Function of MAP20 and MYB103 in cellulose and lignin formation of xylem secondary cell walls

Sammanfattning: Lignocellulose from trees and other crops will have tremendous impact on the next generation of sustainable biofuels and biomaterials. To take advantage of modern breeding tools, it is therefore important to understand the genetic and molecular regulation underlying secondary cell wall formation. Here, functional analysis was performed on two genes specifically involved in secondary cell wall formation, using Arabidopsis and Populus as model species. PttMAP20 was earlier identified as a wood-specific microtubule-associated protein in hybrid aspen, but not functionally assessed [Rajangam et al. (2008). Plant Physiology, pp. 1283–1294]. In this thesis, AtMAP20 was found to be generally expressed in secondary wall forming cell types in Arabidopsis, including xylem cells, and its binding to microtubules was confirmed. A domain-mapping study showed that its central TPX2 domain, together with the N- and/or C-terminal domain, is required for complete microtubule binding. Overexpression of AtMAP20 induced shorter roots and right-handed twisting, mimicking treatment with the microtubule-stabilizing drug taxol. Loss-of-function map20 mutants had longer etiolated hypocotyls and altered cell wall chemistry. This phenotype was interpreted as resulting from mechanical weakening in the secondary walls of their spiral protoxylem vessels. In line with this, overexpression of PttMAP20 in hybrid aspen affected cellulose microfibril angle. Taken together, MAP20 is a novel microtubule-stabilizing protein, specifically active during secondary cell wall formation and important for the patterning of cellulose microfibrils. MYB103 is a xylem-specific transcription factor, previously demonstrated to be directly activated by the secondary wall NAC master switches SND1/NST1 and VND6/VND7 [Zhong et al. (2008). Plant Cell, pp. 2763–2782]. This thesis demonstrates that loss-of-function Arabidopsis myb103 mutants have reduced levels of syringyl lignin in their basal stems. This was compensated for by an increase in guaiacyl lignin, resulting in a modified syringyl to guaiacyl ratio. The altered lignin composition, characterized by Py/GC-MS, FT-IR microspectroscopy and 2D NMR, was caused by a suppression of F5H, a key gene in syringyl lignin biosynthesis. Thus, it is concluded that MYB103 is required for F5H expression. Taken together, this thesis presents novel knowledge on function of genes important for secondary cell wall formation and, hence, wood formation. These findings have the potential to improve wood characteristics to benefit forest growers and industries.

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