Title: Cellular Aspects of Lignin Biosynthesis in Xylem Vessels of Zinnia and Arabidopsis
Defendant: Henrik Srk
Opponent: Prof. Simon Hawkins, Department of Functional and Structural Glycobiology, University of Lille, France.
Time & Place: 16:th of October 2016, 13.30
Lignin is the second most abundant biopolymer on earth and is found in the xylem (wood) of vascular land plants. To transport the hydro-mineral sap, xylem forms specialized 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. The cooperative supply of lignin monomers by the non-TEs was first demonstrated by using in vitro TE cultures and confirmed in whole plants by using lignin monomer synthesis gene mutants that exhibit a reduction in TE lignification. The XP specific complementation of these mutants led to nearly completely rescuing the TE lignin reduction of the mutants. Experiments with in vitro TE cultures further revealed that non-TEs supply reactive oxygen species (ROS) to TEs and that ROS are required for TE post-mortem lignification. Non-TEs exhibit further an enlarged nucleus with increased DNA content, thus indicating that non-TEs are in fact endoreplicated xylem parenchyma (XP) cells. Microscopic analysis of the spatial distribution of lignin in in vitro TE cultures and whole plants revealed that lignification is restricted to TE SCWs in both protoxylem and metaxylem. These specific lignin deposition domains were found to be established by phenoloxidases, i.e. laccases and peroxidases. 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 conduction 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 neighboring XP cells and is restricted to specific deposition domains, defined by the pre-mortem production of phenoloxidases.