Excess boron responsive regulations of antioxidative mechanism at physio-biochemical and molecular levels in Arabidopsis thaliana

Redox regulation of epigenetic and epitranscriptomic gene regulatory pathways in plants

Developmental and environmental constraints influence genome expression through complex networks of regulatory mechanisms. Epigenetic modifications and remodelling of chromatin are some of the major actors regulating the dynamic of gene expression. Unravelling the factors relaying environmental signals that induce gene expression reprogramming under stress conditions is an important and fundamental question. Indeed, many enzymes involved in epigenetic and chromatin modifications are regulated by redox pathways, through post-translational modifications of proteins or by modifications of the flux of metabolic intermediates. Such modifications are potential hubs to relay developmental and environmental changes for gene expression reprogramming. In this review, we provide an update on the interaction between major redox mediators, such as reactive oxygen and nitrogen species and antioxidants, and epigenetic changes in plants. We detail how redox status alters post-translational modifications of proteins, intracellular epigenetic and epitranscriptional modifications, and how redox regulation interplays with DNA methylation, histone acetylation and methylation, miRNA biogenesis, and chromatin structure and remodelling to reprogram genome expression under environmental constraints.

Ecotoxicity Study of Additives Composed of Zinc and Boron

The high use of additives containing zinc borate and their limited solubility in water both lead to its persistence and accumulation in biological systems. On the other hand, soluble forms of boron are easily available to plant roots and are taken up by plants. There are no ecotoxicological data available for zinc borate, the industrial utilization of which is widespread. Therefore, the potential toxicity of zinc borate and its dissociated compounds was evaluated. Based on two different ecotoxicology tests, their effect on plant growth was studied. Firstly, the impact on Lemna minor growth was investigated, including the effect on pigment content. Secondly, the inhibition of the root growth of higher plant species Sinapis alba (mustard), Lactuca sativa (lettuce) and Trifolium pretense (clover) was measured. The growth inhibition test on L. minor was more complex and sensitive compared to the plant seed germination test. Already low concentrations (10 mg/L) of ZnO, B₂O₃ and Zn₃BO₆ led to a decrease in frond growth and to an inhibition of the conversion of chlorophyll a to chlorophyll b. These results suggested that the stress caused by these additives caused damage to the photosynthetic apparatus. The highest inhibition of frond growth was detected in fronds treated with B₂O₃ (92-100%). In ZnO and Zn₃BO₆, the inhibition of frond growth was between 38 and 77%, with Zn₃BO₆ being slightly more toxic. In the seed germination test, the most sensitive species was lettuce, the growth of which was inhibited by 57, 83 and 53% in ZnO, B₂O₃ and Zn₃BO₆ treatments, respectively. However, the inhibitory effect on each plant was different. In lettuce and clover, the seed germination and root elongation decreased with increasing element concentrations. In contrast, in mustard, low concentrations of ZnO and Zn₃BO₆ supported the growth of roots. For that reason, more complex tests are essential to evaluate the additive toxicity in the environment.

Salt Pretreatment-Mediated Alleviation of Boron Toxicity in Safflower Cultivars: Growth, Boron Accumulation, Photochemical Activities, Antioxidant Defense Response

The study aims to elucidate alleviant effects of boron (B) toxicity by salt pretreatment (SP) on growth response, phytoremediation capacity, photosynthesis, and defense mechanisms in two safflower cultivars (Carthamus tinctorius L.; Dinçer and Remzibey-05). Eighteen-day-old plants were divided into two groups: SP (75 mM NaCl for 5 days) and/or B treatment (C, 2, 4, 6, and 8 mM B for 10 days). Depending on the applied B toxicity, B concentrations in roots and leaves of both cultivars, necrotic areas of leaves, ion leakage (RLR), and H2O2 synthesis increased, while shoot and root length as well as biomass, water, chlorophyll a+b, and carotenoid content decreased. In addition, chlorophyll a fluorescence results revealed that every stage of the light reactions of photosynthesis was adversely affected under B toxicity, resulting in decreases in performance indexes (PIABS and PITOT). However, the cultivars tended to induce the synthesis of anthocyanins and flavonoids and increase the activity of antioxidant enzymes (SOD, POD, APX, and GR) to detoxify reactive oxygen species (ROS) under B toxicity. SP mitigated the negative effects of toxic B on biomass, water and pigment content, membrane integrity, photosynthetic activity, and defense systems. Considering all results, Remzibey-05 was able to better overcome the biochemical and physiological changes that may be caused by B toxicity by more effectively rendering B harmless, although it accumulated more B than Dinçer.

