Fucosyltransferase and the Biosynthesis of Storage and Structural Xyloglucan in Developing Nasturtium Fruits

Abstract

Young, developing fruits of nasturtium (Tropaeolum majus L.) accumulate large deposits of nonfucosylated xyloglucan (XG) in periplasmic spaces of cotyledon cells. This "storage" XG can be fucosylated by a nasturtium transferase in vitro, but this does not happen in vivo, even as a transitory signal for secretion. The only XG that is clearly fucosylated in these fruits is the structural fraction (approximately 1% total) that is bound to cellulose in growing primary walls. The two fucosylated subunits that are formed in vitro are identical to those found in structural XG in vivo. The yield of XG-fucosyltransferase activity from membrane fractions is highest per unit fresh weight in the youngest fruits, especially in dissected cotyledons, but declines when storage XG is forming. A block appears to develop in the secretory machinery of young cotyledon cells between sites that galactosylate and those that fucosylate nascent XG. After extensive galactosylation, XG traffic is diverted to the periplasm without fucosylation. The primary walls buried beneath accretions of storage XG eventually swell and lose cohesion, probably because they continue to extend without incorporating components such as fucosylated XG that are needed to maintain wall integrity.

Sugar-nucleotide-binding and autoglycosylating polypeptide(s) from nasturtium fruit: biochemical capacities and potential functions

“…Separate tests showed that the labelled TCA-soluble product in this experiment was not depolymerized by Pronase, but was digested by cellulase to fragments that fractionated on columns of Bio-Gel P2 to positions equivalent to XG subunits. HPLC of these fragments through Dionex columns indicated that "%C was confined to the same two oligosaccharides observed in TCAsoluble products formed from UDP-["%C]Gal by pea enzyme [12], which are different from the subunits found in storage XG [8]. We are obliged to conclude that present tests with nasturtium vesicles only recovered galactosyltransferase activity that was responsible for biosynthesis of structural (wall) XG, not the great bulk of reserve XG.…”

“…Nasturtium fruits or excised cotyledons were harvested at 23 days post-anthesis, when they were about half developed and actively synthesizing reserves like XG [8]. Tissues were homogenized with a high-speed blender in a standard extraction medium [12].…”

“…Assay conditions were similar to those that were effective in demonstrating the incorporation of ["%C]Gal from UDP-["%C]Gal into pea XG subunits. The XG that was employed as potential galactosyl acceptor was derived from nasturtium seed storage XG [8] and partially degalactosylated [12] with nasturtium seed β-galactosidase [13]. Other polysaccharides that were added to reaction mixtures as potential galactosyl acceptors were tamarind seed XG, rye arabinogalactan and potato β-galactan (all from Megazyme, Bray, Ireland), carboxymethyl cellulose (CM-cellulose ; Hercules Powder Co., Wilmington, DE, U.S.A.) and laminarin (Calbiochem).…”

“…Nasturtium fruit and UDP-Gal were used in this study as enzyme and substrate sources because mature nasturtium seeds contain about 100 times as much ' storage ' XG, deposited as a temporary reserve in periplasmic spaces of cotyledon cells, as ' structural ' XG, which is incorporated into primary cell walls [7,8]. The mass of storage XG is much more heavily galactosylated (22 % Gal, w\w) than wall XG (6 % Gal).…”

Biochemical Characterization and Molecular Cloning of an α-1,2-Fucosyltransferase That Catalyzes the Last Step of Cell Wall Xyloglucan Biosynthesis in Pea

“…Pea cell wall XG is thought to be constituted almost completely of alternating -XXXG-(Glc 4 Xyl 3 ) and -XXFG-subunits (4), but recent study (5) has also detected the presence of small amounts of other subunits including two octasaccharides and the decasaccharide -XLFG-(Glc 4 Xyl 3 Gal 2 Fuc). This subunit (-XLFG-) has also been detected in sycamore cell XG (6), Arabidopsis and rapeseed (7), apple fruit (8), and developing nasturtium fruits (9). An unadecasaccharide, -XFFG-(Glc 4 Xyl 3 Gal 2 Fuc 2 ), has additionally been observed in sycamore cell XG (6).…”