No abstract

Calcium is an essential plant nutrient. It is acquired from the soil solution by the root system and translocated to the shoot via the xylem. The root must balance the delivery of calcium to the xylem with the need for individual root cells to use [Ca2+]cyt for intracellular signalling. Here the evidence for the current hypothesis, that Ca2+ travels apoplastically across the root to the Casparian band which it then circumvents via the cytoplasm of the endodermal cell, is critically reviewed. It is noted that, although Ca2+ channels and Ca2+-ATPases are present and could catalyse Ca2+ influx and efflux across the plasma membrane of endodermal cells, their transport capacity is unlikely to be sufficient for xylem loading. Furthermore, there seems to be no competition, or interactions, between Ca2+, Ba2+ and Sr2+ for transport to the shoot. This seems incompatible with a symplastic pathway involving at least two protein-catalysed transport steps. Thus, a quantity of purely apoplastic Ca2+ transport to the xylem is indicated. The relative contributions of these two pathways to the delivery of Ca2+ to the xylem are unknown. However, the functional separation of symplastic Ca2+ fluxes (for root nutrition and cell signalling) and apoplastic Ca2+ fluxes (for transfer to the shoot) would enable the root to fulfil the demand of the shoot for calcium without compromising intracellular [Ca2+]cyt signals. This is also compatible with the observed correlation between transpiration rate and calcium delivery to the shoot.

Calcium channels are involved principally in signal transduction. Their opening results in increased cytoplasmic Ca(2+) concentration, and the spatial and temporal variations in this are thought to elicit specific physiological responses to diverse biotic and abiotic stimuli. Calcium-permeable channels have been recorded in the plasma membrane, tonoplast, endoplasmic reticulum, chloroplast and nuclear membranes of plant cells. This article reviews their electrophysiological properties and discusses their physiological roles. Emphasis is placed on the voltage-dependent and elicitor-activated Ca(2+) channels of the plasma membrane and the depolarisation-activated (SV), hyperpolarisation-activated, IP(3)- and cADPR-dependent Ca(2+) channels of the tonoplast. The closing of stomatal guard cells in the presence of abscisic acid (ABA) is used to illustrate the co-ordination of Ca(2+) channel activities during a physiological response.

SummaryCalcium is a critical structural and regulatory nutrient in plants. However, mechanisms of its uptake by root cells are poorly understood. We have found that Ca 2þ influx in Arabidopsis root epidermal protoplasts is mediated by voltage-independent rapidly activating Ca 2þ -permeable non-selective cation channels (NSCCs). NSCCs showed the following permeability (P) sequence: P Ca (1.00) ¼ P Ba (0.93) > P Zn (0.51), P Ca / P Na ¼ 0.19, P Ca /P K ¼ 0.14. They were inhibited by quinine, Gd 3þ , La 3þ and the His modifier diethylpyrocarbonate, but not by the Ca 2þ or K þ channel antagonists, verapamil and tetraethylammonium (TEA þ ). Single channel conductance measured in 20 mM external Ca 2þ was 5.9 pS. Calcium-permeable NSCCs co-existed with hyperpolarisation-activated Ca 2þ channels (HACCs), which activated 40-60 min after forming the whole-cell configuration. HACCs activated at voltages <À130 to À150 mV, showed slow activation kinetics and were regulated by cytosolic Ca 2þ ([Ca 2þ ] cyt ). Using aequorin-expressing plants, a linear relationship between membrane potential (V m ) and resting [Ca 2þ ] cyt was observed, indicating the involvement of NSCCs. Intact root 45 Ca 2þ influx was reduced by Gd 3þ (NSCC blocker) but was verapamil and TEA þ insensitive. In the root elongation zone, both root net Ca 2þ influx (measured by Ca 2þ -selective vibrating microelectrode) and NSCC activity were increased compared to the mature epidermis, suggesting the involvement of NSCC in growth. A Ca 2þ acquisition system based on NSCC and HACC co-existence is proposed. In mature epidermal cells, NSCC-mediated Ca 2þ influx dominates whereas in specialised root cells (root hairs and elongation zone cells) where elevated [Ca 2þ ] cyt activates HACCs, HACC-mediated Ca 2þ influx predominates.

No abstract