Seminars

The colonization and survival of plants on land are essentially due to the development of a vascular system which conducts water and minerals throughout the plant body but also reinforces the plant organs mechanically to enable upright growth. The conducting cells, named tracheary elements (TEs), represent a complex vascular plumbing system distributing the hydro-mineral sap to all the tissues of the organism. The basic engineering design behind TE formation is achieved by emptying the cell content by programmed cell death (PCD) and reinforcing its sides with a patterned lignified secondary cell wall: to form a corrugated hollow cylinder which can resist the pressure associated to the sap rising. TE key functional features are therefore dependent on the coordination between PCD, cell wall patterning and secondary cell wall polymer synthesis to establish TEs with distinct cell wall patterns fulfilling specific roles during plant development. To unravel the developmental sequence leading to functional TEs, a novel xylogenic in vitro system using habituated Arabidopsis cell suspension cultures was established. This in vitro system enables controlled large production of TE differentiating cells at distinct stages of development. Long time live cell imaging enabled to define the rate and chronology of the TE developmental sequence and surprisingly revealed events occurring before and after PCD. On one hand, TE pre-mortem events include patterned secondary xylan and cellulose depositions which are specifically controlled by microtubules and specific microtubule associated proteins (MAPs). Although individual MAPs have already been associated with TE cell wall patterning, a complete picture of the MAP proteome was made using a systems biology approach along TE formation combining transcriptomics and quantitative proteomics. Classical MAPs as well as newly characterised proteins revealed the complexity of microtubule-dependent TE cell wall patterning. Pharmacological and genetic modifications of microtubules and MAPs resulted in misplaced unpatterned secondary cell walls. On the other hand, TE post-mortem events include the lignification of TE secondary cell wall thickenings which are essentially controlled by cell cooperation. Surrounding parenchyma cells secrete lignin monomers as well as other compounds which are then specifically oxidised into TE secondary cell walls. Pharmacological and genetic modifications of PCD and lignification revealed that both quantity and quality of TE lignin are controlled by non-cell autonomous processes. Perspectives of this research are numerous and will be discussed during the presentation.