Physiological and proteomic analysis reveals the impact of boron deficiency and surplus on alfalfa (Medicago sativa L.) reproductive organs

Boron (B), an essential element for increasing seed yield and germinability in alfalfa (Medicago sativa L.), plays a vital role in its reproductive processes. However, effects of B stress on physiological and proteomic changes in reproductive organs related to alfalfa seed yield and germinability are poorly understood. In order to gain a better insight into B response or tolerance mechanisms, field trials were designed for B deficiency (0 mg B L^-1), B sufficiency (800 mg B L^-1), and B surplus (1600 mg B L^-1) application during alfalfa flowering to analyze the proteomics and physiological responses of alfalfa 'Aohan' reproductive organs. Results showed that B deficiency weakened the stress-responsive ability in these organs, while B surplus reduced the sugar utilization of 'Aohan' flowers and caused lipid membrane peroxidation in 'Aohan' seeds. In addition, four upregulated stress responsive proteins (ADF-like protein, IMFP, NAD(P)-binding Rossmann-fold protein and NAD-dependent ALDHs) might play pivotal roles in the response of 'Aohan' reproductive organs to conditions of B deficiency and B surplus. All of the above results would be helpful to understand the tolerance mechanisms of alfalfa reproductive organs to both B deficiency and B surplus conditions, and also to give insight into the regulatory role of B in improving seed yield and germinability in alfalfa seed production. In summary, B likely plays a structural and regulatory role in relation to lipid metabolism, carbohydrate metabolism, amino acid metabolism, and signal transduction, thus regulates alfalfa reproductive processes eventually affecting the seed yield and germinability of alfalfa seeds.

Myriophyllum elatinoides: A potential candidate for the phytoremediation of water with low level boron contamination

Phytoremediation is considered to be a cost-effective strategy for removing boron (B) from polluted water. In this study, Myriophyllum elatinoides, a widespread submerged or floating macrophyte, was found to survive in 40 mg B/L. Time-dependent kinetics show that the shoot exhibits a much longer period of B uptake and a much higher maximal tissue B concentration than the root. High values of the bioconcentration factor (BCF) and translocation factor (TF) indicate that M. elatinoides is a potential hyperaccumulator of B. Transmission electron micrographs show that excess B damages the cells of M. elatinoides, and the major target organelles are the chloroplast (leaf), mitochondria (stem and root), and nucleolus (root). Energy dispersive spectroscopy (EDS) shows that B is mainly deposited in the cytoplasm and on the surface of the chloroplast of the leaf cell. In the stem and root cells, B is mainly deposited on the mitochondrial membrane and in the vacuoles, respectively. This study indicates that the mechanisms of B toxicity, tolerance, and accumulation in M. elatinoides are involved in the cellular localization of B. Future work should focus on the evaluation of the physiological and genetic mechanisms involved in B tolerance and accumulation in M. elatinoides under different conditions.

Excess boron inhibited the trifoliate orange growth by inducing oxidative stress, alterations in cell wall structure, and accumulation of free boron

The boron (B) is an essential nutrient and plays an important role in the stability of the primary cell wall (CW). Due to the narrow window between B deficiency and toxicity, mismanagement practices lead to B toxicity that inhibit root growth and overall crop productivity. However, the exact cause of root growth inhibition remains unclear. The present study examined the potential causes and targets of B toxicity by studying intercellular mechanism. The trifoliate seedlings were cultured under excess B conditions. The results indicated that plant growth was inhibited by excess B, nevertheless, the effects were prominent on roots and leaves. B toxicity exacerbated oxidative stress and root cell death. The analysis of CW functional groups, CW microstructure and B forms lead to the conclusion that alterations in CW, and accumulation of free-B and carbohydrates might cause inhibition of growth and visible symptoms of B toxicity.

