Haritatos E, Medville R, Turgeon R
Section of Plant Biology, Cornell University, Ithaca, NY 14853, USA.
Planta. 2000 Jun;211(1):105-11. doi: 10.1007/s004250000268.
Leaf and minor vein structure were studied in Arabidopsis thaliana (L.) Heynh. to gain insight into the mechanism(s) of phloem loading. Vein density (length of veins per unit leaf area) is extremely low. Almost all veins are intimately associated with the mesophyll and are probably involved in loading. In transverse sections of veins there are, on average, two companion cells for each sieve element. Phloem parenchyma cells appear to be specialized for delivery of photoassimilate from the bundle sheath to sieve element-companion cell complexes: they make numerous contacts with the bundle sheath and with companion cells and they have transfer cell wall ingrowths where they are in contact with sieve elements. Plasmodesmatal frequencies are high at interfaces involving phloem parenchyma cells. The plasmodesmata between phloem parenchyma cells and companion cells are structurally distinct in that there are several branches on the phloem parenchyma cell side of the wall and only one branch on the companion cell side. Most of the translocated sugar in A. thaliana is sucrose, but raffinose is also transported. Based on structural evidence, the most likely route of sucrose transport is from bundle sheath to phloem parenchyma cells through plasmodesmata, followed by efflux into the apoplasm across wall ingrowths and carrier-mediated uptake into the sieve element-companion cell complex.
对拟南芥的叶片和小叶脉结构进行了研究，以深入了解韧皮部装载机制。叶脉密度（单位叶面积的叶脉长度）极低。几乎所有叶脉都与叶肉紧密相连，可能参与了装载过程。在叶脉的横切面上，每个筛管分子平均有两个伴胞。韧皮部薄壁细胞似乎专门用于将光合产物从维管束鞘输送到筛管分子-伴胞复合体：它们与维管束鞘和伴胞有大量接触，并且在与筛管分子接触的地方有传递细胞壁内突。在涉及韧皮部薄壁细胞的界面处，胞间连丝频率很高。韧皮部薄壁细胞和伴胞之间的胞间连丝在结构上有所不同，即在细胞壁的韧皮部薄壁细胞一侧有几个分支，而在伴胞一侧只有一个分支。拟南芥中大部分转运的糖是蔗糖，但棉子糖也会被运输。基于结构证据，蔗糖最可能的运输途径是从维管束鞘通过胞间连丝进入韧皮部薄壁细胞，然后通过壁内突外流到质外体，并通过载体介导进入筛管分子-伴胞复合体。
