Hyperpolarization-activated calcium channels at the tip of Arabidopsis root hairs

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 ([Ca2+](c)) gradient generated by a local Ca2+ 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 locaIized cell wall laser ablation) to characterize the plasma membrane Ca2+ channels implicated in the tip Ca2+ influx. We have identified a hyperpolarization-activated Ca2+ conductance. This conductance is selective for Ca2+ over K+ and Cl- (P-Ca/P-K = 15; P-Ca/P-Cl = 25) and is fully blocked by <100-mu M trivalent cations (La3+, Al3+ Gd3+). The selectivity sequence among divalent cations (determined by comparisons of the channel unitary conductance) is Ba2+ > Ca2+ (22 pS in 10 mM) approximate to Mg2+ > Mn2+. 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 Ca2+ influx). We therefore propose that this inward-rectifying Ca2+ conductance is inherently involved in the apical Ca2+ influx of growing hairs. The observed enhancement of the conductance by increased [Ca2+](c) may form part of a positive feedback system for continued apical Ca2+ influx during tip growth.