A study on phytogenotoxicity induced by biogenic amines: cadaverine and putrescine

Among the compounds present in necro-leachate, a liquid released during the process of decomposition of the human body, are the biogenic amines cadaverine and putrescine. Although some studies on necro-leachate have indicated a potential ecotoxicological and public health risk associated with it, the research on this type of contamination is still rather limited. This study presents information about the phytotoxic and cytogenotoxic potential of cadaverine and putrescine, evaluated separately and within a mixture. Phytotoxicity was evaluated through a germination test, the initial growth of seedlings with Lactuca sativa, and cytogenotoxicity through chromosomal aberration and micronucleus tests with Allium cepa. The L. sativa results showed a phytotoxic effect for the evaluated amines, by reducing root (> 90%) and hypocotyl (> 80%) elongation. The co-exposure of cadaverine and putrescine potentiated cytogenotoxic activity by aneugenic action in the meristematic cells of A. cepa. From this result, it is possible to infer the eco-toxicogenic potential of cadaverine and putrescine. This study not only highlights the importance of the phytotoxic and cytogenotoxic effects of these amines but also emphasizes the urgent need for further investigation into contamination originating from cemetery environments. By evaluating the risks associated with necro-leachate, this research is aimed at informing global efforts to protect ecological and public health.

Extension of a biotic ligand model for predicting the toxicity of neodymium to wheat: The effects of pH, Ca2+ and Mg2

The damage excessive neodymium (Nd) causes to animals and plants should not be underestimated. However, there is little research on the impact of pH and associated ions on the toxicity of Nd. Here, a biotic ligand model (BLM) was expanded to predict the effects of pH and chief anions on the toxic impact of Nd on wheat root elongation in a simulated soil solution. The results suggested that Nd3+ and NdOH2+ were the major ions causing phytotoxicity to wheat roots at pH values of 4.5-7.0. The Nd toxicity decreased as the activities of H+, Ca2+, and Mg2+ increased but not when the activities of K+ and Na+ increased. The results indicated that H+, Ca2+, and Mg2+ competed with Nd for binding sites. An extended BLM was developed to consider the effects of pH, H+, Ca2+, and Mg2+, and the following stability constants were obtained: logKNdBL = 2.51, logKNdOHBL = 3.90, logKHBL = 4.01, logKCaBL = 2.43, and logKMgBL = 2.70. The results demonstrated that the BLM could predict the Nd toxicity well while considering the competition of H+, Ca2+, Mg2+ and the toxic species Nd3+ and NdOH2+ for binding sites.

Integrated transcriptomic and metabolomic analysis reveals the potential mechanisms underlying indium-induced inhibition of root elongation in wheat plants

Soil contamination by indium, an emerging contaminant from electronics, has a negative impact on crop growth. Inhibition of root growth serves as a valuable biomarker for predicting indium phytotoxicity. Therefore, elucidating the molecular mechanisms underlying indium-induced root damage is essential for developing strategies to mitigate its harmful effects. Our transcriptomic findings revealed that indium affects the expression of numerous genes related to cell wall composition and metabolism in wheat roots. Morphological and compositional analysis revealed that indium induced a 2.9-fold thickening and a 17.5 % increase in the content of cell walls in wheat roots. Untargeted metabolomics indicated a substantial upregulation of the phenylpropanoid biosynthesis pathway. As the major end product of phenylpropanoid metabolism, lignin significantly accumulated in root cell walls after indium exposure. Together with increased lignin precursors, enhanced activity of lignin biosynthesis-related enzymes was observed. Moreover, analysis of the monomeric content and composition of lignin revealed a significant enrichment of p-hydroxyphenyl (H) and syringyl (S) units in root cell walls under indium stress. The present study contributes to the existing knowledge of indium toxicity. It provides valuable insights for developing sustainable solutions to address the challenges posed by electronic waste and indium contamination on agroecosystems.

Alteration of Hordeum vulgare and Sinapis alba germination and early growth in response to airborne low-metallic automotive brake wear debris

Road transportation significantly contributes to environmental pollution, both in terms of exhaust and non-exhaust (brake wear) emissions. As was proven, brake wear debris is released in a wide variety of sizes, shapes, and compositions. Although studies confirming the possible adverse health and environmental impact of brake wear debris were published, there is no standardized methodology for their toxicity testing, and most studies focus only on one type of brake pad and/or one test. The lack of methodology is also related to the very small amount of material released during the laboratory testing. For these reasons, this study deals with the mixture of airborne brake wear debris from several commonly used low-metallic brake pads collected following the dynamometer testing. The mixture was chosen for better simulation of the actual state in the environment and to collect a sufficient amount of particles for thorough characterization (SEM, XRPD, XRF, chromatography, and particle size distribution) and phytotoxicity testing. The particle size distribution measurement revealed a wide range of particle sizes from nanometers to hundreds of nanometers, elemental and phase analysis determined the standard elements and compounds used in the brake pad formulation. The Hordeum vulgare and Sinapis alba were chosen as representatives of monocotyledonous and dicotyledonous plants. The germination was not significantly affected by the suspension of brake wear debris; however, the root elongation was negatively influenced in both cases. Sinapis alba (IC50 = 23.13 g L-1) was more affected than Hordeum vulgare (IC50 was not found in the studied concentration range) the growth of which was even slightly stimulated in the lowest concentrations of brake wear debris. The plant biomass was also negatively affected in the case of Sinapis alba, where the IC50 values of wet and dry roots were determined to be 44.83 g L-1 and 86.86 g L-1, respectively.

Zoom-in to molecular mechanisms underlying root growth and function under heterogeneous soil environment and abiotic stresses

MAIN CONCLUSION

The review describes tissue-specific and non-cell autonomous molecular responses regulating the root system architecture and function in plants. Phenotypic plasticity of roots relies on specific molecular and tissue specific responses towards local and microscale heterogeneity in edaphic factors. Unlike gravitropism, hydrotropism in Arabidopsis is regulated by MIZU KUSSIE1 (MIZ1)-dependent asymmetric distribution of cytokinin and activation of Arabidopsis response regulators, ARR16 and ARR17 on the lower water potential side of the root leading to higher cell division and root bending. The cortex specific role of Abscisic acid (ABA)-activated SNF1-related protein kinase 2.2 (SnRK2.2) and MIZ1 in elongation zone is emerging for hydrotropic curvature. Halotropism involves clathrin-mediated internalization of PIN FORMED 2 (PIN2) proteins at the side facing higher salt concentration in the root tip, and ABA-activated SnRK2.6 mediated phosphorylation of cortical microtubule-associated protein Spiral2-like (SP2L) in the root transition zone, which results in anisotropic cell expansion and root bending away from higher salt. In hydropatterning, Indole-3-acetic acid 3 (IAA3) interacts with SUMOylated-ARF7 (Auxin response factor 7) and prevents expression of Lateral organ boundaries-domain 16 (LBD16) in air-side of the root, while on wet side of the root, IAA3 cannot repress the non-SUMOylated-ARF7 thereby leading to LBD16 expression and lateral root development. In root vasculature, ABA induces expression of microRNA165/microRNA166 in endodermis, which moves into the stele to target class III Homeodomain leucine zipper protein (HD-ZIP III) mRNA in non-cell autonomous manner. The bidirectional gradient of microRNA165/6 and HD-ZIP III mRNA regulates xylem patterning under stress. Understanding the tissue specific molecular mechanisms regulating the root responses under heterogeneous and stress environments will help in designing climate-resilient crops.

Physiological mechanisms of exogenous organic acids to alleviate aluminum toxicity in seedlings of mungbean, buckwheat, and rice

One of the major constraints for crop yield in acidic soils is the phytotoxicity of aluminum ions (Al3+), which primarily affects the roots. To mitigate the harmful effects of Al toxicity, plants use organic acids to chelate Al internally and externally. In this study, the effects of exogenous organic acids on Al toxicity in rice, mung bean, and buckwheat were investigated. Specifically, the study examined the ameliorative effect of three organic acids (oxalic acid, malic acid, and citric acid, each at a concentration of (100 μmol/L) on root elongation, fresh weight, Al content, organic acid key enzymes, and rhizosphere pH in hydroponic media containing (100 μmol/L) Al. The experimental results revealed species-specific responses to aluminum tolerance and the alleviating effects of different organic acids. Buckwheat was found to be the most aluminum-tolerant, followed by mung bean, while rice was the least tolerant. Exogenous application of oxalic acid promoted root elongation, increased root fresh weight, and enhanced the activity of the PEPC enzyme in mung bean. Malic acid, on the other hand, alleviated Al toxicity in rice by promoting root elongation, increasing root fresh weight, enhancing the activity of the PEPC enzyme, and decreasing the activity of the MDH enzyme. In buckwheat, citric acid application reduced Al toxicity by promoting root elongation, increasing root weight, and decreasing the activities of CS and GO enzymes. These findings indicate that different organic acids can reduce Al toxicity in different plant species by employing different physiological mechanisms.

