Phosphate uptake in Chara: membrane transport via Na/Pi cotransport

Abstract

<jats:title>ABSTRACT</jats:title><jats:p>Phosphate uptake in the freshwater charophyte plant <jats:italic>Chara corallina</jats:italic> was found to be strongly dependent on the presence of Na in the external medium. Based on the reciprocal stimulations of <jats:sup>32</jats:sup>Pi uptake by Na and <jats:sup>22</jats:sup>Na uptake by Pi, the logical mechanism for Pi uptake appears to be a <jats:italic>n</jats:italic>Na/Pi symport with a half‐maximal stimulation (<jats:italic>K</jats:italic><jats:sub>m</jats:sub>) for Na of approximately 300 <jats:italic>μ</jats:italic><jats:sc>M</jats:sc> and a <jats:italic>K</jats:italic><jats:sub>m</jats:sub> for Pi of approximately 10 <jats:italic>μ</jats:italic><jats:sc>M</jats:sc>. Comparison of the stimulations of <jats:sup>32</jats:sup>Pi and <jats:sup>22</jats:sup>Na influxes at pH 6 gives a stoichiometry of Na : Pi of 5·68. The reduction in Pi influx with increasing pH is consistent with the transported species being the monovalent H<jats:sub>2</jats:sub>PO<jats:sub>4</jats:sub><jats:sup>−</jats:sup>. In voltage‐clamp experiments, currents elicited by Pi in the presence of Na were equivalent to an influx of positive charge which exceeded the measured influxes of <jats:sup>32</jats:sup>P by a factor of 6·26. Intracellular perfusion was used to examine the dependence of Pi influx on ATP and Na. In perfused cells, Pi influx was low when ATP was absent from the internal medium or Na was absent from the external medium. Addition of ATP alone had little effect whereas addition of Na alone increased the <jats:sup>32</jats:sup>Pi influx slightly. Addition of both ATP and Na together restored Pi influx to rates comparable to those of intact cells. It is suggested that the ATP is required for membrane hyperpolarization which in turn drives the highly electrogenic flux of Pi with up to 6 Na. However, consideration of the electrochemical potential differences for Na and Pi at pH less than 6 shows that <jats:italic>n</jats:italic>Na/Pi would not be feasible. It is suggested that at low pH, H<jats:sup>+</jats:sup> may substitute for Na.</jats:p>