Physiological, transcriptional, and metabolic alterations in spaceflight-subjected Senna obtusifolia

Senna obtusifolia is a widely used medicinal herb in Asian countries. To select elite cultivars, S. obtusifolia seeds were carried by "ShenZhou Ⅷ" recoverable satellite to space. Three spaceflight-subjected lines (SP-lines), namely QC10, QC29, QC46, and their ground control line (GC-line) were cultivated on the ground. Previous studies demonstrated that biological traits and secondary metabolites are different between SP-lines and GC-line. Here, we combined physiological, transcriptional, and metabolic studies to compare the differences between SP-lines and GC-line. The results showed that activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), and monodehydroascorbate reductase (MDHAR) were dramatically increased in SP-lines as compared to that of GC-line. Transcript levels of SOD, POD, CAT, APX, and MDHAR were significantly up-regulated in SP-lines. Malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) contents decreased in SP-lines. Seed yields of QC29 and QC46 were considerably higher than that of GC-line. Besides, QC29 had significantly higher aurantio-obtusin content. Pearson correlation coefficient analysis revealed positive relationships between POD and aurantio-obtusin, as well as APX and aurantio-obtusin. In conclusion, SP-lines have higher antioxidant gene expression level and antioxidant enzyme activity as compared to that of GC-line. With higher seed yield and aurantio-obtusin content, QC29 can be used to breed elite S. obtusifolia cultivars. This study provides a new insight in SP-lines and paves the way to breed elite S. obtusifolia cultivars in the future.

Analysis of physiological responses and expression profiling of boron transporter-like genes in response to excess boron in Populus russkii

Poplars (Populus species) are tolerant to boron (B) toxicity and have phytoremediation potential in B-contaminated soils. However, the detoxification strategy is largely unknown. To screen the key B transporter-like (BOR-like) genes for B compartmentation, Populus russkii plants were exposed to different levels of excess B and the plant growth, physiological responses, B distribution, and the expression patterns of BOR-like genes were characterized. P. russkii showed moderate tolerance to excess B although the plant growth was inhibited. The enhanced proline level and well-regulated antioxidant defense system were associated with B tolerance in leaves. The B absorbed by plants was predominantly allocated to leaves. Ten BOR-like genes were identified and seven of them showed tissue-specific expression patterns. PrBOR7 was identified as an important BOR-like gene possibly involved in the export of B from leaf cytoplasm because it was expressed specifically in leaves and induced by excess B. Yeast experiment assays verified that PrBOR7 functions as an efflux-type transporter and strongly improved cell tolerance to excess B. The expression patterns of BOR-like genes highlight the diversity of the family members in P. russkii, and PrBOR7 has potential as a candidate gene for B detoxification.

The protective effect exerted by dietary borax on toxicity metabolism in rainbow trout (Oncorhynchus mykiss) tissues

The aim of this study was to evaluate the effectiveness of borax (BX) against heavy metal exposure on the transcriptional and biochemical reaction in vivo and alleviating effect on gill and liver tissues of rainbow trout. Due to this aim, fish were fed with different level of BX and/or copper (Cu) (1.25, 2.5 and 5 mg/kg of BX; 500 and 1000 mg/kg of Cu) for 21·days in pre- and co-treatment options. The transcriptional parameters [(heat-shock protein 70 (hsp70), and cytochromes P450 (cyp1a), glutathione peroxidase (GPx), superoxide dismutase (SOD) and catalase (CAT))], antioxidant enzyme activities (SOD, CAT and GPx), malondialdehyde (MDA), oxidative DNA damage (8-hydroxy-2'-deoxyguanosine (8-OHdG) and caspase-3 levels were investigated in different tissues samples of treated and control fish. Our results revealed that antioxidant enzyme activity was increased and levels of 8-OHdG, Caspase-3 and MDA were decreased in the BX and BX combined groups as compared to the copper combination group and to copper-only application during pre- and co-treatment (p < 0.05). Similarly, hsp70 and cyp1a gene expressions were decreased after treatment with BX. As conclusion, we suggest that borax itself is not an antioxidant it supportes antioxidant defense mechanism of fish disrupted by heavy metals.

