Abstract
Extensins are one subfamily of the cell wall hydroxyproline-rich glycoproteins, containing characteristic SerHyp4 glycosylation motifs and intermolecular cross-linking motifs such as the TyrXaaTyr sequence. Extensins are believed to form a cross-linked network in the plant cell wall through the tyrosine-derivatives isodityrosine, pulcherosine, and di-isodityrosine. Overexpression of three synthetic genes encoding different elastin-arabinogalactan protein-extensin hybrids in tobacco suspension cultured cells yielded novel cross-linking glycoproteins that shared features of the extensins, arabinogalactan proteins and elastin. The cell wall properties of the three transgenic cell lines were all changed, but in different ways. One transgenic cell line showed decreased cellulose crystallinity and increased wall xyloglucan content; the second transgenic cell line contained dramatically increased hydration capacity and notably increased cell wall biomass, increased di-isodityrosine, and increased protein content; the third transgenic cell line displayed wall phenotypes similar to wild type cells, except changed xyloglucan epitope extractability. These data indicate that overexpression of modified extensins may be a route to engineer plants for bioenergy and biomaterial production.
Highlights
Plants are the major source of food and chemicals on earth
The decreased cellulose crystallinity appears to be the primary factor leading to the reduced recalcitrance of the 224-EGFP walls as measured by the significantly greater amounts of cellulosic glucose released from the transgenic walls after cellulase treatment compared with WT walls (Fig. 5)
Overexpression of extensin analogs in tobacco primary cell walls resulted in changes of wall properties that were distinct for each transgenic cell line
Summary
Plants are the major source of food and chemicals on earth. With our intensifying desire for sustainable energy, the engineering of plants, in particular their cell walls, to achieve greater biofuel production is a major focus of current cell wall research. Genetic manipulation of plants using gene transformation or selective breeding allows the production plants possessing higher biomass and digestibility [1,2,3]. Examples include the modification of caffeic acid O-methyltransferase (COMT) and caffeoyl CoA 3-O-methyltransferase (CCoAOMT) [4, 5], two enzymes within the lignin biosynthesis pathway, to reduce lignin content and/or modify its structure for higher saccharification of biomass. The modification resulted in low lignin, a low syringyl:guaiacyl lignin ratio in the transgenic plants, and a 8% increase in ethanol production during traditional fermentation. Suppression of CCoAOMT in transgenic poplar tree reduced cell wall lignin content as much as 10% and significantly increased glucose yield from mature poplar wood when subjected to enzymatic digestion [5]
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