Thermodynamic battle for photosynthate acquisition between sieve tubes and adjoining parenchyma in transport phloem
Thermodynamic battle for photosynthate acquisition between sieve tubes and adjoining parenchyma in transport phloem
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DOI：
10.1104/pp.104.058511
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发表时间：
2005-07-01
期刊：
影响因子：
7.4
通讯作者：
van Bel, AJE
中科院分区：
文献类型：
--
作者：
Hafke, JB;van Amerongen, JK;van Bel, AJE
在运输韧皮部中，从筛管逃逸的同化物被释放到筛元/伴胞复合体（SE/CC）和韧皮部薄壁细胞（PPC）之间的质外间隙中。对于摄取各自的检索，PPC和SE/CC利用由质子动力（PMF）供能的质膜转位器。它们的相互竞争，这基本上决定了通过筛管转运的光合同化物的量，因此取决于各自的PMF。测定了SE/CCs和PPC在运输韧皮部中的PMF、膜电位和Δ pH的组成。SE/CCs和PPC在组织切片以及在完整的植物的膜电位分为两类。在第一类包括质外体韧皮部装载物种（e。G. Vicia，Solanum），SE的膜电位比PPC的膜电位更负。在第二组，包括共质体韧皮部加载物种（如葫芦，罗勒），SE的膜电位等于或略高于PPC的。用H+ 1选择性微电极测定了从蚜虫针杆上采集的纯筛管汁液。在我们的实验条件下，筛管汁液的pH值约为7.5，这与薄壁细胞中细胞质隔室的pH值相当。总之，只有膜电位似乎是相关的PMF确定SE/CC和PPC之间的竞争。这一结果可能意味着，轴向汇沿着路径提取更多的同化物从筛管共质负载物种比质外体负载物种。
In transport phloem, photoassimilates escaping from the sieve tubes are released into the apoplasmic space between sieve element (SE)/companion cell (CC) complexes (SE/CCs) and phloem parenchyma cells (PPCs). For uptake respective retrieval, PPCs and SE/CCs make use of plasma membrane translocators energized by the proton motive force (PMF). Their mutual competitiveness, which essentially determines the amount of photoassimilates translocated through the sieve tubes, therefore depends on the respective PMFs. We measured the components of the PMF, membrane potential and Delta pH, of SE/CCs and PPCs in transport phloem. Membrane potentials of SE/CCs and PPCs in tissue slices as well as in intact plants fell into two categories. In the first group including apoplasmically phloem-loading species (e. g. Vicia, Solanum), the membrane potentials of the SEs are more negative than those of the PPCs. In the second group including symplasmically phloem-loading species (e.g. Cucurbita, Ocimum), membrane potentials of SEs are equal to or slightly more positive than those of PPCs. Pure sieve tube sap collected from cut aphid stylets was measured with H+ 1-selective microelectrodes. Under our experimental conditions, pH of the sieve tube saps was around 7.5, which is comparable to the pH of cytoplasmic compartments in parenchymatous cells. In conclusion, only the membrane potential appears to be relevant for the PMF-determined competition between SE/CCs and PPCs. The findings may imply that the axial sinks along the pathway withdraw more photoassimilates from the sieve tubes in symplasmically loading species than in apoplasmically loading species.