β-Radiation Stress Responses on Growth and Antioxidative Defense System in Plants: A Study with Strontium-90 in Lemna minor

In the following study, dose dependent effects on growth and oxidative stress induced by β-radiation were examined to gain better insights in the mode of action of β-radiation induced stress in plant species. Radiostrontium (90Sr) was used to test for β-radiation induced responses in the freshwater macrophyte Lemna minor. The accumulation pattern of 90Sr was examined for L. minor root and fronds separately over a seven-day time period and was subsequently used in a dynamic dosimetric model to calculate β-radiation dose rates. Exposing L. minor plants for seven days to a 90Sr activity concentration of 25 up to 25,000 kBq·L−1 resulted in a dose rate between 0.084 ± 0.004 and 97 ± 8 mGy·h−1. After seven days of exposure, root fresh weight showed a dose dependent decrease starting from a dose rate of 9.4 ± 0.5 mGy·h−1. Based on these data, an EDR10 value of 1.5 ± 0.4 mGy·h−1 was estimated for root fresh weight and 52 ± 17 mGy·h−1 for frond fresh weight. Different antioxidative enzymes and metabolites were further examined to analyze if β-radiation induces oxidative stress in L. minor

<tex>\beta$</tex>-radiation stress responses on growth and antioxidative defense system in plants : a study with strontium-90 in **Lemna minor**

Abstract: In the following study, dose dependent effects on growth and oxidative stress induced by β-radiation were examined to gain better insights in the mode of action of β-radiation induced stress in plant species. Radiostrontium (90Sr) was used to test for β-radiation induced responses in the freshwater macrophyte Lemna minor. The accumulation pattern of 90Sr was examined for L. minor root and fronds separately over a seven-day time period and was subsequently used in a dynamic dosimetric model to calculate β-radiation dose rates. Exposing L. minor plants for seven days to a 90Sr activity concentration of 25 up to 25,000 kBq·L−1 resulted in a dose rate between 0.084 ± 0.004 and 97 ± 8 mGy·h−1. After seven days of exposure, root fresh weight showed a dose dependent decrease starting from a dose rate of 9.4 ± 0.5 mGy·h−1. Based on these data, an EDR10 value of 1.5 ± 0.4 mGy·h−1 was estimated for root fresh weight and 52 ± 17 mGy·h−

Photosynthetic capacity of <i>Arthrospira</i> sp. PCC 8005 after gamma irradiation.

<p>The data obtained for the irradiated samples were normalized against and are shown as percentage of their representative non-irradiated control (which was put at 100%). Data represent the mean of three independent biological replicates (n = 3) and error bars display the standard error of the mean (SEM). The statistical analysis was calculated on raw data, before normalisation to percentages.</p

Temporal Gene Expression of the Cyanobacterium <i>Arthrospira</i> in Response to Gamma Rays

<div><p>The edible cyanobacterium <i>Arthrospira</i> is resistant to ionising radiation. The cellular mechanisms underlying this radiation resistance are, however, still largely unknown. Therefore, additional molecular analysis was performed to investigate how these cells can escape from, protect against, or repair the radiation damage. <i>Arthrospira</i> cells were shortly exposed to different doses of ⁶⁰Co gamma rays and the dynamic response was investigated by monitoring its gene expression and cell physiology at different time points after irradiation. The results revealed a fast switch from an active growth state to a kind of 'survival modus' during which the cells put photosynthesis, carbon and nitrogen assimilation on hold and activate pathways for cellular protection, detoxification, and repair. The higher the radiation dose, the more pronounced this global emergency response is expressed. Genes repressed during early response, suggested a reduction of photosystem II and I activity and reduced tricarboxylic acid (TCA) and Calvin-Benson-Bassham (CBB) cycles, combined with an activation of the pentose phosphate pathway (PPP). For reactive oxygen species detoxification and restoration of the redox balance in <i>Arthrospira</i> cells, the results suggested a powerful contribution of the antioxidant molecule glutathione. The repair mechanisms of <i>Arthrospira</i> cells that were immediately switched on, involve mainly proteases for damaged protein removal, single strand DNA repair and restriction modification systems, while <i>recA</i> was not induced. Additionally, the exposed cells showed significant increased expression of <i>arh</i> genes, coding for a novel group of protein of unknown function, also seen in our previous irradiation studies. This observation confirms our hypothesis that <i>arh</i> genes are key elements in radiation resistance of <i>Arthrospira</i>, requiring further investigation. This study provides new insights into phasic response and the cellular pathways involved in the radiation resistance of microbial cells, in particularly for photosynthetic organisms as the cyanobacterium Arthrospira.</p></div

