Role of Calcium in Serine Transport into Tobacco Cells

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Abstract
The transport of serine into tobacco (Nicotiana tabacum L. cv. Xanthi) cells grown in liquid medium was studied. Serine transport was macimal below pH 4.0. A time-dependent stimulation of transport was observed when cells were incubated in medium containing 0.5 mM Ca2+. Maximum transport rates were achieved after 6 h preincubation in Ca2+. The following 3 distinct roles of Ca2+ in serine transport were demonstrated: time-dependent stimulation of transport rate, maintenance of high transport rates and retention of transported material. Stimulation occurred in the presence of either Ca2+ or Mg2+ and was inhibited by either La3+ or K+. Removal of Ca2+ from the transport medium caused a rapid decline in the rate of serine uptake. This decline was prevented by addition of La3+ and Ca2+ removal. Cells transferred to medium lacking Ca2+ lost substantial amounts of transported serine, this loss was significantly reduced by either La3+ or K+. Cells placed in 45Ca2+ rapidly bound more than 3 .mu.mol of Ca2+/g fresh wt, which was exhangeable within 10 min with medium Ca2+. Of the 45Ca2+ transported into the cells in 4 h 75% was exchanged with medium Ca2+ in the same period. The amount of net Ca2+ transport into tobacco cells is insignificant relative to the total exhangeable Ca2+. Apparently serine transport into tobacco cells involves H+ cotransport and the stimulation by Ca2+ is due to an increase in the proton-motive force.