Effect of polypropylene microplastics on seed germination and nutrient uptake of tomato and cherry tomato plants

Although microplastic (MP) pollution in farmland increased, the effect of MPs on plant growth was not clearly explained. Therefore, the object of the study was to evaluate the effect of polypropylene microplastics (PP-MPs) on plant germination, growth, and nutrient uptake under hydroponic conditions. The effect of PP-MPs on seed germination, shoot and root length, and nutrient uptake were assessed using tomato (Solanum lycopersicum L.) and cherry tomato (Solanum lycopersicum var. cerasiforme) seeds grown in half-strength Hoagland solution. The results showed that PP-MPs did not significantly affect seed germination, but positively affected the shoot and root elongation. In cherry tomato, the root elongation was significantly increased by 34%. Microplastics also affected nutrient uptake by plants, however, the effect varied depending on elements and plant species. The Cu concentration was substantially increased in tomato shoot while it decreased in cherry tomato root. Nitrogen uptake decreased in MP treated plants compared to the control and phosphorus uptake was significantly decreased in the shoot of cherry tomato. However, the root-to-shoot translocation rate of most macro nutrients decreased following exposure to PP-MPs indicating that long-term exposure to MPs may lead to a nutritional imbalance in plants.

Quantitative analysis of dose interval effect of Pb-Cd interaction on Oryza sativa L. root

Combined pollution of cadmium (Cd) and lead (Pb) occurs frequently in agriculture lands, which has received increasing research attention. However, little is known about the interaction behaviors of Cd and Pb at various concentrations in the mixture. This study evaluated the single and combined effects of Cd and Pb on rice (Oryza sativa L.) root elongation through acute exposure test. The combined pollution was analyzed with the concentration addition (CA) model, independent action (IA) model and mathematical statistical methods. The dose-response results revealed that the interaction could weaken the toxicity of both Pb and Cd, and Cd had a more significant inhibitory effect on Pb toxicity. The predicted values of CA and IA models were consistently lower than the observed values in the relative root elongation range of 0-60%. Further, combining the CA or IA model with mathematical statistical methods, the interaction of Pb and Cd at similar concentrations showed a significant antagonistic effect on rice root elongation. At low Pb concentrations (Cd > 0.0195, Pb < 0.015 mg/L), there was a synergistic effect of the mixture on rice root; at high Pb concentrations (Cd < 0.225, Pb ≥ 1.25 mg/L), Pb dominated the toxicity on rice root. This is the first report of a systematic method for assessing heavy metal interaction at different concentration levels, which may facilitate the formulation of control standards of heavy metal combined pollution in agricultural land.

Five NUCLEAR FACTOR-Y subunit B genes in rapeseed (Brassica napus) promote flowering and root elongation in Arabidopsis

Heterologous expression of BnNF-YB2, BnNF-YB3, BnNF-YB4, BnNF-YB5, or BnNF-YB6 from rapeseed promotes the floral process and also affects root development in Arabidopsis. The transcriptional regulator NUCLEAR FACTOR-Y (NF-Y) is a heterotrimeric complex composed of NF-YA, NF-YB, and NF-YC proteins and is ubiquitous in yeast, animal, and plant systems. In this study, we found that five NF-YB proteins from rapeseed (Brassica napus), including BnNF-YB2, BnNF-YB3, BnNF-YB4, BnNF-YB5, and BnNF-YB6 (BnNF-YB2/3/4/5/6), all function in photoperiodic flowering and root elongation. Sequence alignment and phylogenetic analysis showed that BnNF-YB2/3 and BnNF-YB4/5/6 were clustered with Arabidopsis AtNF-YB2 and AtNF-YB3, respectively, implying that these NF-YBs are evolutionarily and functionally conserved. In support of this hypothesis, the heterologous expression of individual BnNF-YB2, 3, 4, 5, or 6 in Arabidopsis promoted early flowering under a long-day photoperiod. Further analysis suggested that BnNF-YB 2/3/4/5/6 elevated the expression of key downstream flowering time genes including CO, FT, LFY and SOC1. Promoter-GUS fusion analysis showed that the five BnNF-YBs were expressed in a variety of tissues at various developmental stages and GFP fusion analysis revealed that all BnNF-YBs were localized to the nucleus. In addition, we demonstrated that the heterologous expression of individual BnNF-YB2/3/4/5/6 in Arabidopsis promoted root elongation and increased the number of root tips formed under both normal and treatment with simulators of abiotic stress conditions.

Investigating and modeling the toxicity of arsenate on wheat root elongation: Assessing the effects of pH, sulfate and phosphate

Excessive arsenic in soil and groundwater will not only seriously affect the growth of plants, but also endanger human health through the food chain. However, there are few studies on the effects of metalloid speciation and anion competition on the toxicity of arsenate [As(Ⅴ)]. To investigate the effects of accompanying anions and pH on the toxicity of As(Ⅴ) on wheat root elongation, wheat roots were exposed to the concentrations of As(Ⅴ) in the solution ranged from 0 to 500 mM and different levels of pH (4.5-8.0) and different accompanying anions (H₂PO₄^-, SO₄^2-, NO₃^- and Cl^-) for five days. The root length of wheat was measured and the biotic ligand model (BLM) was developed to predict the potential toxicity of As(V) speciation to wheat roots. The results illustrated that EC50 of total As(V) (EC50{As(Ⅴ)_T}) values increased from 6.88 to 33.9 μM with increasing pH values from 4.5 to 8.0, suggesting that increasing pH alleviated As(Ⅴ) toxicity. The EC50{AsO₄^3-} and EC50{HAsO₄^2-} values increased from 0.001 to 4342 μM and from 0.0214 to 27.4 μM, respectively, while the EC50{H₂AsO₄^-} and EC50{H₃AsO₄} values sharply decreased from 6.62 to 2.68 μM and from 41.8 μM to 5.34 nm, respectively, when pH increased from 4.5 to 8.0. The toxicity of As(Ⅴ) decreased as the H₂PO₄^- and SO₄^2- activities increased but not when the activities of NO₃^- and Cl^- increased, indicating that SO₄^2- and H₂PO₄^- showed competitive effects with As(Ⅴ) on the binding sites. Based on BLM theory, the stability constants were obtained: [Formula: see text] = 3.70; [Formula: see text] = 4.08; [Formula: see text] = 4.77; [Formula: see text] = 6.50; [Formula: see text] = 2.09 and [Formula: see text] = 1.86, with f_AsBL^50%= 0.30 and β = 1.73. Results implied that BLM performed well in As(Ⅴ) toxicity prediction when coupling toxic species AsO₄^3-, HAsO₄^2-, H₂AsO₄^-, and H₃AsO₄, and the competition of SO₄^2- and H₂PO₄^- for binding sites. The current study provides a useful tool to accurately predict As(V) toxicity to wheat roots.

Glucose-6-phosphate dehydrogenase and abscisic acid mediate programmed cell death induced by aluminum toxicity in soybean root tips

Programmed cell death (PCD) induced by aluminum (Al) is considered an important reason of Al phytotoxicity. However, the underlying mechanism of how Al induces PCD remains largely unknown in plants. The roles of glucose-6-phosphate dehydrogenase (G6PDH) and abscisic acid (ABA) in regulating Al-induced PCD were investigated in soybean roots. Al treatment increased G6PDH activity, while inhibition of G6PDH activity alleviated PCD occurrence and reactive oxygen species (ROS) accumulation under Al stress. Overexpression of cytosolic G6PDH1 enhanced G6PDH activity, thus promoting ROS production and cell death under Al exposure. Inhibition of NADPH oxidase activity mitigated ROS generation and cell death under Al stress. Further investigation demonstrated that G6PDH positively regulated the activity of NADPH oxidase under Al treatment using pharmacological and transgenic approach. Furthermore, Al stress increased ABA production, while inhibition of ABA biosynthesis alleviated PCD occurrence and ROS accumulation under Al stress. Interestingly, ABA upregulated G6PDH1 expression and G6PDH activity under Al stress. These results suggest that G6PDH mediates Al-induced PCD occurrence through the activation of NADPH oxidase-dependent ROS production, and ABA acts upstream of G6PDH in this process. This study will provide novel clues for the improvement of Al phytotoxicity in acidic soils.

Integration of nutrient and water availabilities via auxin into the root developmental program

In most soils, the spatial distribution of nutrients and water in the rooting zone of plants is heterogeneous and changes over time. To access localized resources more efficiently, plants induce foraging responses by modulating individual morphological root traits, such as the length of the primary root or the number and length of lateral roots. These adaptive responses require the integration of exogenous and endogenous nutrient- or water-related signals into the root developmental program. Recent studies corroborated a central role of auxin in shaping root architectural traits in response to fluctuating nutrient and water availabilities. In this review, we highlight current knowledge on nutrient- and water-related developmental processes that impact root foraging and involve auxin as a central player. A deeper understanding and exploitation of these auxin-related processes and mechanisms promises advances in crop breeding for higher resource efficiency.

