Rubidium (potassium) uptake by Arabidopsis: a comparison of uptake by cells in suspension culture and by roots of intact seedlings.

Experiments are reported in which the uptake of (86)Rb(+), 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 (86)Rb(+) 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.