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 40-60min 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 ⁴⁵Ca²⁺ 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 [Ca^Z+]_cyt activates HACCs, HACC-mediated Ca²⁺ influx predominates.