Alleviation of salinity and metal stress using plant growth-promoting rhizobacteria isolated from semiarid Moroccan copper-mine soils

Phytoremediation is an eco-friendly method for rehabilitation of mine tailing. Some heavy metals and salt-tolerant plant growth-promoting rhizobacteria (PGPR) could be beneficial in alleviating soil salinity and heavy metal stress during plant growth. The aim of this work is to select PGPR that could be used in phytoremediation process. Twenty-nine rhizobacteria are examined for their ability to grow at increasing concentrations of NaCl, Zn, Pb, Cu, and Cd. The results showed that seventeen rhizobacteria displayed high salinity and metal tolerance up to 100 g L^-1 of NaCl, 5 mM of Cd, 9 mM of Pb, 10 mM of Zn, and 6 mM of Cu. Moreover, almost all tested bacteria maintained their PGP traits under 10% of NaCl and multi-metal stress. Based on seedling bioassay under metallic and salt stress, using Peganum harmala L. and Lactuca sativa L., beneficial effects of seed inoculation with bacterial consortia (Mesorhizobium tamadayense, Enterobacter xiangfangensis, Pseudomonas azotifigens, and Streptomyces caelestis) have been observed in terms of root and shoot elongation. Our results show that the stress-tolerant consortium used has a great potential to sustain plants establishment in heavily disturbed soils.

Knockdown of Succinate Dehydrogenase Assembly Factor 2 Induces Reactive Oxygen Species-Mediated Auxin Hypersensitivity Causing pH-Dependent Root Elongation

Metabolism, auxin signaling and reactive oxygen species (ROS) all contribute to plant growth, and each is linked to plant mitochondria and the process of respiration. Knockdown of mitochondrial succinate dehydrogenase assembly factor 2 (SDHAF2) in Arabidopsis thaliana lowered succinate dehydrogenase activity and led to pH-inducible root inhibition when the growth medium pH was poised at different points between 7.0 and 5.0, but this phenomenon was not observed in wildtype (WT). Roots of sdhaf2 mutants showed high accumulation of succinate, depletion of citrate and malate and up-regulation of ROS-related and stress-inducible genes at pH 5.5. A change of oxidative status in sdhaf2 roots at low pH was also evidenced by low ROS staining in root tips and altered root sensitivity to H2O2. sdhaf2 had low auxin activity in root tips via DR5-GUS staining but displayed increased indole-3-acetic acid (IAA, auxin) abundance and IAA hypersensitivity, which is most likely caused by the change in ROS levels. On this basis, we conclude that knockdown of SDHAF2 induces pH-related root elongation and auxin hyperaccumulation and hypersensitivity, mediated by altered ROS homeostasis. This observation extends the existing evidence of associations between mitochondrial function and auxin by establishing a cascade of cellular events that link them through ROS formation, metabolism and root growth at different pH values.

Biotic ligand modeling to predict the toxicity of HWO4- and WO42- on wheat root elongation in solution cultures: Effects of pH and accompanying anions

Increasing evidence demonstrates that hexavalent tungsten (W(VI)) can affect the survival of various organisms. This study explored the influences of pH and common anions on W(VI) toxicity on wheat and established a biotic ligand model (BLM) for predicting W(VI) toxicity. It was found that as the pH value increased from 6.0 to 8.5, the EC50[W(VI)]_T values increased greatly from 24.7 to 46.6 μM, indicating that increasing pH values can alleviate W(VI) toxicity. A linear relationship between the ratio of HWO₄^- to WO₄^2- and EC50{WO₄^2-} indicated that WO₄^2- and HWO₄^- were two toxic species of W(VI). The toxicity of W(VI) decreased as the H₂PO₄^- and SO₄^2- activities increased but not when the activities of Cl^- and NO₃^- increased, demonstrating that the competition from H₂PO₄^- and SO₄^2- significantly influenced W(VI) toxicity. By applying BLM theory, the stability constants for HWO₄^-, WO₄^2-, H₂PO₄^-, and SO₄^2- were obtained: logK_WO4BL = 4.08, logK_HWO4BL = 6.44, logK_H2PO4BL = 2.09, and logK_SO4BL = 1.87, f_WBL^50% = 0.300, β = 1.99. Results demonstrated that BLM outperformed the free metal activity model(FIAM) in predicting W(VI) toxicity when considering the influences of pH, W(VI) species, and H₂PO₄^- and SO₄^2- competition for active ligand sites.

Estimating the synergistic and antagonistic effects of dual antibiotics on plants through root elongation test

Antibiotics are recently recognized as a group of emerging environmental contaminants that are frequently detected in various environmental matrixes. Relative root elongation (RRE) test is a rapid and effective strategy to evaluate the water/soil quality and the toxic effects of environmental contaminants on plants. In the present study, we examine the toxicity effect of ciprofloxacin (CIP), norfloxacin (NOR), and tetracycline (TET) to pakchoi individually and in combinations. Both independent action (IA) and concentration addition (CA) models are used for toxicity assessment. Results showed that the EC₅₀ values of CIP, NOR, and TET are 193.59, 60.81, and 40.37 μM, respectively. Combinations of TET + CIP and TET + NOR caused more inhibitory effects on root elongation than those of CIP + NOR. Toxic Unit (TU) and Synergistic Ratio (SR) analysis showed that the relatively lower (higher) EC values are observed in the combinations with lower (higher) antibiotic concentrations, suggesting an effect of low-dose synergism and high-dose antagonism. The reliability of the simulation results from IA and CA models to predict that combined toxicity is highly dependent upon the results from the analysis of TU or SR.

Phytotoxicity test in check: Proposition of methodology for comparison of different method adaptations usually used worldwide

Seed germination tests have been widely used in recent years to indicate the toxicity levels of samples of organic compounds, biosolids, residues and effluents. Lactuca sativa L, commonly known as lettuce, has been one of the main indicative species for these tests due to its high sensitivity to low levels of toxicity, when compared to other seeds and also because it is cultivated worldwide. Although this type of analysis or essay is being widely used, it is necessary to reflect on the various adaptations of methods used by different researchers worldwide. This work presents an innovative methodology that makes it possible to compare the different phytotoxicity methods currently used in the world, through four stages that include the coefficient of variation (CV) as the main classification criterion, also counting on an eliminatory criterion. The existence of a significant difference (P value < 0.05) between the evaluated tests was proven. The phytotoxicity test that presented the lowest CV was T8 (test with lettuce seeds at 25 °C, 60 min agitation, resting overnight, 5 mL of sample on the plate, 90 mm size plate). It has concluded that not all adaptations of this type of test are reliable. It has also concluded that there is a lack of standardization for the phytotoxicity test on a global scale, which makes the various researchers in the field end up promoting variations, adaptations for the phytotoxicity test; therefore, there is an urgent need for ways to compare these variations, as the innovation proposed by this work. With a single standard methodology, we conclude that it will make it possible to compare phytotoxicity in samples directly between countries and continents, being able to generate a worldwide panorama of phytotoxicity, publicizing and comparing the standardized phytotoxicity levels in each region.

Quaternary ammonium iminofullerenes promote root growth and osmotic-stress tolerance in maize via ROS neutralization and improved energy status

In the present study, the role of quaternary ammonium iminofullerenes (IFQA) on the root growth of plant seedlings was investigated. The root elongation of Arabidopsis and maize exposed to 20 and 50 mg/L of IFQA was promoted under normal and osmotic stress conditions, respectively. In the meantime, the root active absorption area and adenosine triphosphate content in roots of maize seedlings were enhanced by IFQA treatment, however, the contents of hydrogen peroxide (H₂O₂) and malondialdehyde in roots were down-regulated. IFQA application improved glutathione transferase and glutathione reductase activities and the ratios of glutathione/oxidized glutathione and ascorbic acid/dehydroascorbic acid, and restored the inhibition of root elongation caused by the excess accumulation of H₂O₂ in roots of maize seedlings under osmotic stress. Furthermore, the expression of 14 proteins involved in cell growth, energy metabolism, and stress response in maize roots was upregulated by two-dimensional electrophoresis combined with mass spectrometry. This analysis revealed that IFQA stimulated the redox pathway to maintain balance levels of reactive oxygen species to ensure normal cell metabolism, promote energy production for root growth, and enhance osmotic-stress tolerance. It provided crucial information to elucidate the mechanism of the root growth of crop seedlings enhanced by water-soluble fullerene-based nanomaterials.

Genomic appraisal of Klebsiella PGPB isolated from soil to enhance the growth of barley

BACKGROUND

PGPR has substituted chemical fertilizers to enhance the nutrient profile of the soil. Although gene encoding for PGP activity is present in PGPB their activity changes in response to conditions.

OBJECTIVE

To study comparative genomics for three Klebsiella strains and their PGPR activity in response to in vitro and soil condition.