Rhizophagus irregularis MUCL 41833 can colonize and improve P uptake of Plantago lanceolata after exposure to ionizing gamma radiation in root organ culture

Long-lived radionuclides such as 90Sr and 137Cs can be naturally or accidentally deposited in the upper soil layers where they emit β/γ radiation. Previous studies have shown that arbuscular mycorrhizal fungi (AMF) can accumulate and transfer radionuclides from soil to plant, but there have been no studies on the direct impact of ionizing radiation on AMF. In this study, root organ cultures of the AMF Rhizophagus irregularis MUCL 41833 were exposed to 15.37, 30.35, and 113.03 Gy gamma radiation from a 137Cs source. Exposed spores were subsequently inoculated to Plantago lanceolata seedlings in pots, and root colonization and P uptake evaluated. P. lanceolata seedlings inoculated with non-irradiated AMF spores or with spores irradiated with up to 30.35 Gy gamma radiation had similar levels of root colonization. Spores irradiated with 113.03 Gy gamma radiation failed to colonize P. lanceolata roots. P content of plants inoculated with non-irradiated spores or of plants inoculated with spores irradiated with up to 30.35 Gy gamma radiation was higher than in non-mycorrhizal plants or plants inoculated with spores irradiated with 113.03 Gy gamma radiation. These results demonstrate that spores of R. irregularis MUCL 41833 are tolerant to chronic ionizing radiation at high doses

Effects of environmental parameters on Lemna minor growth: An integrated experimental and modelling approach

Pollution of surface waters is a worldwide problem for people and wildlife. Remediation and phytoremediation approaches can offer a solution to deal with specific scenarios. Lemna minor, commonly known as duckweed, can absorb and accumulate pollutants in its biomass. To evaluate if L. minor could be applied for phytoremediation purposes, it is necessary to further investigate its remediation capability and to identify which parameters affect the remediation process. Such a model must include both plant growth and pollutant exchange. A remediation model based on a robust experimental study can help to evaluate L. minor as a proper remediation strategy and to predict the outcome of a L. minor based remediation system. To set up this model, this paper focusses on a detailed experimental study and a comprehensive mathematical modelling approach to represent L. minor growth as a function of biomass, temperature, light irradiation and variable nutrient concentrations. The influence of environmental conditions on L. minor growth was studied, by composing 7 days growth curves. Plants were grown under predefined environmental conditions (25°C, 14h photoperiod, 220 μmol m−2 s−1 light intensity and a modified Hoagland solution with 23.94 mg N L−1 and 3.10 mg P L−1 (N:P ratio of 7.73)) as standard for all experiments. The influence of different temperatures (6, 10, 15, 20, 25, 30 and 35°C), light intensities (63, 118, 170, 220 and 262 μmol m−2 s−1), photoperiods (12h and 14h) and N:P ratios (1.18, 3.36, 7.73 and 29.57) were tested in the model. As a result, a growth model was optimised using separate datasets for temperature, light intensity, photoperiod and nutrients and validated by further integrated testing. The growth model is a stable platform for application in phytoremediation of radionuclides in contaminated water, to be extended in future studies with information of pollutant uptake, pollutant-nutrient interactions and transfer to the biomass

General overview of the main transcriptional response events of <i>Arthrospira</i> sp. PCC 8005 after exposure to ⁶⁰Co gamma rays.

