title: Active fungal GH115 α-glucuronidase produced in Arabidopsis thaliana affects only the UX1-reactive glucuronate decorations on native glucuronoxylans.
abstract: BACKGROUND
  Expressing microbial polysaccharide-modifying enzymes in plants is an attractive approach to custom tailor plant lignocellulose and to study the importance of wall structures to plant development. Expression of α-glucuronidases in plants to modify the structures of glucuronoxylans has not been yet attempted. Glycoside hydrolase (GH) family 115 α-glucuronidases cleave the internal α-D-(4-O-methyl)glucopyranosyluronic acid ((Me)GlcA) from xylans or xylooligosaccharides. In this work, a GH115 α-glucuronidase from Schizophyllum commune, ScAGU115, was expressed in Arabidopsis thaliana and targeted to apoplast. The transgene effects on native xylans' structures, plant development, and lignocellulose saccharification were evaluated and compared to those of knocked out glucuronyltransferases AtGUX1 and AtGUX2.
  
  
  RESULTS
  The ScAGU115 extracted from cell walls of Arabidopsis was active on the internally substituted aldopentaouronic acid (XUXX). The transgenic plants did not show any change in growth or in lignocellulose saccharification. The cell wall (Me)GlcA and other non-cellulosic sugars, as well as the lignin content, remained unchanged. In contrast, the gux1gux2 double mutant showed a 70% decrease in (Me)GlcA to xylose molar ratio, and, interestingly, a 60% increase in the xylose content. Whereas ScAGU115-expressing plants exhibited a decreased signal in native secondary walls from the monoclonal antibody UX1 that recognizes (Me)GlcA on non-acetylated xylan, the signal was not affected after wall deacetylation. In contrast, gux1gux2 mutant was lacking UX1 signals in both native and deacetylated cell walls. This indicates that acetyl substitution on the xylopyranosyl residue carrying (Me)GlcA or on the neighboring xylopyranosyl residues may restrict post-synthetic modification of xylans by ScAGU115 in planta.
  
  
  CONCLUSIONS
  Active GH115 α-glucuronidase has been produced for the first time in plants. The cell wall-targeted ScAGU115 was shown to affect those glucuronate substitutions of xylan, which are accessible to UX1 antibody and constitute a small fraction in Arabidopsis, whereas majority of (Me)GlcA substitutions were resistant, most likely due to the shielding by acetyl groups. Plants expressing ScAGU115 did not show any defects under laboratory conditions indicating that the UX1 epitope of xylan is not essential under these conditions. Moreover the removal of the UX1 xylan epitope does not affect lignocellulose saccharification.
authors.affiliation: Department of Food and Environmental Sciences, Faculty of Agriculture and Forestry, University of Helsinki, P.O. Box 27, Helsinki, 00014, Finland. sun-li.chong@helsinki.fi.
authors.forename: Sun-Li
authors.initials: SL
authors.lastname: Chong
authors.affiliation: Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, Umeå, 901-83, Sweden. marta.derba-maceluch@slu.se.
authors.forename: Marta
authors.initials: M
authors.lastname: Derba-Maceluch
authors.affiliation: Department of Food and Environmental Sciences, Faculty of Agriculture and Forestry, University of Helsinki, P.O. Box 27, Helsinki, 00014, Finland. sanna.koutaniemi@helsinki.fi.
authors.forename: Sanna
authors.initials: S
authors.lastname: Koutaniemi
authors.affiliation: Center for Novel Agricultural Products Department of Biology, University of York, York, YO10 5DD, UK. leonardo.gomez@york.ac.uk.
authors.forename: Leonardo D
authors.initials: LD
authors.lastname: Gómez
authors.affiliation: Center for Novel Agricultural Products Department of Biology, University of York, York, YO10 5DD, UK. simon.mcqueenmason@york.ac.uk.
authors.forename: Simon J
authors.initials: SJ
authors.lastname: McQueen-Mason
authors.affiliation: Department of Food and Environmental Sciences, Faculty of Agriculture and Forestry, University of Helsinki, P.O. Box 27, Helsinki, 00014, Finland. maija.tenkanen@helsinki.fi.
authors.forename: Maija
authors.initials: M
authors.lastname: Tenkanen
authors.affiliation: Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, Umeå, 901-83, Sweden. ewa.mellerowicz@slu.se.
authors.forename: Ewa J
authors.initials: EJ
authors.lastname: Mellerowicz
mesh_terms: D017360:Arabidopsis / Q000201:enzymology; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D002473:Cell Wall / Q000201:enzymology; D015971:Gene Expression Regulation, Enzymologic; D005965:Glucuronates / Q000378:metabolism; D020723:Glucuronic Acid / Q000378:metabolism; D006026:Glycoside Hydrolases / Q000096:biosynthesis* / Q000235:genetics; D016695:Glycosyltransferases / Q000235:genetics / Q000378:metabolism; D008031:Lignin / Q000235:genetics / Q000378:metabolism*; D009844:Oligosaccharides / Q000378:metabolism; D011134:Polysaccharides / Q000378:metabolism; D012567:Schizophyllum / Q000201:enzymology*; D014990:Xylans / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D005965:Glucuronates; D009844:Oligosaccharides; D011134:Polysaccharides; D014990:Xylans; C570991:xylooligosaccharide; C036909:lignocellulose; C038910:glucuronoxylan; D020723:Glucuronic Acid; D008031:Lignin; C553365:GUX1 protein, Arabidopsis; C553366:GUX2 protein, Arabidopsis; D016695:Glycosyltransferases; D006026:Glycoside Hydrolases; C096734:alpha-glucuronidase