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 (Csagar) on which Arabidopsis were grown reduced shoot growth. Increasing the K concentration in the agar (Kagar) increased the Csagar at which Cs toxicity was observed. However, although increasing Csagar reduced shoot K concentration (Kshoot), the decrease in shoot growth appeared unrelated to Kshoot 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 Kshoot, increasing Csagar also increased shoot Cs concentration (Csshoot), but shoot growth appeared unrelated to Csshoot per se. The relationship between shoot growth and Csshoot/Kshoot suggested that, at a nontoxic Csshoot, growth was determined by Kshoot but that the growth of Cs-intoxicated plants was related to the Csshoot/Kshoot quotient. This is consistent with Cs intoxication resulting from competition between K+ and Cs+ for K+-binding sites on essential proteins.