Desveaux D, Faik A, Maclachlan G
Biology Department, McGill University, 1205 Docteur Penfield Avenue, Montreal, Quebec, Canada H3A 1B1.
Plant Physiol. 1998 Nov;118(3):885-94. doi: 10.1104/pp.118.3.885.
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.
幼年发育中的旱金莲（Tropaeolum majus L.）果实，在子叶细胞的周质空间积累大量非岩藻糖基化木葡聚糖（XG）沉积物。这种“储存”XG在体外可被旱金莲转移酶岩藻糖基化，但在体内不会发生，即使作为分泌的短暂信号也不会。在这些果实中唯一明显被岩藻糖基化的XG是与正在生长的初生壁中的纤维素结合的结构部分（约占总量的1%）。体外形成的两种岩藻糖基化亚基与体内结构XG中发现的亚基相同。膜组分中XG-岩藻糖基转移酶活性的产量，以单位鲜重计在最幼嫩的果实中最高，尤其是在解剖的子叶中，但在储存XG形成时会下降。在幼年子叶细胞的分泌机制中，半乳糖基化位点和新生XG岩藻糖基化位点之间似乎出现了阻断。在广泛的半乳糖基化之后，XG的运输被转移到周质而不进行岩藻糖基化。埋在储存XG沉积物之下的初生壁最终会膨胀并失去凝聚力，这可能是因为它们在不纳入维持壁完整性所需的成分（如岩藻糖基化XG）的情况下继续伸展。
