Chiou T J, Bush D R
Department of Plant Biology, U.S. Department of Agriculture-Agricultural Research Service, 190 Madigan Laboratories, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
Proc Natl Acad Sci U S A. 1998 Apr 14;95(8):4784-8. doi: 10.1073/pnas.95.8.4784.
The proton-sucrose symporter mediates the key transport step in the resource distribution system that allows many plants to function as multicellular organisms. In the results reported here, we identify sucrose as a signaling molecule in a previously undescribed signal-transduction pathway that regulates the symporter. Sucrose symporter activity declined in plasma membrane vesicles isolated from leaves fed exogenous sucrose via the xylem transpiration stream. Symporter activity dropped to 35-50% of water controls when the leaves were fed 100 mM sucrose and to 20-25% of controls with 250 mM sucrose. In contrast, alanine symporter and glucose transporter activities did not change in response to sucrose treatments. Decreased sucrose symporter activity was detectable after 8 h and reached a maximum by 24 h. Kinetic analysis of transport activity showed a decrease in Vmax. RNA gel blot analysis revealed a decrease in symporter message levels, suggesting a drop in transcriptional activity or a decrease in mRNA stability. Control experiments showed that these responses were not the result of changing osmotic conditions. Equal molar concentrations of hexoses did not elicit the response, and mannoheptulose, a hexokinase inhibitor, did not block the sucrose effect. These data are consistent with a sucrose-specific response pathway that is not mediated by hexokinase as the sugar sensor. Sucrose-dependent changes in the sucrose symporter were reversible, suggesting this sucrose-sensing pathway can modulate transport activity as a function of changing sucrose concentrations in the leaf. These results demonstrate the existence of a signaling pathway that can control assimilate partitioning at the level of phloem translocation.
质子 - 蔗糖同向转运体介导了资源分配系统中的关键转运步骤，该系统使许多植物能够作为多细胞生物发挥功能。在本文报道的结果中，我们确定蔗糖是一种信号分子，参与了一条以前未被描述的调节同向转运体的信号转导途径。从通过木质部蒸腾流供给外源蔗糖的叶片中分离出的质膜囊泡中，蔗糖同向转运体活性下降。当给叶片供给100 mM蔗糖时，同向转运体活性降至降至至水对照的35 - 50%，供给250 mM蔗糖时降至对照的20 - 25%。相比之下，丙氨酸同向转运体和葡萄糖转运体活性对蔗糖处理没有变化。8小时后可检测到蔗糖同向转运体活性降低，24小时时达到最大值。转运活性的动力学分析显示Vmax降低。RNA凝胶印迹分析显示同向转运体信使水平下降，表明转录活性下降或mRNA稳定性降低。对照实验表明，这些反应不是渗透压条件变化的结果。等摩尔浓度的己糖不会引发这种反应，己糖激酶抑制剂甘露庚酮也不会阻断蔗糖效应。这些数据与一条不通过己糖激酶作为糖传感器介导的蔗糖特异性反应途径一致。蔗糖同向转运体中依赖蔗糖的变化是可逆的，这表明这种蔗糖感应途径可以根据叶片中蔗糖浓度的变化调节转运活性。这些结果证明了存在一条能够在韧皮部转运水平上控制同化物分配的信号通路。
