Potassium and its role in cesium transport in plants

Kinetic properties of 137Cs uptake by the cesium-accumulating eustigmatophycean microalga

Cesium-137 (¹³⁷Cs) is one of the radioactive substances that was released into the environment as a result of the Fukushima nuclear disaster. Radiocesium exposure is of great concern due to its potential environmental implications. However, research on ¹³⁷Cs removal using algae is still limited. This is the first report to describe the kinetic properties of ¹³⁷Cs uptake by Vacuoliviride crystalliferum in the presence and absence of potassium. In this work, we studied the kinetic properties of ¹³⁷Cs uptake using a freshwater microalga, V. crystalliferum (NIES 2860). We also analyzed the effects of temperature, light, and potassium (K) on the ¹³⁷Cs uptake. Results showed that V. crystalliferum can remove up to 90% of 157 nM ¹³⁷Cs within an hour. At 20 °C, the removal increased by up to 96%, compared to less than 10% at 5 °C. However, the removal was inhibited by nearly 90% in the dark compared to the removal in the light, implying that V. crystalliferum cells require energy to accumulate ¹³⁷Cs. In the inhibition assay, K concentrations ranged from 0 to 500 µM and the inhibitory constant (K_i) for K was determined to be 16.7 µM. While in the uptake assay without potassium (- K), the Michaelis constant (K_m) for Cs was 45 nM and increased to 283 nM by the addition of 20 µM potassium (+ K), indicating that V. crystalliferum had a high affinity for ¹³⁷Cs. In addition, the maximum uptake velocity (V_max) also increased from 6.75 to 21.10 nmol (mg Chl h)^-1, implying the existence of Cs active transport system. In conclusion, V. crystalliferum is capable of removing radioactive ¹³⁷Cs from the environment and the removal was favorable at both normal temperature and in the light.

Arabidopsis thaliana Accessions from the Chernobyl Exclusion Zone Show Decreased Sensitivity to Additional Acute Irradiation

Chronic ionising radiation exposure is a main consequence of radioactive pollution of the environment. The development of functional genomics approaches coupled with morphological and physiological studies allows new insights into plant adaptation to life under chronic irradiation. Using morphological, reproductive, physiological, and transcriptomic experiments, we evaluated the way in which Arabidopsis thaliana natural accessions from the Chernobyl exclusion zone recover from chronic low-dose and acute high-dose γ-irradiation of seeds. Plants from radioactively contaminated areas were characterized by lower germination efficiency, suppressed growth, decreased chlorophyll fluorescence, and phytohormonal changes. The transcriptomes of plants chronically exposed to low-dose radiation indicated the repression of mobile genetic elements and deregulation of genes related to abiotic stress tolerance. Furthermore, these chronically irradiated natural accessions showed higher tolerance to acute 150 Gy γ-irradiation of seeds, according to transcriptome and phytohormonal profiles. Overall, the lower sensitivity of the accessions from radioactively contaminated areas to acute high-dose irradiation may come at the cost of their growth performance under normal conditions.

Phytoremediation of 137Cs: factors and consequences in the environment

Radionuclide contamination is a concerning threat due to unexpected nuclear disasters and authorized discharge of radioactive elements, both in the past and in present times. Use of atomic power for energy generation is associated with unresolved issues concerning storage of residues and contaminants. For example, the nuclear accidents in Chernobyl 1986 and Fukushima 2011 resulted in considerable deposition of cesium (Cs) in soil, along with other radionuclides. Among Cs radioactive variants, the anthropogenic radioisotope ¹³⁷Cs (t_½ = 30.16 years) is of serious environmental concern, owing to its rapid incorporation into biological systems and emission of β and γ radiation during the decaying process. To remediate contaminated areas, mostly conventional techniques are applied that are not eco-friendly. Hence, an alternative green technology, i.e., phytoremediation, should in future be considered and implemented. This sustainable technology generates limited secondary waste and its objectives are to utilize hyper-accumulating plants to extract, stabilize, degrade, and filter the radionuclides. The review highlights plant mechanisms for up-taking radionuclides and influences of different environmental factors involved in the process, while considering its long-term effects.