METHODS

We evaluated the activity of three Klebsiella spp. in two different conditions, specific nitrogen-deficient MS media and greenhouse experiment. Applying comparative genomics, genes encoding for PGP traits were identified from the whole-genome sequencing of the three strains. With the help of the RAST tool kit and functional annotation, a total number of genes encoding for cell wall capsule, nitrogen metabolism, sulfur genes and many other functional groups were identified. With the help of blast circular genome, similarity between GC content, pseudogene and tRNA was represented. The percentage of gene similarity of SSN1 was generated against BLAST with M5a1 and SGM81. Other methods like synteny alignment and orthologous gene clusters were applied to understand the homologous present in three strains.

RESULTS

SSN1 was actively producing the maximum amount of ammonia 10.97 ± 0.29 µmol/mL compared to the other two strains. K. oxytoca M5a1 was considered negative for all PGP traits except ammonia production. The activity of SSN1 was showing a consistent pattern both the conditions whereas M5a1 was only active in vitro condition. Gene encoding for allantoin metabolism allD, allC, allB, allA, allE, allR, allH were identified in SSN1 and M5a1 but was absent in SGM81. The highest COG was shared between SGM81 and SSN1 predicting a maximum number of similar genes. The nif gene cluster was 98 % identical to the M5a1 strain.

CONCLUSIONS

Comparatively, SSN1 expressed the additional gene for various PGP traits which suggest higher efficiency of strain in nitrogen deficiency stress.

Studies on the performance of bentonite and its composite as phosphate adsorbent and phosphate supplementation for plant

Organo-bentonite (OrB) was prepared by modifying bentonite with chitosan, and natural surfactant extracted from Sapindus rarak fruit. The physical alteration post-modification, performance of phosphates (Pi) adsorption, and possibility as a Pi-supplementation for plants of OrB were assessed and compared to acid-activated bentonite (AAB). The physical alteration due to modification of bentonite was characterized. SEM images were not indicating significant morphology differences between OrB and AAB. Existence of chitosan layers in OrB causes a decrease in basal spacing as characterized using XRD. The BET surface area of OrB was decreased compared to AAB due to pore coverage by chitosan. Adsorption studies reveal that OrB has a higher adsorption capacity towards Pi than AAB, which is 97.608 and 131.685 mg/g at 323 K for AAB and OrB, respectively. The H-shape isotherm curve indicates that chemisorption is dominantly controlling the adsorption. The isotherm and kinetics adsorption were well fitted to Langmuir and Pseudo-second order models, respectively. Performance of AAB and OrB as Pi-supplementation was assessed based on growth phenotypes of Arabidopsis thaliana; seedlings show that supplementation of Pi@AAB and Pi@OrB (at half doses) can promote primary root extension. These results also demonstrate the safety of direct disposal of the materials into the soil.

Low nitrogen induces root elongation via auxin-induced acid growth and auxin-regulated target of rapamycin (TOR) pathway in maize

Under low nitrogen (N) supply, an important adaption of the maize root system is to promote the root elongation so as to increase N uptake from a larger soil space. The underlying physiological mechanism is largely unknown. In the present study, two maize inbred lines (Ye478 and Wu312) were used to study the possible involvement of the auxin and target of rapamycin (TOR) pathway in low-N-induced root elongation. Compared to Wu312, primary root elongation of Ye478 was more sensitive to low nitrate supply. Correspondingly, more auxin was accumulated in the root tip, and more protons were secreted, increasing the acidity of the apoplast space. On the other hand, low-N-induced root elongation was greatly reduced when shoot-to-root auxin transport was inhibited by applying N-1-naphthylphthalamic acid (NPA) at the plant base or by pruning the top leaf where auxin is mostly synthesized. Furthermore, exogenous application of TOR inhibitor also eliminated the response of root elongation under low N. The content of TOR kinase and the expression of TOR pathway-related genes were significantly changed when shoot-to-root auxin transport was reduced by NPA treatment. Taken together, it is concluded that low-N stress increases shoot-to-root auxin transport which enhances root elongation via auxin-dependent acid growth and the auxin-regulated TOR pathway in maize.

Phytotoxicity indexes and removal of color, COD, phenols and ISA from pulp and paper mill wastewater post-treated by UV/H2O2 and photo-Fenton

Pulp and paper mill wastewater (PPMWW) contains high concentrations of recalcitrant compounds that cause toxicity to organisms. Advanced oxidation processes (AOPs) have the ability to degrade these compounds and reduce overall toxicity. Physicochemical characterization and Lactuca sativa toxicity test were conducted to compare the effectiveness of two post-treatments: UV/H₂O₂ and photo-Fenton. A comparison of four phytotoxicity indexes was carried out. PPMWW from a Brazilian treatment plant was characterized by high values of phenols, color, integrated spectral area (ISA), and chemical oxygen demand (COD), and caused significant inhibition to seedling development. The use of both post-treatments allowed the removal of over 75% of phenols, color, ISA, and COD. Although UV/H₂O₂ was more effective in removing phenols and ISA, photo-Fenton better reduced phytotoxicity. The most sensitive phytotoxicity indexes were RGIC_0.8 and GIC_80%, whereas SGC₀, REC_-0.25 and REC_-0.50 better showed the effectiveness of the post-treatments. We suggest the combined use of two phytotoxicity indexes: one that evaluates the effects on seed germination and, another, on root elongation, e.g., SGC₀ and RGIC_0.8. Additionally, we recommend the use of ISA for monitoring programs of wastewater treatments because it is a cost-effective approach that allows narrowing down the search and identification of compounds present in complex mixtures.

Comparative study of alleviation effects of DMTU and PCIB on root growth inhibition in two tall fescue varieties under cadmium stress

In plants, tolerance to cadmium (Cd) stress is closely related to indole-3-acetic acid (IAA) and hydrogen peroxide (H₂O₂). However, it is unclear whether Cd-resistant and -sensitive varieties respond differently to Cd stress. In this study, the effects of dimethylthiourea (DMTU, a H₂O₂ scavenger) and p-chlorophenoxy isobutyric acid (PCIB, an IAA signaling inhibitor) on root growth, endogenous hormones and antioxidant system were investigated to decipher how DMTU and PCIB treatments alleviate the inhibition of root elongation in Cd-resistant (Commander) and -sensitive (Crossfire III) tall fescue varieties under Cd stress. Both varieties subjected to 10 μM Cd treatments for 12 h presented a substantial decrease in root elongation coupled with a reduction in brassinosteroid (BR) and zeatin riboside (ZR) contents, but the changes in IAA and abscisic acid (ABA) contents under Cd stress were opposite in the two varieties. In addition, the H₂O₂ content and antioxidant enzyme activities significantly increased in both varieties. However, pretreatment with PCIB or DMTU mitigated the inhibition of root elongation caused by Cd, accompanied by the significant changes of aforementioned physiological parameters. PCIB significantly reduced the IAA content in 'Commander', while DMTU significantly increased the IAA content in 'Crossfire III' and effectively relieved the inhibition of root elongation. But both treatments decreased the Cd-induced H₂O₂ accumulation. These results indicated that DMTU or PCIB can alleviate the Cd-inhibited root elongation in two varieties whose resistance differed under Cd stress, but they presented differences in the response of hormones, especially IAA, which may be due to the different adaptation mechanisms of two varieties in response to Cd stress.

Phytotoxicity of Cu2+ and Cd2+ to the roots of four different wheat cultivars as related to charge properties and chemical forms of the metals on whole plant roots

The relationship between the chemical forms of Cu²⁺ and Cd²⁺ adsorbed on the roots of different wheat cultivars and their phytotoxic effects on the plants were investigated. The wheat varieties Dunmaiwang (DMW), Tekang 6 (TK6), Zhongmai895 (ZM895), and Chaojixiaomai (AK68) were used. The zeta potentials of wheat roots, measured by the streaming potential method, were used to characterize root charge properties. Results indicated that the changes in zeta potential at pH 4.01-6.61 were 14.7, 15.53, 13.01, and 12.06 mV for ZM895, AK68, DMW, and TK6, respectively. The negative charge and functional groups on ZM895 and AK68 roots were greater than on DMW and TK6 roots, which led to more exchangeable and complexed Cu²⁺ and Cd²⁺ on ZM895 and AK68 roots and increased Cu²⁺ and Cd²⁺ toxicity compared to DMW and TK6. Coexisting cations, such as Ca²⁺, Mg²⁺, K⁺, and NH₄⁺, alleviated Cu²⁺ and Cd²⁺ toxicity to wheat roots through competition for adsorption sites on the roots, which decreased exchangeable and complexed Cu²⁺ and Cd²⁺ on wheat roots. The Ca²⁺ and Mg²⁺ were most effective in alleviating heavy metal toxicity and they decreased exchangeable Cu²⁺ on AK68 roots by 39.14% and 47.82%, and exchangeable Cd²⁺ by 8.51% and 28.23%, respectively.

