Haritatos Edith, Keller Felix, Turgeon Robert
Section of Plant Biology, Cornell University, 14853, Ithaca, NY, USA.
Institute of Plant Biology, University of Zürich, Zollikerstrasse 107, CH-8008, Zürich, Switzerland.
Planta. 1996 Apr;198(4):614-622. doi: 10.1007/BF00262649. Epub 2017 Mar 18.
Raffinose, stachyose, and galactinol are synthesized in intermediary cells (specialized companion cells) of the minor-vein phloem of cucurbits. To better understand the role of these carbohydrates and the regulation of their synthesis and transport, we measured the concentrations of each of the components of the raffinose oligosaccharide synthetic pathway in mesophyll and sieve element-intermediary cell complexes (SE-ICCs) in the leaves of melon (Cucumis melo L. cv. Hale's Best Jumbo). These concentrations are consistent with a polymer-trapping mechanism for phloem loading, with sucrose diffusing from mesophyll into intermediary cells and being made into raffinose and stachyose, which are too large to diffuse back to the mesophyll. To determine carbohydrate concentrations, we developed a method involving microdissected tissues. Blind endings of areoles, and mesophyll surrounding these veins, were separately removed from lyophilized leaf tissue. Carbohydrates were quantitated by high-performance liquid chromatography with pulsed amperometric detection. A small amount of mesophyll remained attached to the blind endings; the carbohydrate contribution of these cells to the vein sample was eliminated by subtraction, based on the amount of chlorophyll. Volumes of cells and subcellular compartments were calculated by morphometric analysis and were used to calculate carbohydrate concentrations. Assuming no subcellular compartmentation, the additive concentration of sugars in the SE-ICCs of minor veins is about 600 mM. Stachyose and raffinose concentrations are about 330 mM and 70 mM, respectively, in SE-ICCs; concentrations of these sugars are much lower in mesophyll (0.2 and 0.1 mM). This is consistent with the view that stachyose and raffinose are unable to pass through the plasmodesmata between intermediary cells and bundle-sheath cells. Sucrose levels appear to be higher in the SE-ICC (about 130mM) than in the mesophyll (about 10 mM), but if compartmentation is taken into account the gradient for sucrose is probably downhill from mesophyll to intermediary cells. Flux through plasmodesmata between the bundle sheath and intermediary cells was calculated and was found to be within the range of values of flux through plasmodesmata reported in the literature.
棉子糖、水苏糖和棉子糖醇在葫芦科植物小叶脉韧皮部的中间细胞（特化的伴胞）中合成。为了更好地理解这些碳水化合物的作用及其合成和运输的调控机制，我们测定了甜瓜（Cucumis melo L. cv. Hale's Best Jumbo）叶片叶肉和筛管分子-中间细胞复合体（SE-ICC）中棉子糖寡糖合成途径各组分的浓度。这些浓度与韧皮部装载的聚合物陷阱机制一致，即蔗糖从叶肉扩散到中间细胞，并被合成为棉子糖和水苏糖，它们太大而无法扩散回叶肉。为了测定碳水化合物浓度，我们开发了一种涉及显微切割组织的方法。从冻干的叶片组织中分别去除叶腋的盲端以及围绕这些叶脉的叶肉。通过高效液相色谱-脉冲安培检测法定量碳水化合物。少量叶肉仍附着在盲端；根据叶绿素含量，通过减法消除这些细胞对叶脉样品中碳水化合物的贡献。通过形态计量分析计算细胞和亚细胞区室的体积，并用于计算碳水化合物浓度。假设不存在亚细胞区室化，小叶脉SE-ICC中糖的累加浓度约为600 mM。SE-ICC中水苏糖和棉子糖的浓度分别约为330 mM和70 mM；这些糖在叶肉中的浓度要低得多（0.2和0.1 mM）。这与水苏糖和棉子糖无法穿过中间细胞和维管束鞘细胞之间的胞间连丝的观点一致。SE-ICC中的蔗糖水平似乎高于叶肉（约130 mM），但如果考虑区室化，蔗糖的梯度可能是从叶肉到中间细胞呈下坡。计算了维管束鞘和中间细胞之间通过胞间连丝的通量，发现其在文献报道的通过胞间连丝的通量值范围内。
