Hemicellulosic polymers of cell walls of zea coleoptiles.
Hemicellulosic polymers comprised about 43% of the primary walls of Zea mays L. cv WF9 x Bear 38 coleoptiles; these polymers were separated by an alkali-gradient into three major fractions. Fraction 1 (GAX I) was solubilized from walls with 0.01 to 0.045 n KOH and consisted of novel glucuronoarabino(galacto)xylans. Nearly six of every seven residues of these xylans were substituted predominantly with single arabinosyl sidegroups. Fraction 2 (GAX II), material released by 0.45 to 0.8 n KOH, was also enriched with glucuronoarabinoxylan, but only two of every three xylose residues was substituted. This xylan was similar to those found in Zea and other Graminaceous species. Both of these xylan fractions contained uronic acid, terminal- and 4-linked galactosyl, and small amounts of 2-, 3-, 5-, and 3,5-linked arabinosyl units. Fraction 3 (MG-GAX) was released by 2.0 to 3.0 n KOH and consisted of about 60% mixed-linked glucan and about 40% glucuronoarabinoxylan. This fraction represented about half of the total hemicellulosic material of the primary walls of these coleoptiles.The molecular weight of the highly substituted GAX I was approximately 21 kilodaltons as determined by the ratio of reducing sugar to total sugar, but ultracentrifugation studies and gel chromatography on Sepharose 4B-200 indicated that GAX I formed larger aggregates of primarily 50 to 90 kilodaltons, whereas most of the GAX II and virtually all of the MG-GAX materials were excluded by Sepharose 4B; exclusion from the Sepharose was correlated with the presence of mixed-linked glucan. Only GAX II and MG-GAX material demonstrated any appreciable binding to cellulose in vitro.
Carpita
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Department of Botany and Plant Pathology, Purdue University, West Lafayette, Indiana 47907.