Véry A A, Davies J M
Department of Plant Sciences, University of Cambridge, United Kingdom.
Proc Natl Acad Sci U S A. 2000 Aug 15;97(17):9801-6. doi: 10.1073/pnas.160250397.
The root hair elongative growth phase ("tip growth"), like that of other tip-growing systems such as pollen tubes, algal rhizoids, and fungal hyphae, is associated with an apex-high cytosolic free calcium (Ca(2+)) gradient generated by a local Ca(2+) influx at the tip. This gradient has been shown to be a fundamental regulator of tip growth. Here, we have performed patch-clamp experiments at root hair apices of Arabidopsis thaliana (after localized cell wall laser ablation) to characterize the plasma membrane Ca(2+) channels implicated in the tip Ca(2+) influx. We have identified a hyperpolarization-activated Ca(2+) conductance. This conductance is selective for Ca(2+) over K(+) and Cl(-) (P(Ca)/P(K) = 15; P(Ca)/P(Cl) = 25) and is fully blocked by < 100-microM trivalent cations (La(3+), Al(3+), Gd(3+)). The selectivity sequence among divalent cations (determined by comparisons of the channel unitary conductance) is Ba(2+) > Ca(2+) (22 pS in 10 mM) approximately Mg(2+) > Mn(2+). This conductance was operative at typical growing hair apical resting membrane potentials. Moreover, it was seen to be down-regulated in growing hair subapical regions, as well as at the tip of mature hairs (known not to exhibit Ca(2+) influx). We therefore propose that this inward-rectifying Ca(2+) conductance is inherently involved in the apical Ca(2+) influx of growing hairs. The observed enhancement of the conductance by increased Ca(2+) may form part of a positive feedback system for continued apical Ca(2+) influx during tip growth.
根毛伸长生长阶段（“顶端生长”），与其他顶端生长系统如花粉管、藻类假根和真菌菌丝一样，与顶端局部Ca²⁺内流产生的顶端高细胞质游离钙（[Ca²⁺]c）梯度相关。该梯度已被证明是顶端生长的基本调节因子。在这里，我们在拟南芥根毛顶端（局部细胞壁激光消融后）进行了膜片钳实验，以表征参与顶端Ca²⁺内流的质膜Ca²⁺通道。我们鉴定出一种超极化激活的Ca²⁺电导。该电导对Ca²⁺的选择性高于K⁺和Cl⁻（P(Ca)/P(K)=15；P(Ca)/P(Cl)=25），并被<100 μM的三价阳离子（La³⁺、Al³⁺、Gd³⁺）完全阻断。二价阳离子之间的选择性序列（通过比较通道单位电导确定）为Ba²⁺>Ca²⁺（10 mM时为22 pS）≈Mg²⁺>Mn²⁺。该电导在典型的生长根毛顶端静息膜电位下起作用。此外，在生长根毛的亚顶端区域以及成熟根毛的顶端（已知不表现出Ca²⁺内流）观察到该电导下调。因此，我们提出这种内向整流Ca²⁺电导本质上参与了生长根毛的顶端Ca²⁺内流。观察到的[Ca²⁺]c增加导致的电导增强可能构成顶端生长期间持续顶端Ca²⁺内流的正反馈系统的一部分。