The physiological mechanism underlying root elongation in response to nitrogen deficiency in crop plants

In response to low nitrogen stress, multiple hormones together with nitric oxide signaling pathways work synergistically and antagonistically in crop root elongation. Changing root morphology allows plants to adapt to soil nutrient availability. Nitrogen is the most important essential nutrient for plant growth. An important adaptive strategy for crops responding to nitrogen deficiency is root elongation, thereby accessing increased soil space and nitrogen resources. Multiple signaling pathways are involved in this regulatory network, working together to fine-tune root elongation in response to soil nitrogen availability. Based on existing research, we propose a model to explain how different signaling pathways interact to regulate root elongation in response to low nitrogen stress. In response to a low shoot nitrogen status signal, auxin transport from the shoot to the root increases. High auxin levels in the root tip stimulate the production of nitric oxide, which promotes the synthesis of strigolactones to accelerate cell division. In this process, cytokinin, ethylene, and abscisic acid play an antagonistic role, while brassinosteroids and auxin play a synergistic role in regulating root elongation. Further study is required to identify the QTLs, genes, and favorable alleles which control the root elongation response to low nitrogen stress in crops.

Chemical characterization and phytotoxicity assessment of peri-urban soils using seed germination and root elongation tests

The peri-urban soil is exposed to pollutants because of its proximity to the city, which may influence the quality of agricultural products. In this study, the occurrence of 16 trace elements (TEs), 16 polycyclic aromatic hydrocarbons (PAHs), and 33 contaminants of emerging concern (CECs) was analyzed in two soil sites of the peri-urban area of Barcelona (Spain) (S2 and S3) and a pristine site (S1). Levels of Pb (S2 164 and S3 150 mg kg-1) are around 2.5 times higher than the guideline values. Values for Cu (178 mg kg-1) in S2 are 1.8-fold higher, whereas for Zn, levels are slightly above the threshold in S2 (208 mg kg-1) and S3 (217 mg kg-1). The total concentrations of PAHs are significantly below the limits: 24 ng g-1 dw (S1), 38 ng g-1 dw (S2), 49 ng g-1 dw (S3), whereas only some CECs are detected with low concentrations. We also developed a simple and rapid method to assess soil pollution. Here, we use two plant growth indexes (seed germination rate and root elongation at the initial stage) of three seeds (lettuce, tomato, and cauliflower) to assess soil chemical contamination on agriculture. In the peri-urban soil, the concentration of Pb was 2.5 times higher than the guideline values, whereas for Cu and Zn, values were slightly above their limits, while only few PAHs and CECs were detected. Results for principal component analysis suggest that root elongation is a more sensitive measurement endpoint than germination rate, especially for lettuce. The germination rate of tomato relied on the nitrate in the soil and decreased sharply in the site with pollution of Cu and As. Under the specific conditions of this study, cauliflower should not be recommended to assess environmental pollution due to its low sensitivity to pollutants. In conclusion, this is a low-cost, simple, and rapid method for evaluating the effects of chemical pollution of agriculture soils on seed growth.

An improved biotic ligand model (BLM) for predicting Co(II)-toxicity to wheat root elongation: The influences of toxic metal speciation and accompanying ions

In order to explore the effects of pH and accompanying ions on divalent cobalt (Co(II)) toxicity to the wheat root elongation, an improved biotic ligand model (BLM) to predict Co(II) toxicity was developed in solution culture. The results showed that the Co(II)-toxicity decreased with the increases of K⁺, Ca²⁺ and Mg²⁺ activities, and the activity of Na⁺ had no impact on the Co(II)-toxicity. High H⁺ activity reduced the Co(II)-toxicity by the competitive effect of H⁺, while low H⁺ activity affected the toxicity by the change in the type of Co(II) in culture medium. Co²⁺ and CoOH⁺ were toxic to wheat root elongation, and Co(OH)₂ was not. On the basis of BLM theory, the conditional equilibrium constants for Co²⁺, CoOH⁺, H⁺, Mg²⁺, Ca²⁺, K⁺ were obtained: logK_CoBL = 4.65, logK_CoOHBL = 6.62, logK_HBL = 4.53, logK_MgBL = 3.65, logK_CaBL = 2.36 and logK_KBL = 2.17. Free Co²⁺ and CoOH⁺, and the competitions of K⁺, Mg²⁺ and Ca²⁺ were suggested to be considered when developing the Co(II)-BLM.

Milli-channel array for direct and quick reading of root elongation bioassays

A novel platform to perform systematic analysis and direct reading of root elongation bioassays is presented. The device was designed to include multiplexed microenvironments for the germination and growth of individual seeds, which allows observation by the naked eye or by optical systems, notably cellphone cameras. Prototypes were fabricated by laser micromachining on a highly transparent material that is fully compatible with biological systems. The effectiveness of the milli-channel array was verified against the conventional system (Petri dish). Lactuca sativa was chosen as a model species and glyphosate as a typical toxic agent. All tests were run according to standardized procedures and data analysis was carried out through different statistical indicators such as the root elongation and germination indexes. Results attained in the milli-channel array were identical to those in Petri dish, with the remarkable benefit that several steps required in the conventional system were avoided, which enormously decreases the operation time and the possibility of experimental errors. Further advantages of the milli-channel array are also reported, such as the capability to achieve live imaging of plant organs growth through a simple experiment. The developed device has been proven to be effective, versatile, easy-to-use, and integrable to cellphones, which naturally provide facilities for data recording, analysis, and networking. These improvements open the route to novel applications of bioassays in the wide field of ecotoxicology and environmental studies.

Specific roles of Os4BGlu10, Os6BGlu24, and Os9BGlu33 in seed germination, root elongation, and drought tolerance in rice

Morphological, physiological, and gene expression analyses showed that Os4BGlu10, Os6BGlu24, and Os9BGlu33 played specific roles in seed germination, root elongation, and drought tolerance of rice, with various relations with indole-3-acetic acid (IAA) and abscisic acid (ABA) signaling. β-Glucosidases (BGlus) belong to glycoside hydrolase family 1 and have many functions in plants. In this study, we investigated the function of three BGlus in seed germination, drought tolerance, and root elongation using the loss-of-function mutants bglu10, bglu24, and bglu33. These mutants germinated slightly later under normal conditions and had significantly longer roots than the wild type. In the presence of ABA, bglu10 and bglu24 exhibited a higher germination inhibition percentage, whereas bglu33 had a lower germination inhibition percentage, compared to the wild type. All of the mutants exhibited less drought tolerance, with the survival rates significantly lower than that of the wild type, which was also confirmed by a decrease in relative leaf water content and Fv/Fm ratio after drought treatment. The root length of bglu10 did not respond to IAA, whereas that of bglu24 responded to a high (0.25 µM) concentration of IAA, and that of bglu33 to a low (0.05 µM) concentration of IAA. The root length of bglu10 and bglu24 did not respond to ABA, whereas that of bglu33 increased significantly in response to a high (0.05 µM) concentration of ABA. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis showed that expression of Os4BGlu10 was up-regulated by polyethylene glycol (PEG), whereas that of Os6BGlu24 was up-regulated by 0.25 µM IAA, and Os9BGlu33 was up-regulated by PEG, IAA, and ABA. Taken together, we demonstrate that Os4BGlu10, Os6BGlu24, and Os9BGlu33 play specific roles in seed germination, root elongation, and drought tolerance with various relation with IAA and ABA signaling.

Effects of Copper on Root Morphology, Cations Accumulation, and Oxidative Stress of Grapevine Seedlings

In the present study, a hydroponic experiment was conducted to investigate the oxidative stress and the copper (Cu) accumulation in grapevines exposed to three Cu levels (0, 5, and 15 µM) for 1, 2, and 3 days. The results showed that the root elongation was stunted at the highest-exposure concentration. The Cu accumulation in the grapevines increased with increasing Cu treatments, while the other nutrient elements (Ca, Mg and K) absorbed by the grapevines decreased. Most of the Cu taken up by the grapevines was accumulated in the roots. Compared to the data for 1 day after treatment, the Cu-addition significantly decreased the Mg and K concentration in the roots and leaves, yet increased the superoxide dismutase activity in the leaves after 3 days of treatment. For the reactive oxygen species, the malondialdehyde increased with increasing Cu levels in the roots and leaves; however, both the Cu-addition and exposure duration reduced the H₂O₂ level in the root. Additionally, the Cu-induced accumulation of ^·O₂^- and H₂O₂ in the grapevine leaves can be observed by the histochemical staining of nitroblue tetrazolium and diaminobenzidine, respectively. In conclusion, the present results indicate that excess Cu results in a change of the root morphology and leads to oxidative stress for the grapevine leaves and roots.

General control non-repressible 20 (GCN20) functions in root growth by modulating DNA damage repair in Arabidopsis

BACKGROUND

Most ABC transporters are engaged in transport of various compounds, but its subfamily F lacks transmembrane domain essential for chemical transportation. Thus the function of subfamily F remains further elusive.

