Role of Calcium in Serine Transport into Tobacco Cells

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.