Lignin is the second most abundant biopolymer on earth and is found in the wood (xylem) of vascular land plants. To transport the hydro-mineral sap, xylem forms specialized conduit cells, called tracheary elements (TEs), which are hollow dead cylinders reinforced with lateral secondary cell walls (SCW). These SCWs incorporate lignin to gain mechanical strength, water impermeability and resistance against pathogens. The aim of this thesis is to understand the spatio-temporal deposition of lignin during TE differentiation and the relationship with its neighbouring cells. In vitro TE differentiating cell cultures of Zinnia elegans and Arabidopsis thaliana are ideal tools to study this process: cells differentiate simultaneously into 30-50% TEs while the rest remain parenchymatic (non-TEs). Live-cell imaging of such TEs indicated that lignification occurs after programmed cell death (PCD), in a non-cell autonomous manner, in which the non-TEs provide the lignin monomers. This thesis confirms that lignification occurs and continues long after TE PCD in both in vitro TE cultures and whole plants using biochemical, pharmacological and cytological methods. The cooperative supply of lignin monomers by the non-TEs was demonstrated by using Zinnia and Arabidopsis in vitro TE cultures. Inhibitor experiments revealed further that the non-TEs supply reactive oxygen species (ROS) to TEs and that ROS are required for TE post-mortem lignification. Characterization of the non-TEs showed an enlarged nucleus with increased DNA content, thus indicating that non-TEs are in fact endoreplicated xylem parenchyma cells (XP). The cooperative lignification was confirmed in whole plants by using knock-out mutants in a lignin monomer synthesis gene, which exhibit reduced TE lignification. The XP specific complementation of these mutants led to nearly completely rescuing the TE lignin reduction. Using microscopic techniques, the spatial distribution of lignin was analyzed in TEs from in vitro cultures and whole plants, revealing that lignification is restricted to TE SCWs in both protoxylem and metaxylem. These specific deposition domains were established by phenoloxidases, i.e. laccases localized to SCWs and peroxidases, present in SCWs and the apoplastic space. Laccases were cell-autonomously produced by developing TEs, indicating that the deposition domains are defined before PCD. Altogether, these results highlight that the hydro-mineral sap transport through TEs is enabled by the spatially and temporally controlled lignification of the SCW. Lignification occurs post-mortem by the supply of monomers and ROS from neighbouring XP cells and is restricted to specific deposition domains, defined by the pre-mortem sequestration of phenoloxidases.

The reform of the labor process in Uruguay. The return to the autonomous labor processThe reform of the Uruguayan labor process and the return to an autonomous legal regime represent a radical and fundamental change in the national legislation. The simplicity of new procedural structures governed by principles and owns norms of labor discipline give the new regime a dogmatic autonomous space lost for many years. The procedural labor reform is built on the adjectival character of its nature prioritizing the substantive law and adjusting the procedural rule to its characteristics. The principles of Labor Law (substantive and procedural) are the foundation of the new regime / La reforma del proceso laboral uruguayo y la vuelta a un régimen jurídico autónomo constituye un cambio radical y fundamental en la legislación nacional. La sencillez de las nuevas estructuras procesales regidas por principios y normas propias de la disciplina laboral le dan al nuevo régimen un espacio de autonomía dogmática por muchos años perdido. El sistema laboral procesal se construye sobre el carácter adjetivo de su naturaleza priorizando el derecho sustantivo y ajustando la norma procesal a las características del mismo.Los principios del derecho del trabajo (sustantivos y procesales) son labase del nuevo régimen