cerevisiae provides a unicellular and simple test system to study Cs + uptake, as it has been shown to utilize cation transporters and channels similar to plant cells 8. thaliana accessions, which might be helpful for determining additional, possibly multigenic, factors in Cs + homeostasis 13, 14. In addition, recent analyses of the natural genetic variation of Cs + accumulation have revealed several quantitative trait loci in A. Therefore, a biotechnological approach based on mutagenesis should target crucial root uptake and/or export proteins to achieve an improved K +/Cs + specificity and selectivity 5. A discriminative uptake mechanism for a non-essential and rare cation would also be surprising from an evolutionary point of view, as toxic concentrations are not reached in nature. Nevertheless, attempts to specifically manipulate Cs + accumulation in plants have failed owing to interference with K + homeostasis leading to detrimental, pleiotropic effects 5. Therefore, alternative transport processes may enable a discrimination between these closely related cations. Previous studies have shown that Cs + may either inhibit or compete with K +-dependent processes, and that K + and Cs + concentrations are not strictly correlated 9, 11, 12. However, Cs + exhibits toxicity already at a rather low concentration with respect to the high intracellular K + concentration, whereas Rb + can even replace K + in a potassium-depleted scenario 10. Mutual interference of Cs + with K + or Rb + accumulation has further substantiated it as a substrate for K + uptake proteins in plants and other organisms 5, 6, 8, 9. Low-affinity uptake, although less relevant for ecological scenarios, can be mediated through CNGC-type channels 5. High-affinity Cs + uptake has been attributed to HAK/KUP-type K + transporters 5. Uptake and transport across root plasma membranes are considered to be the key aspect and thereby a promising target for manipulating Cs + uptake 5. To this end, Cs + uptake has been intensely studied in plants. Conversely, it has been suggested to reduce Cs + input into the food chain by developing ‘safer crops’, which discriminate against Cs + (ref. Bioremediation could be based on plants accumulating higher amounts of Cs +. Agriculture in contaminated areas needs to be restricted or remediation strategies have to be developed 6. Therefore, Cs + and Rb + uptake is mediated by potassium transport systems and Cs + radioisotopes easily enter the food chain via plants 4, 5. Among the alkali metal cations, K +, Rb + and Cs + have comparable physico-chemical characteristics owing to similar radii of their hydrated ions 3. However, it has acquired enormous ecological importance owing to the anthropogenic release of its persisting radioisotope 137Cs ( T ½=30.1 years), in particular by catastrophic events like the nuclear meltdown and explosions of the Chernobyl or Fukushima nuclear power plants 1, 2. As it is the rarest alkali cation that does not accumulate in soil, no detrimental effects are ascribed to Cs + in natural conditions. This finding provides a new strategy to reduce radiocaesium entry into the food chain.Ĭs + is considered to be non-essential for organisms. A developmentally controlled loss-of-function mutant of the orthologous Arabidopsis thaliana SEC22 phenocopies the reduced Cs + uptake without affecting plant growth. Biochemical fractionation confirms this and indicates a new feature of Sec22p in enhancing non-selective cation deposition. Mathematical modelling of the uptake time course predicts a compromised vacuolar Cs + deposition in sec22Δ. Loss of Saccharomyces cerevisiae Sec22p does not affect K + homeostasis, yet halves Cs + concentration compared with the wild type. Here we show that the SNARE protein Sec22p/SEC22 specifically impacts Cs + accumulation in yeast and in plants. The apparent lack of a Cs +-specific uptake mechanism has obstructed attempts to manipulate Cs + accumulation without causing pleiotropic effects. Cs + accumulates owing to its chemical similarity to the potassium ion (K +). Thus, anthropogenically released radiocaesium is of concern to agriculture. The non-essential cation caesium (Cs +) is assimilated by all organisms.
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