Potassium supply reduces cesium uptake in Konara oak not by an alteration of uptake mechanism, but by the uptake competition between the ions

Arabidopsis transcriptomic analysis reveals cesium inhibition of root growth involves abscisic acid signaling

MAIN CONCLUSION

This is the first report on the involvement of abscisic acid signaling in regulating post-germination growth under Cs stress, not related to potassium deficiency. Cesium (Cs) is known to exert toxicity in plants by competition and interference with the transport of potassium (K). However, the precise mechanism of how Cs mediates its damaging effect is still unclear. This fact is mainly attributed to the large effects of lower K uptake in the presence of Cs that shadow other crucial effects by Cs that were not related to K. RNA-seq was conducted on Arabidopsis roots grown to identify putative genes that are functionally involved to investigate the difference between Cs stress and low K stress. Our transcriptome data demonstrated Cs-regulated genes only partially overlap to low K-regulated genes. In addition, the divergent expression trend of High-affinity K+ Transporter (HAK5) from D4 to D7 growth stage suggested participation of other molecular events besides low K uptake under Cs stress. Potassium deficiency triggers expression level change of the extracellular matrix, transfer/carrier, cell adhesion, calcium-binding, and DNA metabolism genes. Under Cs stress, genes encoding translational proteins, chromatin regulatory proteins, membrane trafficking proteins and defense immunity proteins were found to be primarily regulated. Pathway enrichment and protein network analyses of transcriptome data exhibit that Cs availability are associated with alteration of abscisic acid (ABA) signaling, photosynthesis activities and nitrogen metabolism. The phenotype response of ABA signaling mutants supported the observation and revealed Cs inhibition of root growth involved in ABA signaling pathway. The rather contrary response of loss-of-function mutant of Late Embryogenesis Abundant 7 (LEA7) and Translocator Protein (TSPO) further suggested low K stress and Cs stress may activate different salt tolerance responses. Further investigation on the crosstalk between K transport, signaling, and salt stress-responsive signal transduction will provide a deeper understanding of the mechanisms and molecular regulation underlying Cs toxicity.

Radiocesium mobility in different parts of the two major tree species in Fukushima

Radiocesium (¹³⁷Cs) released in the Fukushima Dai-ichi Nuclear Power Plant accident is still cycling in the forest ecosystem. We examined the mobility of ¹³⁷Cs in the external parts-leaves/needles, branches, and bark-of the two major tree species in Fukushima, Japanese cedar (Cryptomeria japonica) and konara oak (Quercus serrata). This variable mobility will likely lead to spatial heterogeneity of ¹³⁷Cs and difficulty in predicting its dynamics for decades. We conducted leaching experiments on these samples by using ultrapure water and ammonium acetate. In Japanese cedar, the ¹³⁷Cs percentage leached from current-year needles was 26-45% (ultrapure water) and 27-60% (ammonium acetate)-similar to those from old needles and branches. In konara oak, the ¹³⁷Cs percentage leached from leaves was 47-72% (ultrapure water) and 70-100% (ammonium acetate)-comparable to those from current-year and old branches. Relatively poor ¹³⁷Cs mobility was observed in the outer bark of Japanese cedar and in organic layer samples from both species. Comparison of the results from corresponding parts revealed greater ¹³⁷Cs mobility in konara oak than in Japanese cedar. We suggest that more active cycling of ¹³⁷Cs occurs in konara oak.

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.

Strategic roadmap to assess forest vulnerability under air pollution and climate change

Although it is an integral part of global change, most of the research addressing the effects of climate change on forests have overlooked the role of environmental pollution. Similarly, most studies investigating the effects of air pollutants on forests have generally neglected the impacts of climate change. We review the current knowledge on combined air pollution and climate change effects on global forest ecosystems and identify several key research priorities as a roadmap for the future. Specifically, we recommend (1) the establishment of much denser array of monitoring sites, particularly in the South Hemisphere; (2) further integration of ground and satellite monitoring; (3) generation of flux-based standards and critical levels taking into account the sensitivity of dominant forest tree species; (4) long-term monitoring of N, S, P cycles and base cations deposition together at global scale; (5) intensification of experimental studies, addressing the combined effects of different abiotic factors on forests by assuring a better representation of taxonomic and functional diversity across the ~73,000 tree species on Earth; (6) more experimental focus on phenomics and genomics; (7) improved knowledge on key processes regulating the dynamics of radionuclides in forest systems; and (8) development of models integrating air pollution and climate change data from long-term monitoring programs.

Untangling radiocesium dynamics of forest-stream ecosystems: A review of Fukushima studies in the decade after the accident

Forest-stream ecosystems are widespread and biodiverse terrestrial landscapes with physical and social connections to downstream human activities. After radiocesium is introduced into these ecosystems, various material flows cause its accumulation or dispersal. We review studies conducted in the decade after the Fukushima nuclear accident to clarify the mechanisms of radiocesium transfer within ecosystems and to downstream areas through biological, hydrological, and geomorphological processes. After its introduction, radiocesium is heavily deposited in the organic soil layer, leading to persistent circulation due to biological activities in soils. Some radiocesium in soils, litter, and organisms is transported to stream ecosystems, forming contamination spots in depositional habitats. While reservoir dams function as effective traps, radiocesium leaching from sediments is a continual phenomenon causing re-contamination downstream. Integration of data regarding radiocesium dynamics and contamination sites, as proposed here, is essential for contamination management in societies depending on nuclear power to address the climate crisis.

