Cesium toxicity in Arabidopsis

Cesium (Cs) is chemically similar to potassium (K). However, although K is an essential element, Cs is toxic to plants. Two contrasting hypotheses to explain Cs toxicity have been proposed: (1) extracellular Cs⁺prevents K⁺ uptake and, thereby, induces K starvation; and (2) intracellular Cs⁺ interacts with vital K⁺-binding sites in proteins, either competitively or noncompetitively, impairing their activities. We tested these hypotheses with Arabidopsis (Arabidopsis thaliana). Increasing the Cs concentration in the agar ([Cs]_agar) on which Arabidopsis were grown reduced shoot growth. Increasing the K concentration in the agar ([K]_agar) increased the [Cs]_agar at which Cs toxicity was observed. However, although increasing [Cs]_agar reduced shoot K concentration ([K]_shoot), the decrease in shoot growth appeared unrelated to [K]_shoot per se. Furthermore, the changes in gene expression in Cs-intoxicated plants differed from those of K-starved plants, suggesting that Cs intoxication was not perceived genetically solely as K starvation. In addition to reducing [K]_shoot, increasing [Cs]_agar also increased shoot Cs concentration ([Cs]_shoot), but shoot growth appeared unrelated to [Cs]_shoot per se. The relationship between shoot growth and [Cs]_shoot/[K]_shoot suggested that, at a nontoxic [Cs]_shoot, growth was determined by [K]_shoot but that the growth of Cs-intoxicated plants was related to the [Cs]_shoot/[K]_shoot quotient. This is consistent with Cs intoxication resulting from competition between K⁺ and Cs⁺ for K⁺-binding sites on essential proteins.