RESULTS

Here, we identified General Control Non-Repressible 20 (GCN20), a member of subfamily F, as new factor for DNA damage repair in root growth. While gcn20-1 mutant had a short primary root with reduced meristem size and cell number, similar primary root lengths were assayed in both wild-type and GCN20::GCN20 gcn20-1 plants, indicating the involvement of GCN20 in root elongation. Further experiments with EdU incorporation and comet assay demonstrated that gcn20-1 displays increased cell cycle arrest at G2/M checkpoint and accumulates more damaged DNA. This is possible due to impaired ability of DNA repair in gcn20-1 since gcn20-1 seedlings are hypersensitive to DNA damage inducers MMC and MMS compared with the wild type plants. This note was further supported by the observation that gcn20-1 is more sensitive than the wild type when subjected to UV treatment in term of changes of both fresh weight and survival rate.

CONCLUSIONS

Our study indicates that GCN20 functions in primary root growth by modulating DNA damage repair in Arabidopsis. Our study will be useful to understand the functions of non-transporter ABC proteins in plant growth.

Boron reduces aluminum-induced growth inhibition, oxidative damage and alterations in the cell wall components in the roots of trifoliate orange

Aluminum (Al) toxicity is a major restriction for crops production on acidic soils. The primary symptom of aluminum toxicity is visible in the roots of plants. Recently, several studies reported the alleviation of Al toxicity by the application of Boron (B), however, the information how B alleviates Al toxicity is not well understood. Thus, we investigated the ameliorative response of B on Al-induced growth inhibition, oxidative damages, and variations in the cell wall components in trifoliate orange roots. The results indicated that plants under Al stress experienced a substantial decrement in root length and overall plant growth. The supply of B improved the root elongation by eliminating oxidative stress, membrane peroxidation, membrane leakage, and cell death produced under Al toxicity. Moreover, accumulation of Al on the cell wall and alteration in the cell wall components might be one of the causes resulting in the quick inhibition of root elongation under B-starvation circumstances by providing susceptible negative charges on pectin matrix for binding of Al. The results provide a useful understanding of the insight into mechanisms of B-induced mitigation of Al toxicity especially in the trifoliate orange that might be helpful in the production of crops on acidic soils.

Testing the toxicity of metals, phenol, effluents, and receiving waters by root elongation in Lactuca sativa L

Phytotoxicity tests using higher plants are among the most simple, sensitive, and cost-effective of the methods available for ecotoxicity testing. In the present study, a hydroponic-based phytotoxicity test using seeds of Lactuca sativa was used to evaluate the water quality of receiving waters and effluents near two industrial sites (Soyo and Daejon) in Korea with respect to the toxicity of 10 metals (As, Cd, Cr, Cu, Fe, Pb, Mn, Hg, Ni, Zn) and phenol, and of the receiving waters and effluents themselves. First, the L. sativa hydroponic bioassay was used to determine whether the receiving water or effluents were toxic; then, the responsible toxicant was identified. The results obtained with the L. sativa bioassay ranked the EC50 toxicities of the investigated metal ions and phenol as: Cd > Ni > Cu > Zn > Hg > phenol > As > Mn > Cr > Pb > Fe. We found that Zn was the toxicant principally responsible for toxicity in Daejeon effluents. The Daejeon field effluent had a higher Zn concentration than permitted by the effluent discharge criteria of the Ministry of Environment of Korea. Our conclusion on the importance of Zn toxicity was supported by the results of the L. sativa hydroponic assay, which showed that the concentration of Zn required to inhibit root elongation in L. sativa by 50% (EC50) was higher in the Daejeon field effluent than that of pure Zn. More importantly, we proved that the L. sativa hydroponic test method can be applied not only as an alternative tool for determining whether a given waste is acceptable for discharge into public water bodies, but also as an alternative method for measuring the safety of aquatic environments using EC20 values, with respect to the water pollutants investigated (i.e., Cd, Cr, Cu, Pb, Mn, Hg, Ni, Zn, and phenol).

Defining appropriate methods for studying toxicities of trace metals in nutrient solutions

The use of inappropriate experimental conditions for examining trace metal phytotoxicity results in data of questionable value. The present study aimed to identify suitable parameters for study of phytotoxic metals in nutrient solutions. First, the literature was reviewed to determine the concentration of six metals (Cd, Cu, Hg, Ni, Pb, and Zn) from solution of contaminated soils. Next, the effects of pH, P, Cl, NO3, and four Fe-chelators were investigated by using thermodynamic modelling and by examining changes in root elongation rate of soybean (Glycine max cv. Bunya). The literature review identified that the solution concentrations of metals in soils were low, ranging from (µM) 0.069-11Cd, 0.19-15.8 Cu, 0.000027-0.000079 Hg, 1.0-8.7 Ni, 0.004-0.55 Pb, and 0.4-36.3 Zn. For studies in nutrient solution, pH should generally be low given its effects on solubility and speciation, as should the P concentration due to the formation of insoluble phosphate salts. The concentrations of Cl, NO3, and various chelators also influence metal toxicity through alteration of metal speciation. The nutrient solutions used to study metal toxicity should consider environmentally-relevant conditions especially for metal concentrations, with concentrations of other components added at levels that do not substantially alter metal toxicity.

Plant assays and avoidance tests with collembola and earthworms demonstrate rehabilitation success in bauxite residue

Bauxite residues are a by-product of alumina manufacture from bauxite ore and are commonly disposed of in purpose-built bauxite residue disposal areas (BRDAs). Revegetation is viewed as the most effective way to landscape and rehabilitate closed BRDAS and physicochemical assessment remains the primary indicator of rehabilitation success. Little is known about the ability of keystone mesofaunal species to colonise and establish in these environments yet the long-term success of rehabilitation is dependent on residues becoming suitable habitats for such groups. Using six different residue treatments (untreated, leached, organic application, organic amended, and two revegetated field treatments) together with OECD test soil, this study assessed the characteristics of residues with plant germination and seedling development using the Rhizotest™ approach with Lepidium sativum, Sinapis alba, and Sorghum bicolor. Avoidance tests with soil mesofauna Eisenia foetida together with growth and reproduction tests for Folsomia candida were conducted to determine possibility of inhibition in residue soils. Unamended residue is inhibitory to plant growth and mesofaunal establishment. Amendment improves the physicochemical properties of the residue, and data revealed that both gypsum and organic addition is required to promote conditions favourable to plant growth and mesofauna establishment. Earthworms avoid residues with elevated Na content but will choose substrate with high soluble Ca content. F. candida preferentially moves to residues from field treatments, and both mortality and reproduction rates are comparable or superior to OECD soil. On the basis of these assays, we propose that bauxite residue can be transformed to a soil-like medium capable of supporting keystone species.

A root penetration model of Arabidopsis thaliana in phytagel medium with different strength

Phytagel media were evaluated as systems to mechanically impede roots of A. thaliana. Studying mechanical properties of Phytagel and exploring the root response to mechanical stimulation can facilitate plant culture and plant development. Breaking strengths of 0.5-2.0% phytagel media were tested by uniaxial compression test. Different phytagel concentrations were set to alter the strength of layers in growth medium. Negative correlations were observed between root length, straightness and medium strength. When roots elongated through soft upper-layer (0.6%), penetration ratio decreased with the increase of lower-layer strength (0.6-1.2%) and all roots couldn't penetrate into lower-layer with concentration ≥1.2%. Roots could grow into soft lower-layer (0.6%) from hard upper-layer (0.6-1.2%), with decreased penetration ratio. When roots grew in soft lower-layer, the growth rate linked with upper-layer strength increased to peak. Roots penetration capability into 1.2% lower-layer was improved by growing plants through moderate layer inserted between soft and hard layer, and roots in 0.8% moderate medium have a significant higher penetration ratio than that in 1.0%. It was concluded that the Phytagel systems studied were suitable for studying the effect of mechanical impedance on the elongation of A. thaliana roots. The medium strength affected root penetration significantly and acclimation can improve root penetration capability.

Characterization of multifarious plant growth promoting traits of rhizobacterial strain AR6 under Chromium (VI) stress

Plant growth promoting rhizobacteria (PGPR) can increase the host plant tolerance to cope up with heavy metal induced stress, which can be improve plant growth. Thus, the present study was designed to isolate Cr(VI) tolerant PGPR strain and evaluate its plant growth promoting (PGP) properties under Cr(VI) stress. Rhizobacterial strain AR6 was isolated from the rhizosphere of Phaseolus vulgaris L. and showed 99% homology with Cellulosimicrobium funkei (KM032184) in BLASTn analysis. Strain AR6 was specifically selected due to its high Cr(VI) tolerance (1200μg/ml) and substantial production of PGP substances. Strain AR6 produced 36.75μg/ml of indole acetic acid (IAA), 60.40μg/ml of ammonia and 14.23μg/ml of exopolysaccharide (EPS). Moreover, strain AR6 showed positive results for catalase, protease, amylase, lipase production and phosphate solubilization. A trend of Cr(VI) concentration dependent progressive decline for PGP traits of strain AR6 was observed excluding EPS which was regularly increased on increasing concentrations of Cr(VI). Among the four tested Cr(VI) concentrations, 250μg/ml showed the maximum toxicity to PGP activities of strain AR6. Inoculation of rhizobacterial strain AR6 significantly increased the root length of test crops in the presence of Cr(VI) and produced a considerable number of colonizes on the root of versatile dicot and monocot plants. Moreover, strain AR6 exhibited strong antagonistic activity against phytopathogen Aspergillus niger. Thus, the present study suggests that metal tolerant and PGP activities of the rhizobacterial strain AR6 could be exploited for environmental and agricultural issues.

