Calcium ion transport in higher plant mitochondria (Helianthus tuberosus)

A study on Ca2+ transport by mitochondria isolated from Jerusalem artichoke (Helianthus tuberosus L. cv. OB1) tubers is presented. By following the distribution of Ca2+ under respiratory conditions, we have been able to show that Ca2+ accumulation into the matrix space depends on membrane potential (ΔΨ) since the uptake is not affected by the protonophore nigericin but fully blocked by valinomycin and carbonyl cyanide‐p‐trifluoromethoxy phenylhydrazone (FCCP). Ca2+ uptake requires phosphate (Pi) and is inhibited by mersalyl and by ruthenium red (RR). In addition to a Ca2+ influx route, mitochondria from H. tuberosus possess an RR‐insensitive Ca2+ efflux pathway which is not stimulated by external Na+, Ca2+ is rapidly released from Ca2+‐loaded mitochondria in the presence of ionophores such as A23187 and valinomycin and of the uncoupler FCCP. The Pi‐transport inhibitor mersalyl also induces a massive Ca2+ release through reversal of the uptake route, the latter process being blocked by RR. Thus Jerusalem artichoke mitochondria possess a Ca2+ cycling mechanism which is different from that of animal mitochondria and certain other plant species.