It was suggested that AtHAK5 was responsible for Cs uptake and Cs accumulation under low K conditions 14. 14, using Arabidopsis thaliana as a model, previously reported that K ion deficiency induced the expression of HIGH-AFFINITY K TRANSPORTER5 ( AtHAK5), which is a member of the Arabidopsis K UPTAKE PERMEASE (AtKUP) family. The mechanism regulating how Cs ions enter and accumulates in cells, however, has not been completely elucidated. Notably, our previous study also demonstrated that application of cysteine increases Cs accumulation in plants. We have reported that Cs stress alters a large set of metabolites in plants, including several amino acids, and especially cysteine levels 13. Cs can also inactivate proteins by interacting with potassium-binding sites, as well as alter gene expression and microRNA processing 3, 4, 9, 10, 11, 12. This is because Cs ions can accumulate and function as an analogue of K ion by entering plant cells through the K transport system, consequently resulting in K deficiency in organisms 1, 2, 4, 5, 6, 7, 8.
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Exposure to high levels of Cs also causes diseases in humans, such as acute cardiac arrest and hypokalemia. Although natural environments do not contain large amounts of Cs, high levels of Cs are toxic and have a negative impact on plant growth 3, 4. In addition, AtHAK5 also appears to be involved in this response.Ĭesium (Cs) has similar physical and chemical properties to potassium (K) 1, 2. Our data indicate that AtβGLU23 regulates plant response to Cs stress and that CsToAcE1 enhances Cs tolerance by repressing AtβGLU23. Notably, application of CsToAcE1 resulted in a reduction of Cs-induced AtβGLU23 expression in wild-type plants, while this was not observed in a high affinity transporter mutant, athak5. Interestingly, Arabidopsis atβglu23 mutants exhibited enhanced tolerance to Cs stress but did not respond to the application of CsToAcE1. Our analysis identified one of the interacting target proteins of CsToAcE1 to be BETA-GLUCOSIDASE 23 (AtβGLU23).
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In the present study, potential target proteins of CsToAcE1 were isolated from Arabidopsis to determine the mechanism by which CsToAcE1 alleviates Cs stress, while enhancing Cs accumulation. Treatment of plants with CsToAcE1 resulted in greater Cs and K accumulation and also alleviated Cs-induced growth retardation in Arabidopsis. Among them, a small chemical compound (C 17H 19F 3N 2O 2), named CsToAcE1, was confirmed to enhance Cs tolerance while increasing Cs accumulation in plants. In previous studies, we identified chemicals that increase Cs tolerance in plants. Cs and K compete in cells due to the chemical similarity of Cs to potassium (K), and can induce K deficiency in cells. Cesium (Cs) is found at low levels in nature but does not confer any known benefit to plants.