Redox Balance-DDR-miRNA Triangle: Relevance in Genome Stability and Stress Responses in Plants

Plants are continuously faced with complex environmental conditions which can affect the oxidative metabolism and photosynthetic efficiency, thus leading to the over-production of reactive oxygen species (ROS). Over a certain threshold, ROS can damage DNA. DNA damage, unless repaired, can affect genome stability, thus interfering with cell survival and severely reducing crop productivity. A complex network of pathways involved in DNA damage response (DDR) needs to be activated in order to maintain genome integrity. The expression of specific genes belonging to these pathways can be used as indicators of oxidative DNA damage and effective DNA repair in plants subjected to stress conditions. Managing ROS levels by modulating their production and scavenging systems shifts the role of these compounds from toxic molecules to key messengers involved in plant tolerance acquisition. Oxidative and anti-oxidative signals normally move among the different cell compartments, including the nucleus, cytosol, and organelles. Nuclei are dynamically equipped with different redox systems, such as glutathione (GSH), thiol reductases, and redox regulated transcription factors (TFs). The nuclear redox network participates in the regulation of the DNA metabolism, in terms of transcriptional events, replication, and repair mechanisms. This mainly occurs through redox-dependent regulatory mechanisms comprising redox buffering and post-translational modifications, such as the thiol-disulphide switch, glutathionylation, and S-nitrosylation. The regulatory role of microRNAs (miRNAs) is also emerging for the maintenance of genome stability and the modulation of antioxidative machinery under adverse environmental conditions. In fact, redox systems and DDR pathways can be controlled at a post-transcriptional level by miRNAs. This review reports on the interconnections between the DDR pathways and redox balancing systems. It presents a new dynamic picture by taking into account the shared regulatory mechanism mediated by miRNAs in plant defense responses to stress.

Phytoremediation potential of poplar and willow species in small scale constructed wetland for boron removal

Boron (B) pollution is an expanding environmental problem throughout the world due to intensive mining practices and extensive usage of B in agricultural chemicals and industrial products in recent years. The purpose of this study was to investigate B removal performance of four poplar and four willow species in small scale Constructed Wetland (CW). Rooted cuttings of tested species were treated with simulated wastewater having five elevated B concentrations (0.5, 5, 10, 20 and 40 ppm). All the tested species could resist up to 20 ppm wastewater B supply and could regrow from their roots in the soil having maximum 15 mg/kg B content. The result of the study indicated that 65% ± 5.3 of B was removed from the wastewater in 5 ppm B treatment while the same efficiency decreased to 45% ± 4.6 at 40 ppm B supply. The average effect of sediment on B removal was found to be approximately 20% for all B treatments while the remaining part of the loaded B was removed from the CW within effluent (35-54%). Therefore, actual effects of plant species on B removal was ranged from 45% to 25% between 5 and 40 ppm B treatments. Mass B removal within plant body (phytextraction) comprised the 13-10% of total loaded B in CW while the remaining part of the loaded B (31-15%) was stabilized into the sediment with the effects of poplar and willow roots. These results presented clear understanding of effective B purification mechanisms in CWs. Boron phytextraction capacity of a plant species was less effective than its phytstabilization efficiency which increase filtering capacity of the sediment and stabilization of more B around the rhizosphere. In terms of their B removal ability, P.nigra and S.anatolica had the highest B removal capacities with phytextraction (20-11%) while S.alba, P.alba and S.babylonica had more phytstabilizaiton performance (40-15%) in CW. Disposal of B loaded plant material create another environmental costs for CW applications. Therefore, B loaded wood and leaf tissues were mixed and used for production of wooden panels in the study. Then a combustion test was applied on these panels to test their fire resistance. The results of the tests revealed much higher burning tolerance of the B loaded panels (5-20%) compared to controls. Annual harvesting, fast growing and deep rooting ability of the poplar and willow species with their high phytstabilization and phytextraction efficiencies make these species excellent tools to remove B from the polluted waters. Utilization of these species for B removal in large scale CWs is quite possible which should be also investigated in further studies.