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Rubidium (Potassium) Uptake by Arabidopsis

A Comparison of Uptake by Cells in Suspension Culture and by Roots of Intact Seedlings ¹

^a Department of Biology, Washington University, St. Louis, Missouri 63130

Experiments are reported in which the uptake of ⁸⁶Rb⁺, used as an analog of K⁺, into cultured cells of Arabidopsis thaliana is investigated. A single transport system is found with K_m = 0.34 millimolar and V_max = 14 nmoles per milligram of protein per hour. This system is blocked by the metabolic inhibitor carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and by cold. At high concentrations of external K⁺ (above 1 millimolar), a significant fraction of total uptake is energy-independent. No evidence is found for more than one energy-dependent uptake system or for concentration-dependent modifications of a carrier as postulated in multiphasic transport models.

Rb⁺ uptake was also examined in cultured cells derived from an "osmotic mutant" of Arabidopsis. The system closely resembles that found in wild type cells with the exception that the Michaelis-Menten constants are higher: K_m = 1 millimolar and V_max = 32 nanomoles per milligram of protein per hour.

The possibility that these results are artifacts associated with use of cultured cells was checked by examining ⁸⁶Rb⁺ uptake by roots of intact seedlings of wild type Arabidopsis. A single energy-dependent transport system is found with K_m = 0.42 millimolar which is not significantly different from the K_m of cultured cells. There is also energy-independent uptake at high external ion concentration.