New regulators of xylem lignification in arabidopsis

Sammanfattning: The ability of land plants to grow upright, bear their own weight and withstand adverse environmental conditions is largely dependent on the secondary xylem tissues of the stem. The xylem cells acquire thick secondary cell walls which are composed of cellulose, hemicellulose and lignin. The chemical structure of lignin renders the secondary cell wall rigid and waterproof, facilitating the transport of water and solutes through the vascular system. Lignin is a polyphenolic polymer composed of three different types of lignin units, guaiacyl (G), syringyl (S) and p-hydroxyphenyl (H), derived from the coniferyl, sinapyl and p-coumaryl alcohol, respectively. Lignin biosynthesis, monolignol transport and lignin polymerization (collectively called as ”lignification”) are controlled by numerous transcription factors and other regulators.This thesis work uncovers three novel regulators of lignification in the secondary xylem tissues of Arabidopsis (Arabidopsis thaliana) stem and hypocotyl. The cupin domain containing protein PIRIN2 (PRN2) suppresses S-type lignin accumulation. PRN2 functions in a non-cell-autonomous fashion: it is expressed in the cells next to the xylem vessel elements, but affects the lignin composition of the vessel and fiber cell walls of the neighbouring cells. Two protein interactors of PRN2 are characterized here in connection to lignification. Opposite to the function of PRN2, the chromatin-modifying protein HISTONE MONOUBIQUITINATION2 (HUB2) promotes S-type lignin deposition. In line with this, PRN2 and HUB2 antagonistically regulate the expression of FERULATE-5-HYDROXYLASE1 which encodes the key S-type lignin-biosynthetic enzyme. Possibly, PRN2 antagonizes the S-lignin promoting function of HUB2 to ensure that the cell walls of the vessel elements get enriched in G-type lignin. Finally, identification of a potential diurnal modulator of lignin biosynthesis is described in this work. The PRN2-interacting basic helix-loop-helix transcription factor (PIB) does not influence the lignin content or composition of the secondary cell walls. However, PIB affects the diurnal expression pattern and promoter activity of some lignin-biosynthetic genes. Altogether, PRN2, HUB2 and PIB highlight the importance of intercellular co-operation in lignification, and uncover novel regulatory aspects of this process.