Induction of Solute Release from Nicotiana tabacum Tissue 10

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, leakedfrom treated tissue culture cell suspensions of Nicotiana tabacumwere measured. The kinetics of leakage from cells treated witheither of the polymyxins was biphasic, whereas kinetics forcells treated with EDTA was monophasic. Only K⁺ leaked frompolymyxin-treated cells during the first phase, and all threesolutes leaked during the second phase. The slower first phaseis interpreted as leakage of K⁺ from the Donnan free space andcytoplasm, and the faster second phase as the leakage of solutesfrom the vacuole. The monophasic kinetics of EDTA treatmentindicated that solutes were leaking simultaneously from cytoplasmand vacuole. Of the divalent cations tested, only Ca⁺⁺ and Mn⁺⁺counteracted the effects of polymyxin and EDTA. Ca⁺⁺ even restoredP¹ and sugar uptake. Addition of Mg⁺⁺ or Sr⁺⁺ to polymyxin-treatedcells did not stop solute leakage but actually enhanced theleakage rates. A model is presented that suggests that polymyxinor EDTA induces solute leakage by forming pores in plant cellmembranes. The effects of divalent cations on membranes oncethe pores are formed are also discussed. Key words: Polymyxin, EDTA, Nicotiana tabacum, Solute leakage