Demidchik Vadim, Essah Pauline Adobea, Tester Mark
Department of Plant Sciences, University of Cambridge, Downing Street, CB2 3EA, Cambridge, UK.
Planta. 2004 May;219(1):167-75. doi: 10.1007/s00425-004-1207-8. Epub 2004 Feb 7.
The effect of glutamate on plant plasma membrane cation transport was studied in roots of Arabidopsis thaliana (L.) Heynh. Patch-clamp experiments using root protoplasts, (22)Na(+) unidirectional fluxes into intact roots and measurements of cytosolic Ca(2+) activity using plants expressing cytosolically-targeted aequorin in specific cell types were carried out. It was demonstrated that low-millimolar concentrations of glutamate activate within seconds both Na(+) and Ca(2+) currents in patch-clamped protoplasts derived from roots. The probability of observing glutamate-activated currents increased with increasing glutamate concentration (up to 29% at 3 mM); half-maximal activation was seen at 0.2-0.5 mM glutamate. Glutamate-activated currents were voltage-insensitive, 'instantaneous' (completely activated within 2-3 ms of a change in voltage) and non-selective for monovalent cations (Na(+), Cs(+) and K(+)). They also allowed the permeation of Ca(2+). Half-maximal Na(+) currents occurred at 20-30 mM Na(+). Glutamate-activated currents were sensitive to non-specific blockers of cation channels (quinine, La(3+), Gd(3+)). Although low-millimolar concentrations of glutamate did not usually stimulate unidirectional influx of (22)Na(+) into intact roots, they reliably caused an increase in cytosolic Ca(2+) activity in protoplasts isolated from the roots of aequorin-transformed Arabidopsis plants. The response of cytosolic Ca(2+) activity revealed a two-phase development, with a rapid large transient increase (lasting minutes) and a prolonged subsequent stage (lasting hours). Use of plants expressing aequorin in specific cell types within the root suggested that the cell types most sensitive to glutamate were in the mature epidermis and cortex. The functional significance of these glutamate-activated currents for both cation uptake into plants and cell signaling remains the subject of speculation, requiring more knowledge about the dynamics of apoplastic glutamate in plants.
在拟南芥（Arabidopsis thaliana (L.) Heynh.）的根中研究了谷氨酸对植物质膜阳离子转运的影响。使用根原生质体进行膜片钳实验、测量完整根中（22）Na（+）的单向通量以及使用在特定细胞类型中表达靶向胞质的水母发光蛋白的植物来测量胞质Ca（2+）活性。结果表明，低毫摩尔浓度的谷氨酸在数秒内就能激活源自根的膜片钳原生质体中的Na（+）和Ca（2+）电流。观察到谷氨酸激活电流的概率随谷氨酸浓度的增加而增加（在3 mM时高达29%）；在0.2 - 0.5 mM谷氨酸时出现半数最大激活。谷氨酸激活的电流对电压不敏感、“瞬时”（在电压变化后2 - 3毫秒内完全激活）且对单价阳离子（Na（+）、Cs（+）和K（+））无选择性。它们也允许Ca（2+）通透。半数最大Na（+）电流出现在20 - 30 mM Na（+）时。谷氨酸激活的电流对阳离子通道的非特异性阻滞剂（奎宁、La（3+）、Gd（3+））敏感。尽管低毫摩尔浓度的谷氨酸通常不会刺激（22）Na（+）单向流入完整根中，但它们确实可靠地导致了从水母发光蛋白转化的拟南芥植物根中分离出的原生质体中胞质Ca（2+）活性增加。胞质Ca（2+）活性的反应呈现出两阶段发展，先是快速大幅短暂增加（持续数分钟），随后是持续较长时间的阶段（持续数小时）。在根内特定细胞类型中使用表达水母发光蛋白的植物表明，对谷氨酸最敏感的细胞类型位于成熟表皮和皮层。这些谷氨酸激活电流对于植物阳离子吸收和细胞信号传导的功能意义仍是推测的主题，需要更多关于植物质外体谷氨酸动态的知识。