Emerging concepts of potassium homeostasis in plants

Potassium (K+) is an essential cation in all organisms that influences crop production and ecosystem stability. Although most soils are rich in K minerals, relatively little K+ is present in forms that are available to plants. Moreover, leaching and run-off from the upper soil layers contribute to K+ deficiencies in agricultural soils. Hence, the demand for K fertilizer is increasing worldwide. K+ regulates multiple processes in cells and organs, with K+ deficiency resulting in decreased plant growth and productivity. Here, we discuss the complexity of the reactive oxygen species-calcium-hormone signalling network that is responsible for the sensing of K+ deficiency in plants, together with genetic approaches using K+ transporters that have been used to increase K+ use efficiency (KUE) in plants, particularly under environmental stress conditions such as salinity and heavy metal contamination. Publicly available rice transcriptome data are used to demonstrate the two-way relationship between K+ and nitrogen nutrition, highlighting how each nutrient can regulate the uptake and root to shoot translocation of the other. Future research directions are discussed in terms of this relationship, as well as prospects for molecular approaches for the generation of improved varieties and the implementation of new agronomic practices. An increased knowledge of the systems that sense and take up K+, and their regulation, will not only improve current understanding of plant K+ homeostasis but also facilitate new research and the implementation of measures to improve plant KUE for sustainable food production.

Regulation of AtKUP2 Expression by bHLH and WRKY Transcription Factors Helps to Confer Increased Salt Tolerance to Arabidopsis thaliana Plants

Potassium transporters play an essential role in maintaining cellular ion homeostasis, turgor pressure, and pH, which are critical for adaptation under salt stress. We identified a salt responsive Avicennia officinalis KUP/HAK/KT transporter family gene, AoKUP2, which has high sequence similarity to its Arabidopsis ortholog AtKUP2. These genes were functionally characterized in mutant yeast cells and Arabidopsis plants. Both AoKUP2 and AtKUP2 were induced by salt stress, and AtKUP2 was primarily induced in roots. Subcellular localization revealed that AoKUP2 and AtKUP2 are localized to the plasma membrane and mitochondria. Expression of AtKUP2 and AoKUP2 in Saccharomyces cerevisiae mutant strain (BY4741 trk1Δ::loxP trk2Δ::loxP) helped to rescue the growth defect of the mutant under different NaCl and K⁺ concentrations. Furthermore, constitutive expression of AoKUP2 and AtKUP2 conferred enhanced salt tolerance in Arabidopsis indicated by higher germination rate, better survival, and increased root and shoot length compared to the untreated controls. Analysis of Na⁺ and K⁺ contents in the shoots and roots showed that ectopic expression lines accumulated less Na⁺ and more K⁺ than the WT. Two stress-responsive transcription factors, bHLH122 and WRKY33, were identified as direct regulators of AtKUP2 expression. Our results suggest that AtKUP2 plays a key role in enhancing salt stress tolerance by maintaining cellular ion homeostasis.

A through year behavior of 137Cs in a Japanese flowering cherry tree in relation to that of potassium

To understand the transfer of radiocesium (¹³⁷Cs) in inside of deciduous trees, changes in ¹³⁷Cs activity concentrations, primarily derived from the Fukushima accident in March 2011, were observed in the upper parts of a Japanese flowering cherry tree (Prunus x yedoensis cv. Somei-Yoshino) between 2015 and 2018. The sampling of the foliar parts occurred over the entire leaf life span from winter bud to litterfall and those of the branches were distinguished based on emergence years (2017, 2016, 2015, 2014-2011, and 2010/before). First, every tissue demonstrated a clear seasonal variation in ¹³⁷Cs activity concentration. Second, a synchrony of seasonal variations in ¹³⁷Cs activity concentration with those in the biological analogue of K concentration was observed in foliar parts during their growth season, but not in branches nor during the other seasons. With respect to the timing of changes in each tissue with tree phenology, it is possible that K and ¹³⁷Cs alternate between leaves and branches via the same translocation mechanisms. The resorption efficiencies (i.e., 1 - [the concentrations in the last litterfall]/[the maximum concentrations in green leaves]) of K and ¹³⁷Cs were 76% and 46% in average, respectively. In addition, both leaf buds and branches played an important role as reservoirs during dormancy. The buds storage ratio before and after bud burst (i.e., [the inventories in buds at the end of defoliation]/[those before and after bud burst]) for K were 0.57 and 0.10 in median, respectively, and those for and ¹³⁷Cs were 1.14 and 0.14 in median, respectively. Consequently, the transfer of ¹³⁷Cs in inside of trees was still visible seven years after deposition, even though the annual reduction in ¹³⁷Cs activity concentration was apparent in each tissue.