Seasonal changes in radiocesium and potassium concentrations in current-year shoots of saplings of three tree species in Fukushima, Japan

We studied seasonal changes in radiocesium (¹³⁷Cs) activity and potassium concentrations in current-year leaves and branches of Pinus densiflora (naturally regenerated saplings), Cryptomeria japonica (planted saplings) and Quercus serrata (planted saplings and coppice shoots) in Fukushima, Japan. We collected current-year shoots from 10 individuals of each species over two growing seasons at intervals of 1-4 months, between June 2016 and December 2017. For the deciduous species Q. serrata, we also collected dead leaves that remained attached to branches in December to investigate reabsorption of ¹³⁷Cs. All collected shoots were divided into leaves and branches, oven-dried, and ground; dry weights of each sample were recorded. ¹³⁷Cs activity concentrations were measured using a germanium semiconductor detector. Potassium concentrations were quantified using inductively coupled plasma optical emission spectrometry (ICP-OES). Increases in dry weight were observed in both leaves and branches between May/June and August; growth then slowed considerably and virtually ceased after October. Clear seasonal changes in ¹³⁷Cs activity concentrations were observed in both 2016 and 2017, regardless of tree species. Concentrations were higher in young leaves and branches during May and June, then decreased and changed relatively little from August to winter. Reduced ¹³⁷Cs activity concentrations in dead leaves of Q. serrata were observed only in December 2017 (approximately 15% lower than in October). This reduction may indicate reabsorption of ¹³⁷Cs in leaves prior to shedding. The changes in potassium concentrations were similar to those in ¹³⁷Cs in both years. Potassium concentrations were higher in young leaves than in mature leaf and branch samples collected later in the year. A reduction of about 50% in the potassium concentrations in dead leaves of Q. serrata was also observed in December. A positive relationship between ¹³⁷Cs and potassium concentrations in leaves and branches was observed in all species, except for planted Q. serrata. This relationship may indicate that ¹³⁷Cs moves in tree shoots with potassium. Leaf and branch weight correlated negatively with ¹³⁷Cs and potassium concentrations. Reduced concentrations may indicate dilution of these elements as a result of biomass increases over the growing season. Our results imply that irrespective of species, ¹³⁷Cs exhibits seasonal variations resulting from dilution; these variations correspond with trends in potassium, with higher levels in young organs and decreased levels in older organs.

New predictions of 137Cs dynamics in forests after the Fukushima nuclear accident

Most of the area contaminated by the Fukushima Daiichi Nuclear Power Plant accident is covered by forest. In this paper, we updated model predictions of temporal changes in the ¹³⁷Cs dynamics using the latest observation data and newly provided maps of the predicted ¹³⁷Cs activity concentration for wood, which is the most commercially important part of the tree body. Overall, the previous prediction and latest observation data were in very good agreement. However, further validation revealed that the migration from the soil surface organic layer to the mineral soil was overestimated for evergreen needleleaf forests. The new prediction of the ¹³⁷Cs inventory showed that although the ¹³⁷Cs distribution within forests differed among forest types in the first 5 years, the difference diminished in the later phase. Besides, the prediction of the wood ¹³⁷Cs activity concentrations reproduced the different trends of the ¹³⁷Cs activity concentrations for cedar, oak, and pine trees. Our simulation suggests that the changes of the wood ¹³⁷Cs activity concentration over time will slow down after 5–10 years. Although the model uncertainty should be considered and monitoring and model updating must continue, the study provides helpful information on the ¹³⁷Cs dynamics within forest ecosystems and the changes in wood contamination.

Potassium in Root Growth and Development

Potassium is an essential macronutrient that has been partly overshadowed in root science by nitrogen and phosphorus. The current boom in potassium-related studies coincides with an emerging awareness of its importance in plant growth, metabolic functions, stress tolerance, and efficient agriculture. In this review, we summarized recent progress in understanding the role of K+ in root growth, development of root system architecture, cellular functions, and specific plant responses to K+ shortage. K+ transport is crucial for its physiological role. A wide range of K+ transport proteins has developed during evolution and acquired specific functions in plants. There is evidence linking K+ transport with cell expansion, membrane trafficking, auxin homeostasis, cell signaling, and phloem transport. This places K+ among important general regulatory factors of root growth. K+ is a rather mobile element in soil, so the absence of systemic and localized root growth response has been accepted. However, recent research confirms both systemic and localized growth response in Arabidopsis thaliana and highlights K+ uptake as a crucial mechanism for plant stress response. K+-related regulatory mechanisms, K+ transporters, K+ acquisition efficiency, and phenotyping for selection of K+ efficient plants/cultivars are highlighted in this review.