Toxicity of Inorganic Mercury to Native Australian Grass Grown in Three Different Soils

In this study, three native Australian grasses namely Iseilema membranaceum (Barcoo), Dichanthium sericeum (Queensland Blue) and Sporobolus africanus (Tussock) were grown in three different soils spiked with different concentrations of inorganic mercury and the root elongation was monitored up to 28 days following the germination. Results showed that mercury at certain concentrations significantly inhibited the root growth of all three tested native grasses grown in three soils, however, the toxicity was less in the soil with high organic carbon content and acidic pH. The calculated EC₅₀ values ranged from 10 to 224 mg/kg total Hg in soil. However, the EC₁₀ values indicated that existing guideline values for mercury may be of protective to the native Australian vegetation. Considering their tolerance to soil mercury, these grass species have the potential for their use in rehabilitation of mercury contaminated sites.

The toxicity of HCrO4- and CrO42- to barley root elongation in solution culture: pH effect and modelling

The influence of pH on the toxicity of Cr(VI) to barley root elongation was studied in solution culture to better understand the toxicity of different species of Cr(VI). Results showed that the values of EC50{CrO₄^2-} (the free CrO₄^2- that results in50% of barley root elongation with respect to the control) increased when the pH increased from 4.5 to 6.5; however, it was not significantly different in the high-pH range from 7.0 to 8.5. The nonlinear relationship between EC50{CrO₄^2-} and OH^- activity indicated that OH^- competition with Cr(VI) on cell membrane ligands was not strong. There was a good linear relationship (R² = 0.99) between the ratio of HCrO₄^- activity to CrO₄^2- activity and Cr(VI) toxicity to barley root elongation when the toxicity of HCrO₄^- were considered, indicating that the observed toxicity of Cr(VI) in the high pH range may be caused by HCrO₄^- and CrO₄^2- in solution. It was found that HCrO₄^- had a greater binding affinity than CrO₄^2- on the biotic ligand sites. The logistic dose-response curves showed that consideration of Cr(VI) dose as HCrO₄^- and CrO₄^2- significantly improved the data fit compared to consideration of the activity of HCrO₄^- or CrO₄^2- only. The present study suggested that HCrO₄^- was highly toxic to the root of barely, and both HCrO₄^- and CrO₄^2- species needed to be considered when predicting the toxicity of Cr(VI) under different pH conditions.

Nitrogen remobilisation facilitates adventitious root formation on reversible dark-induced carbohydrate depletion in Petunia hybrida

BACKGROUND

Adventitious root (AR) formation in axillary shoot tip cuttings is a crucial physiological process for ornamental propagation that is utilised in global production chains for young plants. In this process, the nitrogen and carbohydrate metabolisms of a cutting are regulated by its total nitrogen content (N_t), dark exposure during transport and irradiance levels at distinct production sites and phases through a specific plasticity to readjust metabolite pools. Here, we examined how elevated N_t contents with a combined dark exposure of cuttings influence their internal N-pools including free amino acids and considered early anatomic events of AR formation as well as further root development in Petunia hybrida cuttings.

RESULTS

Enhanced N_t contents of unrooted cuttings resulted in elevated total free amino acid levels and in particular glutamate (glu) and glutamine (gln) in leaf and basal stem. N-allocation to mobile N-pools increased whereas the allocation to insoluble protein-N declined. A dark exposure of cuttings conserved initial N_t and nitrate-N, while it reduced insoluble protein-N and increased soluble protein, amino- and amide-N. The increase of amino acids mainly comprised asparagine (asn), aspartate (asp) and arginine (arg) in the leaves, with distinct tissue specific responses to an elevated N supply. Dark exposure induced an early transient rise of asp followed by a temporary increase of glu. A strong positive N effect of high N_t contents of cuttings on AR formation after 384 h was observed. Root meristematic cells developed at 72 h with a negligible difference for two N_t levels. After 168 h, an enhanced N_t accelerated AR formation and gave rise to first obvious fully developed roots while only meristems were formed with a low N_t. However, dark exposure for 168 h promoted AR formation particularly in cuttings with a low N_t to such an extent so that the benefit of the enhanced N_t was almost compensated. Combined dark exposure and low N_t of cuttings strongly reduced shoot growth during AR formation.

CONCLUSIONS

The results indicate that both enhanced N_t content and dark exposure of cuttings reinforced N signals and mobile N resources in the stem base facilitated by senescence-related proteolysis in leaves. Based on our results, a model of N mobilisation concomitant with carbohydrate depletion and its significance for AR formation is postulated.

Drought-Up-Regulated TaNAC69-1 is a Transcriptional Repressor of TaSHY2 and TaIAA7, and Enhances Root Length and Biomass in Wheat

A well-known physiological adaptation process of plants encountering drying soil is to achieve water balance by reducing shoot growth and maintaining or promoting root elongation, but little is known about the molecular basis of this process. This study investigated the role of a drought-up-regulated Triticum aestivum NAC69-1 (TaNAC69-1) in the modulation of root growth in wheat. TaNAC69-1 was predominantly expressed in wheat roots at the early vegetative stage. Overexpression of TaNAC69-1 in wheat roots using OsRSP3 (essentially root-specific) and OsPIP2;3 (root-predominant) promoters resulted in enhanced primary seminal root length and a marked increase in maturity root biomass. Competitive growth analysis under water-limited conditions showed that OsRSP3 promoter-driven TaNAC69-1 transgenic lines produced 32% and 35% more above-ground biomass and grains than wild-type plants, respectively. TaNAC69-1 overexpression in the roots down-regulated the expression of TaSHY2 and TaIAA7, which are from the auxin/IAA (Aux/IAA) transcriptional repressor gene family and are the homologs of negative root growth regulators SHY2/IAA3 and IAA7 in Arabidopsis. The expression of TaSHY2 and TaIAA7 in roots was down-regulated by drought stress and up-regulated by cytokinin treatment, which inhibited root growth. DNA binding and transient expression analyses revealed that TaNAC69-1 bound to the promoters of TaSHY2 and TaIAA7, acted as a transcriptional repressor and repressed the expression of reporter genes driven by the TaSHY2 or TaIAA7 promoter. These data suggest that TaNAC69-1 is a transcriptional repressor of TaSHY2 and TaIAA7 homologous to Arabidopsis negative root growth regulators and is likely to be involved in promoting root elongation in drying soil.

Mixture toxicity and interactions of copper, nickel, cadmium, and zinc to barley at low effect levels: Something from nothing?

Metal contamination is mostly a mixture of different metals, and these multicomponent mixtures can produce significant mixture effects. The present study was set up to investigate the toxicity of multiple metal mixtures of Cu, Ni, Cd, and Zn to plants at metal doses individually causing low-level phytotoxic effects. Barley (Hordeum vulgare L.) root elongation toxicity tests were performed in resin-buffered nutrient solutions to control metal speciation. Treatments included single-metal concentrations and binary, ternary, and quaternary mixtures. Mixtures of different metals at free ion concentrations, each causing <10% inhibition of root elongation, yielded significant mixture effects, with inhibition reaching up to 50%. The independent action (IA) model predicted mixture toxicity statistically better than the concentration addition (CA) model, but some synergisms relative to the IA model were observed. These synergisms relative to IA were most pronounced in quaternary mixtures and when the dose-response curves had steep slopes. Generally, antagonistic interactions relative to the CA model were observed. Increasing solution Zn concentrations shifted metal interactions (CA based) from additive or slightly synergistic at background Zn concentrations to antagonistic at higher Zn concentrations, suggesting a protective effect of Zn. Overall, the present study shows that the CA model can be used as a conservative model to predict metal mixture toxicity to barley. Environ Toxicol Chem 2016;35:2483-2492. © 2016 SETAC.

The sensitivity of an hydroponic lettuce root elongation bioassay to metals, phenol and wastewaters

The root elongation bioassay is one of the most straightforward test methods used for environmental monitoring in terms of simplicity, rapidity and economy since it merely requires filter paper, distilled water and Petri dishes. However, filter paper as a support material is known to be problematic as it can reduce the sensitivity of the test. The newly developed hydroponic method reported here differs from the conventional root elongation method (US EPA filter paper method) in that no support material is used and the exposure time is shorter (48 h in this test versus 120 h in the US EPA test). For metals, the hydroponic test method was 3.3 (for Hg) to 57 (for Cu) times more sensitive than the US EPA method with the rank orders of sensitivity, estimated from EC50 values, being Cu≥Cd>Ni≥Zn≥Hg for the former and Hg≥Cu≥Ni≥Cd≥Zn for the latter methods. For phenol, the results did not differ significantly; EC50 values were 124 mg L(-1) and 108-180 mg L(-1) for the hydroponic and filter paper methods, respectively. Lettuce was less sensitive than daphnids to wastewaters, but the root elongation response appears to be wastewater-specific and is especially sensitive for detecting the presence of fluorine. The new hydroponic test thus provides many practical advantages, especially in terms of cost and time-effectiveness requiring only a well plate, a small volume of distilled water and short exposure period; furthermore, no specialist expertise is required. The method is simpler than the conventional EPA technique in not using filter paper which can influence the sensitivity of the test. Additionally, plant seeds have a long shelf-life and require little or no maintenance.

