Arabidopsis thaliana root non-selective cation channels mediate calcium uptake and are involved in growth

Calcium is a critical structural and regulatory nutrient in plants. However, mechanisms of its uptake by root cells are poorly understood. We have found that Ca2+ influx in Arabidopsis root epidermal protoplasts is mediated by voltage-independent rapidly activating Ca2+- permeable non-selective cation channels (NSCCs). NSCCs showed the following permeability (P) sequence: P-Ca (1.00) = P-Ba (0.93)> P-Zn (0.51), P-Ca/ P-Na = 0.19, P-Ca/ P-K = 0.14. They were inhibited by quinine, Gd3+, La3+ and the His modifier diethylpyrocarbonate, but not by the Ca2+ or K+ channel antagonists, verapamil and tetraethylammonium (TEA(+)). Single channel conductance measured in 20 mM external Ca2+ was 5.9 pS. Calcium-permeable NSCCs co-existed with hyperpolarisation-activated Ca2+ channels (HACCs), which activated 40-60 min after forming the whole-cell configuration. HACCs activated at voltages < -130 to -150 mV, showed slow activation kinetics and were regulated by cytosolic Ca2+ ([Ca2+](cyt)). Using aequorin-expressing plants, a linear relationship between membrane potential (V-m) and resting [Ca2+](cyt) was observed, indicating the involvement of NSCCs. Intact root Ca-45(2+) influx was reduced by Gd3+ (NSCC blocker) but was verapamil and TEA(+) insensitive. In the root elongation zone, both root net Ca2+ influx ( measured by Ca2+-selective vibrating microelectrode) and NSCC activity were increased compared to the mature epidermis, suggesting the involvement of NSCC in growth. A Ca2+ acquisition system based on NSCC and HACC co-existence is proposed. In mature epidermal cells, NSCC-mediated Ca2+ influx dominates whereas in specialised root cells ( root hairs and elongation zone cells) where elevated [Ca2+](cyt) activates HACCs, HACC- mediated Ca2+ influx predominates.