Wang N, Nobel PS
Department of Biology, University of California, Los Angeles, California 90095-1606
Plant Physiol. 1998 Feb 1;116(2):709-14. doi: 10.1104/pp.116.2.709.
Four oligofructans (neokestose, 1-kestose, nystose, and an un-identified pentofructan) occurred in the vascular tissues and phloem sap of mature leaves of Agave deserti. Fructosyltransferases (responsible for fructan biosynthesis) also occurred in the vascular tissues. In contrast, oligofructans and fructosyltransferases were virtually absent from the chlorenchyma, suggesting that fructan biosynthesis was restricted to the vascular tissues. On a molar basis, these oligofructans accounted for 46% of the total soluble sugars in the vascular tissues (sucrose [Suc] for 26%) and for 19% in the phloem sap (fructose for 24% and Suc for 53%). The Suc concentration was 1.8 times higher in the cytosol of the chlorenchyma cells than in the phloem sap; the nystose concentration was 4.9 times higher and that of pentofructan was 3.2 times higher in the vascular tissues than in the phloem sap. To our knowledge, these results provide the first evidence that oligofructans are synthesized and transported in the phloem of higher plants. The polymer-trapping mechanism proposed for dicotyledonous C3 species may also be valid for oligofructan transport in monocotyledonous species, such as A. deserti, which may use a symplastic pathway for phloem loading of photosynthates in its mature leaves.
四种低聚果糖（新蔗果三糖、蔗果三糖、蔗果四糖和一种未鉴定的戊聚果糖）存在于沙漠龙舌兰成熟叶片的维管组织和韧皮部汁液中。果糖基转移酶（负责果聚糖生物合成）也存在于维管组织中。相比之下，薄壁组织中几乎不存在低聚果糖和果糖基转移酶，这表明果聚糖生物合成仅限于维管组织。以摩尔计算，这些低聚果糖占维管组织中总可溶性糖的46%（蔗糖占26%），在韧皮部汁液中占19%（果糖占24%，蔗糖占53%）。薄壁组织细胞胞质溶胶中的蔗糖浓度比韧皮部汁液中的高1.8倍；维管组织中的蔗果四糖浓度比韧皮部汁液中的高4.9倍，戊聚果糖浓度比韧皮部汁液中的高3.2倍。据我们所知，这些结果首次证明了低聚果糖在高等植物韧皮部中合成并运输。为双子叶C3植物提出的聚合物截留机制可能也适用于单子叶植物如沙漠龙舌兰中的低聚果糖运输，沙漠龙舌兰在其成熟叶片中可能利用共质体途径进行光合产物的韧皮部装载。
