Acetylation of polysaccharides in plant cell wall : with a focus on woody species
Sammanfattning: Plant cell wall in woody tissues is a complex matrix, which consists of cellulose, matrix polysaccharides and lignin. The matrix polysaccharides are substituted with acetyl group that are hypothesised to play important roles in determining properties of these polysaccharides. The aim of this thesis was to understand the role of O-acetylation in plants and investigate possibilities for improvement of woody lignocellulose for biorefinery applications by reducing wood O-acetylation. To alter acetylation specifically in woody tissues, a promoter from Glycosyl Transferase 43 family (GT43) in Populus was isolated that had a very specific expression in secondary cell wall forming cells. This xylem specific promoter (pGT43B) was more effective in modification of wood acetylation by overexpression and suppression strategies as compared to 35S promoter (Paper I). To reduce xylan acetylation using transgenic approach, acetyl xylan esterase from Aspergillus niger, AnAXE1, was targetted specifically to the cell wall in Arabidopsis (Paper II) and in Populus (Paper III). Plants expressing AnAXE1 grew as well as wild type and had increased acetyl esterase activity. This has led to reduction in cell wall acetyl content and in xylan O-acetylation. Moreover, transgenic Arabidopsis exhibited increased resistance against a biotrophic pathogen Hyaloperonospora arabidopsidis. Both transgenic plants had improved sugar yields in saccharification with different pretreatments and without pretreatment. To reduce acetylation using cisgenic approach, Populus Reduced Wall Acetylation (RWA) gene family was characterised by suppression of the two clades RWA-AB, and RWA-CD (Paper IV). Both clades were shown to be involved in xylan acetylation in the wood. Both clades were therefore suppressed under control of xylem specific promoter pGT43B to improve wood saccharification potential. Transgenic plants had reduced wood acetyl content, normal growth, and increased sugar yield and glucose conversion % in saccharification without pretreatment. Glucose yield was also slightly increased in saccharification after acid pretreatment. These results show that reduction of cell wall acetylation by 10-30% does not alter plant growth and development, but improves yields in lignocellulose saccharification (with and without pretreatment) (Papers II, III and IV). To identify Quantitative Trait Loci (QTLs) related to cell wall acetyl content and other chemical traits in Salix, the mapping population of 463 progenies of S. viminalis and S. schwerinii was analysed by FT-IR and acetyl content assay (Paper V). 28 QTLs were identified for different cell wall chemical traits, which were co-located with several cell wall related genes and gene clusters. These QTLs and genes can be used in the future to improve wood chemical traits in Salix and Populus for biofuel production by breeding.
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