosion, sediment transport. Between 2009 and 2013, 35 gullies were mapped in a polygon terrace in the valley of the Glacier C-79 on Bylot Island, Nunavut (Canada), one of which was monitored for its hydrology. A gully (R08p) initiated in 1999 in a low-center polygon terrace. Between 1999 and 2013, 202 polygons over a surface of 28 891 m ^2 were breached by gullying. Overall, 1401 polygons were similarly breached on the terrace in the valley before 2013. R08p is fed by a 1.74 km ^2 watershed and the hydrological regime is characterized by peak flows of 0.69 m ^3 s ^−1 and a cumulative volume of 229 662 m ^3 for 2013. Historic aerial photography from 1972 and recent field surveys showed a change in the paths of water tracks and an increase in channelized flow in the gully area from none to 35% of the overall flow path of the section. The overall eroded area for the studied gullies in the valley up to 2013 was estimated at 158 000 m ^2 and a potential volume close to 200 000 m ^3 . Gullying processes increased drainage of wetlands and the hydrological connectivity in the valley, while lowering residence time of water near gullied areas