<p>Schemes represent a global gene expression response (A) immediately after irradiation; (B) after 2H and 5H of recovery period. Blue colour, stand for down-regulated genes. Red colour stand for up-regulated genes, Green colour stand for restored expression of the initial silenced genes. (A) The largest changes in transcription occurred upon irradiation, as part of a kind of an “Emergency Response”. Cells displayed a <b>reduced transcription</b> for photosynthesis and energy production (PSII, PSI, ATP), and for carbon and nitrogen metabolism during irradiation. The CO₂ fixation via the Calvin-Benson-Bassham cycle (CBB), glycogen biosynthesis (gluconeogenesis) and the tricarboxylic acid cycle (TCA) were repressed. The transcription of the SigE regulator acting as nitrogen-dependent activator for catabolic genes towards glycogen degradation (glycolysis) was induced. Also a re-routing of the metabolic flux to glycolysis and the pentose phosphate pathway (PPP) was seen. A synthesis of carbon storage molecules (PHA) and compatible solutes (trehalose) was seen. The expression of polyamine import (<i>potBC)</i>, well known as a group of nitrogen-containing C-compounds which help in cell survival during stress, was recorded. The import of nitrate or cyanate as N-sources was repressed (<i>nrtABCD</i>, <i>cynBD</i>). In parallel also the metabolism of agmatine, a known competitive inhibitor of polyamine transport, was repressed. The cellular protection, detoxification, and repair were <b>enhanced</b> immediately after irradiation. In an effort to maintain the intracellular redox balance while provide sufficient metal-cofactors for enzymes, selective metal export (<i>copA)</i> and import (<i>feoAB</i>, <i>cutA</i>, <i>corA</i>, <i>mtgC</i>, <i>cbiQ1</i>, <i>cbiQ2</i>, <i>znuA</i>) was induced. There was upregulation of isiA gene encoding the CP43’ protein, which is an auxiliary antenna complex, to compensate for the loss of phycobilisomes. This protein may also serve as a chlorophyll storage molecule contributing to the reassembly of reaction centres during recovery. In addition, ROS detoxification was activated via the expression of the peroxiredoxine enzyme (<i>ahpC</i>) and the glutathione synthesis genes. The generation of glutathione starts at T0H via the formation of glutamate from proline by <i>hyuA</i>, from aspartate by aspartate aminotransferase <i>(aat1)</i>, from 1-pyrroline-5-carboxylate by (<i>putA</i>), and from 2-oxoglutarate via GLDH (see Fig 8B). Glutamate synthesis via the GS/GOGAT cycle was repressed. The final synthesis of glutathione from glutamate occurred via glutathione (GSH) synthase <i>(gshB)</i>, which continued during recovery (see Fig 8B). Chaperones (<i>dnaK1</i>, <i>dnaK2</i>, <i>hspA</i>, <i>cbpA</i>) and proteases (<i>clpB2</i>) were also significantly induced during this stage, to remove damaged proteins. The free amino-acids released from protein degradation, likely lead to the production of urea, and the urease (<i>ureABC)</i> activity, transforming urea to ammonium, was induced. In parallel <i>Arthrospira</i> enhance some genes related to DNA repair system (<i>uvrBCD</i> for nucleotide excision and repair, <i>ruvB</i> resolving holiday junction, and <i>recJ</i>, <i>dnaG and mod</i> genes). The DNA-repair mechanism of <i>Arthrospira</i> included also enzymatic restriction modification (<i>hsdr</i>) and endonucleases. (B) During the <u><b>later phase</b></u><i>Arthrospira</i> cells try to <u><b>recover from the damage; which lead to a slowly restored expression</b></u> of the genes related to photosynthesis and energy production, carbon fixation via the CBBn cycle and gluconeogenesis, TCA cycle. Expression of the hydrogenase genes (<i>hypA1</i>, <i>hypB1and hoxW</i>). Metal chaperone proteins HypA and HypB are required for the nickel insertion step of [NiFe]-hydrogenase maturation. In parallel slight reactivation of amino-acid transport (<i>aapJPQ</i>, <i>argGHJ</i>, <i>iaaA</i>) occurred. The genes for import of taurine (<i>tauABC</i>) known as organic sulphur and amino source were highly induced. The restoration of agmatinase, the key enzyme of agmatine hydrolysis was seen in recovery period. ROS detoxification was maintained efficiently via the expression for glutathione biosynthesis (GSH). Few genes related to protein damage clean up (proteases and chaperones) and DNA repair maintained their expression during recovery. The expression of gene cluster <i>arhABCDEF</i>, enriched during recovery, was seen.</p

Scatter plot showing the differentially expressed genes of <i>Arthrospira</i> sp. PCC 8005 in response to gamma irradiation plotted accordingly to their change in mRNA concentration (Log₂ fold change values), for 3 radiation doses (800, 1600 and 3200 Gy) and 3 time points after radiation (0 hours, 2 hours, 5 hours).

<p>Scatter plot showing the differentially expressed genes of <i>Arthrospira</i> sp. PCC 8005 in response to gamma irradiation plotted accordingly to their change in mRNA concentration (Log₂ fold change values), for 3 radiation doses (800, 1600 and 3200 Gy) and 3 time points after radiation (0 hours, 2 hours, 5 hours).</p