Induction of Solute Release fromNicotiana tabacumTissue 10

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Abstract
For determination of the effects of polymyxin B, polymyxin E, or ethylenediamine tetra-acetic acid (EDTA) on plant cell membranes, the rates at which three solutes, K+, P1, and sugar, leaked from treated tissue culture cell suspensions of Nicotiana tabacum were measured. The kinetics of leakage from cells treated with either of the polymyxins was biphasic, whereas kinetics for cells treated with EDTA was monophasic. Only K+ leaked from polymyxin-treated cells during the first phase, and all three solutes leaked during the second phase. The slower first phase is interpreted as leakage of K+ from the Donnan free space and cytoplasm, and the faster second phase as the leakage of solutes from the vacuole. The monophasic kinetics of EDTA treatment indicated that solutes were leaking simultaneously from cytoplasm and vacuole. Of the divalent cations tested, only Ca++ and Mn++ counteracted the effects of polymyxin and EDTA. Ca++ even restored P1 and sugar uptake. Addition of Mg++ or Sr++ to polymyxin-treated cells did not stop solute leakage but actually enhanced the leakage rates. A model is presented that suggests that polymyxin or EDTA induces solute leakage by forming pores in plant cell membranes. The effects of divalent cations on membranes once the pores are formed are also discussed.