Thiel G, Brüdern A, Gradmann D
Pflanzenphysiologisches Institut der Universität, Göttingen, Germany.
J Membr Biol. 1996 Jan;149(1):9-20. doi: 10.1007/s002329900002.
Plant growth requires a continuous supply of intracellular solutes in order to drive cell elongation. Ion fluxes through the plasma membrane provide a substantial portion of the required solutes. Here, patch clamp techniques have been used to investigate the electrical properties of the plasma membrane in protoplasts from the rapid growing tip of maize coleoptiles. Inward currents have been measured in the whole cell configuration from protoplasts of the outer epidermis and from the cortex. These currents are essentially mediated by K+ channels with a unitary conductance of about 12 pS. The activity of these channels was stimulated by negative membrane voltage and inhibited by extracellular Ca2+ and/or tetraethylammonium-CI (TEA). The kinetics of voltage- and Ca(2+)-gating of these channels have been determined experimentally in some detail (steady-state and relaxation kinetics). Various models have been tested for their ability to describe these experimental data in straightforward terms of mass action. As a first approach, the most appropriate model turned out to consist of an active state which can equilibrate with two inactive states via independent first order reactions: a fast inactivation/activation by Ca(2+)-binding and -release, respectively (rate constants >> 10(3) sec-1) and a slower inactivation/activation by positive/negative voltage, respectively (voltage-dependent rate constants in the range of 10(3) sec-1). With 10 mM K+ and 1 mM Ca2+ in the external solution, intact coleoptile cells have a membrane voltage (V) of -105 +/- 7 mV. At this V, the density and open probability of the inward-rectifying channels is sufficient to mediate K+ uptake required for cell elongation. Extracellular TEA or Ca2+, which inhibit the K+ inward conductance, also inhibit elongation of auxin-depleted coleoptile segments in acidic solution. The comparable effects of Ca2+ and TEA on both processes and the similar Ca2+ concentration required for half maximal inhibition of growth (4.3 mM Ca2+) and for conductance (1.2 mM Ca2+) suggest that K+ uptake through the inward rectifier provides essential amounts of solute for osmotic driven elongation of maize coleoptiles.
植物生长需要持续供应细胞内溶质以驱动细胞伸长。通过质膜的离子通量提供了很大一部分所需溶质。在此，膜片钳技术已被用于研究来自玉米胚芽鞘快速生长尖端的原生质体质膜的电学性质。在外表皮和皮层的原生质体的全细胞模式中测量到内向电流。这些电流主要由单通道电导约为12 pS的钾离子通道介导。这些通道的活性受到负膜电压的刺激，并受到细胞外钙离子和/或四乙铵 - 氯（TEA）的抑制。这些通道的电压门控和钙门控动力学已通过实验进行了相当详细的测定（稳态和弛豫动力学）。已经测试了各种模型以其用质量作用的直接术语描述这些实验数据的能力。作为第一种方法，最合适的模型结果是由一个活性状态组成，该活性状态可以通过独立的一级反应与两个非活性状态达到平衡：分别通过钙离子结合和释放进行快速失活/激活（速率常数>> 10³秒⁻¹）以及分别通过正/负电压进行较慢的失活/激活（电压依赖性速率常数在10³秒⁻¹范围内）。在外部溶液中含有10 mM钾离子和1 mM钙离子时，完整的胚芽鞘细胞具有 -105±7 mV的膜电压（V）。在此V值下，内向整流通道的密度和开放概率足以介导细胞伸长所需的钾离子摄取。抑制钾离子内向电导的细胞外TEA或钙离子，也抑制酸性溶液中生长素耗尽的胚芽鞘切段的伸长。钙离子和TEA对这两个过程的类似影响以及生长半最大抑制（4.3 mM钙离子）和电导（1.2 mM钙离子）所需的类似钙离子浓度表明，通过内向整流器摄取钾离子为玉米胚芽鞘的渗透驱动伸长提供了必需量的溶质。