Phytotoxicity of veterinary antibiotics to seed germination and root elongation of crops

Large quantities of veterinary antibiotics (VAs) are being used worldwide in agricultural fields through wastewater irrigation and manure application. They cause damages to the ecosystem when discharged into the environment, but there is a lack of information on their toxicity to plants and animals. This study evaluated the phytotoxic effects of five major VAs, namely tetracycline (TC), sulfamethazine (SMZ), norfloxacin (NOR), erythromycin (ERY) and chloramphenicol (CAP), on seed germination and root elongation in lettuce, tomato, carrot and cucumber, and investigated the relationship between their physicochemical properties and phytotoxicities. Results show that these compounds significantly inhibited root elongation (p<0.05), the most sensitive endpoint for the phytotoxicity test. TC was associated with the highest level of toxicity, followed by NOR, ERY, SMZ and CAP. Regarding crop species, lettuce was found to be sensitive to most of the VAs. The median effect concentration (EC50) of TC, SMZ, NOR, ERY and CAP to lettuce was 14.4, 157, 49.4, 68.8 and 204 mg/L, respectively. A quantitative structure-activity relationship (QSAR) model has been established based on the measured data. It is evident that hydrophobicity was the most important factor governing the phytotoxicity of these compounds to seeds, which could be explained by the polar narcosis mechanism. Lettuce is considered a good biomarker for VAs in the environment. According to the derived equation, phytotoxicities of selected VA compounds on different crops can be calculated, which could be applicable to other VAs. Environmental risks of VAs were summarized based on the phytotoxicity results and other persistent factors.

Assessment of phenolic herbicide toxicity and mode of action by different assays

A phytotoxicity assay based on seed germination/root elongation has been optimized and used to evaluate the toxic effects of some phenolic herbicides. The method has been improved by investigating the influence of experimental conditions. Lepidium sativum was chosen as the most suitable species, showing high germinability, good repeatability of root length measurements, and low sensitivity to seed pretreatment. DMSO was the most appropriate solvent carrier for less water-soluble compounds. Three dinitrophenols and three hydroxybenzonitriles were tested: dinoterb, DNOC, 2,4-dinitrophenol, chloroxynil, bromoxynil, and ioxynil. Toxicity was also determined using the Vibrio fischeri Microtox® test, and a highly significant correlation was found between EC50 values obtained by the two assays. Dinoterb was the most toxic compound. The toxicity of hydroxybenzonitriles followed the order: ioxynil >bromoxynil >chloroxynil; L. sativum exhibited a slightly higher sensitivity than V. fischeri to these compounds. A QSAR analysis highlighted the importance of hydrophobic, electronic, and hydrogen-bonding interactions, in accordance with a mechanism of toxic action based on protonophoric uncoupling of oxidative phosphorylation. The results suggest that the seed germination/root elongation assay with L. sativum is a valid tool for the assessment of xenobiotic toxicity and can be recommended as part of a test battery.

Identification of key genes involved in root development of tomato using expressed sequence tag analysis

Root system of plants are actually fascinating structures, not only critical for plant development, but also important for storage and conduction. Due to its agronomic importance, identification of genes involved in root development has been a subject of intense study. Tomato is the one of the most consumed vegetables in the world. Tomato has been used as model system for dicot plants because of its small genome, well-established transformation techniques and well-constructed physical map. The present study is targeted to identify of root specific genes expressed temporally and also gene(s) involved in lateral root and profuse root development. A total of 890 ESTs were identified from five EST libraries constructed using SSH approach which included temporal gene regulation (early and late) and genes involved in morphogenetic traits (lateral and profuse rooting). One hundred sixty-one unique ESTs identified from various libraries were categorized based on their putative functions and deposited in NCBI-dbEST database. In addition, 36 ESTs were selected for validation of their expression by RT-PCR. The present findings will help in shedding light to the unexplored developmental process of root growth in tomato and plant in general.

Development and validation of a terrestrial biotic ligand model for Ni toxicity to barley root elongation for non-calcareous soils

A Terrestrial Biotic Ligand Model (TBLM) for Ni toxicity to barley root elongation (RE) developed from experiments conducted in sand culture was used to predict toxicity in non-calcareous soils. Ca(2+) and Mg(2+) concentrations and pH in sand solution were varied individually and TBLM parameters were computed. EC50 increased as Mg(2+) increased, whereas the effect of Ca(2+) was insignificant. TBLM parameters developed from sand culture were validated by toxicity tests in eight Ni-amended, non-calcareous soils. Additional to Ni(2+) toxicity, toxicity from all solution ions was modelled independently as an osmotic effect and needed to be included for soil culture results. The EC50s and EC10s in soil culture were predicted within twofold of measured results. These are close to the results obtained using parameters estimated from the soil culture data itself.

How do roots elongate in a structured soil?

In this review, we examine how roots penetrate a structured soil. We first examine the relationship between soil water status and its mechanical strength, as well as the ability of the soil to supply water to the root. We identify these as critical soil factors, because it is primarily in drying soil that mechanical constraints limit root elongation. Water supply to the root is important because root water status affects growth pressures and root stiffness. To simplify the bewildering complexity of soil-root interactions, the discussion is focused around the special cases of root elongation in soil with pores much smaller than the root diameter and the penetration of roots at interfaces within the soil. While it is often assumed that the former case is well understood, many unanswered questions remain. While low soil-root friction is often viewed as a trait conferring better penetration of strong soils, it may also increase the axial pressure on the root tip and in so doing reduce the rate of cell division and/or expansion. The precise trade-off between various root traits involved in root elongation in homogeneous soil remains to be determined. There is consensus that the most important factors determining root penetration at an interface are the angle at which the root attempts to penetrate the soil, root stiffness, and the strength of the soil to be penetrated. The effect of growth angle on root penetration implicates gravitropic responses in improved root penetration ability. Although there is no work that has explored the effect of the strength of the gravitropic responses on penetration of hard layers, we attempt to outline possible interactions. Impacts of soil drying and strength on phytohormone concentrations in roots, and consequent root-to-shoot signalling, are also considered.

Copper regulates primary root elongation through PIN1-mediated auxin redistribution

The heavy metal copper (Cu) is an essential microelement required for normal plant growth and development, but it inhibits primary root growth when in excess. The mechanism underlying how excess Cu functions in this process remains to be further elucidated. Here, we report that a higher concentration of CuSO4 inhibited primary root elongation of Arabidopsis seedlings by affecting both the elongation and meristem zones. In the meristem zone, meristematic cell division potential was reduced by excess Cu. Further experiments showed that Cu can modulate auxin distribution, resulting in higher auxin activities in both the elongation and meristem zones of Cu-treated roots based on DR5::GUS expression patterns. This Cu-mediated auxin redistribution was shown to be responsible for Cu-mediated inhibition of primary root elongation. Additional genetic and physiological data demonstrated that it was PINFORMED1 (PIN1), but not PIN2 or AUXIN1 (AUX1), that regulated this process. However, Cu-induced hydrogen peroxide accumulation did not contribute to Cu-induced auxin redistribution for inhibition of root elongation. When the possible role of ethylene in this process was analyzed, Cu had a similar impact on the root elongation of both the wild type and the ein2-1 mutant, implying that Cu-mediated inhibition of primary root elongation was not due to the ethylene signaling pathway.

Seedling root anatomy and morphology: an examination of ecological differentiation with rainfall using phylogenetically independent contrasts

We examined patterns of seedling root architecture, morphology and anatomy in Australian perennial plants chosen as phylogenetically independent contrasts (PICs) for rainfall in the areas they inhabit. Our objective was to assess whether there are consistent evolutionary patterns in structure of seedling root systems in species from different rainfall environments when examined across multiple evolutionary lineages. Seedlings were grown to a standardised developmental stage under controlled conditions. We found that seedling root systems of species restricted to low rainfall environments are characterised by greater proportional allocation to main root axis and have proportionally smaller main root axis diameter and areas of stele and xylem. Species of low rainfall environments also had higher specific root length (SRL) of the main axis, but lower SRL when the entire root system was considered. Seedling root system elongation rates were higher in species of high rainfall relative to those of low rainfall environments, paralleling expected differences in relative growth rate. The higher root system elongation rates in species of high rainfall environments were associated with greater numbers of growing tips in the root system, but not with differences in elongation rates of individual tips, relative to species of low rainfall environments.