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 ([Ca(2+)](c)) 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+)](c) may form part of a positive feedback system for continued apical Ca(2+) influx during tip growth.
Véry
A A
AA
Department of Plant Sciences, University of Cambridge, United Kingdom. very@ensam.inra.fr
Davies
J M
JM
D017360:Arabidopsis / Q000166:cytology / Q000187:drug effects / Q000254:growth & development / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D015220:Calcium Channels / Q000378:metabolism*; D002412:Cations / Q000378:metabolism / Q000494:pharmacology; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism / Q000494:pharmacology; D015536:Down-Regulation; D004553:Electric Conductivity; D015640:Ion Channel Gating / Q000187:drug effects; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D018517:Plant Roots / Q000166:cytology / Q000187:drug effects / Q000254:growth & development / Q000378:metabolism*; D013379:Substrate Specificity
D015220:Calcium Channels; D002412:Cations; D002712:Chlorides; D002118:Calcium