FUCOSYL-TRANSFERASE AND THE BIOSYNTHESIS OF STORAGE AND STRUCTURAL XYLOGLUCAN IN DEVELOPING NASTURTIUM FRUITS

Young, developing fruits of nasturtium (Tropaeolum majus L.) accumulat e large deposits of nonfucosylated xyloglucan (XG) in periplasmic spac es of cotyledon cells. This ''storage'' XG can be fucosylated by a nas turtium transferase in vitro, but this does not happen in vivo, even a s a transitory signal for secretion. The only XG that is clearly fucos ylated in these fruits is the structural fraction (approximately 1% to tal) that is bound to cellulose in growing primary walls. The two fuco sylated subunits that are formed in vitro are identical to those found in structural XG in vivo. The yield of XC-fucosyltransferase activity from membrane fractions is highest per unit fresh weight in the young est fruits, especially in dissected cotyledons, but declines when stor age XG is forming. A block appears to develop in the secretory machine ry of young cotyledon cells between sites that galactosylate and those that fucosylate nascent XG. After extensive galactosylation, XG traff ic is diverted to the periplasm without fucosylation. The primary wall s buried beneath accretions of storage XG eventually swell and lose co hesion, probably because they continue to extend without incorporating components such as fucosylated XG that are needed to maintain wall in tegrity.