Tubulin Tyrosination Is Required for the Proper Organization and Pathfinding of the Growth Cone

International audienceBACKGROUND: During development, neuronal growth cones integrate diffusible and contact guidance cues that are conveyed to both actin and microtubule (MT) cytoskeletons and ensure axon outgrowth and pathfinding. Although several post-translational modifications of tubulin have been identified and despite their strong conservation among species, their physiological roles during development, especially in the nervous sytem, are still poorly understood. METHODOLOGY/FINDINGS: Here, we have dissected the role of a post-translational modification of the last amino acid of the alpha-tubulin on axonal growth by analyzing the phenotype of precerebellar neurons in Tubulin tyrosin ligase knock-out mice (TTL(-/-)) through in vivo, ex vivo and in vitro analyses. TTL(-/-) neurons are devoid of tyrosinated tubulin. Their pathway shows defects in vivo, ex vivo, in hindbrains open-book preparations or in vitro, in a collagen matrix. Their axons still orient toward tropic cues, but they emit supernumerary branches and their growth cones are enlarged and exhibit an emission of mis-oriented filopodia. Further analysis of the TTL(-/-) growth cone intracellular organization also reveals that the respective localization of actin and MT filaments is disturbed, with a decrease in the distal accumulation of Myosin IIB, as well as a concomitant Rac1 over-activation in the hindbrain. Pharmacological inhibition of Rac1 over-activation in TTL(-/-) neurons can rescue Myosin IIB localization. CONCLUSIONS/SIGNIFICANCE: In the growth cone, we propose that tubulin tyrosination takes part in the relative arrangement of actin and MT cytoskeletons, in the regulation of small GTPases activity, and consequently, in the proper morphogenesis, organization and pathfinding of the growth cone during development

Effect of root water and solute uptake on solute transport in soils: a 3D simulation study

Plant transpiration is an important component of the hydrological cycle. Through root water uptake, plants do not only affect the three-dimensional soil water flow distribution, but also solute movement. This numerical study aims at investigating how solute fate is impacted by root uptake using the three-dimensional model R-SWMS (Javaux et al., 2008). This model combines the threedimensional Richards equation describing water flow in soil with an equation describing flow in root xylem vessels. The coupling was implemented by defining the water flow along water potential gradients in the root-soil continuum. For solute transport simulations, the three-dimensional random walk particle tracking algorithm PARTRACE (Bechtold et al., 2011) was used. Here, the water flow velocity from the Richards equation and a random displacement for dispersion are used to move large numbers of solute particles through the soil. In a numerical study, we investigated how root and solute uptake by plants affects solute movement in soil. Therefore, we simulated three-dimensional virtual steady-state breakthrough curves (BTC) experiments in soils with transpiring plants. Simulated BTCs of averaged concentrations in a horizontal cross section of the simulation domain were then fitted with a 1D numerical flow and transport model under steady-state conditions to obtain apparent transport parameters of the 1-D model: apparent velocity and dispersivity. In these virtual experiments, the impact of root architecture, solute uptake mechanism and transpiration rate on the apparent disperisivity and velocity could be evaluated. Our simulation results show, that both, apparent velocity and dispersivity length are affected by water and solute root uptake. Under high exclusion processes, solute accumulates around roots and generates a long tailing to the breakthrough curves, which cannot be reproduced by 1D models that simulate root water uptake with solute exclusion. This observation may have an important impact on how to model pollutant mass transfer to groundwater at larger scales

A coupled modeling approach for soil-root interaction processes

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