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 ofArabidopsis thaliana (after localized 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− (PCa /PK = 15; PCa /PCl = 25) and is fully blocked by <100-μM 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) ≈ 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.