Effects of silicon supply on apoplastic manganese concentrations in leaves and their relation to manganese tolerance in cowpea (Vigna unguiculata (L.) Walp.)

The effects of silicon (Si) supply on manganese (Mn) toxicity symptoms and Mn and Si concentrations in the leaf apoplast in a Mn-sensitive cowpea cultivar (Vigna unguiculata (L.) Walp. cv. TVu 91) were investigated in solution culture experiments. When 1.44 mM Si was supplied concurrently with 50 M Mn, the Mn toxicity symptoms were clearly avoided without decreasing the total Mn concentration. On the other hand, the symptoms were not completely alleviated when the plants were pretreated with 1.44 mM Si and then exposed to 50 M Mn without concurrent Si supply. Plants of both of these treatments exhibited lower Mn concentrations in the apoplastic washing fluids but higher amounts of adsorbed Mn on the cell walls than the plants treated with 50 M Mn without Si supply. However, the difference in Mn concentration between plants with continuous and interrupted Si supply was not significant. Moreover, the Mn concentration in the apoplastic washing fluids of the plants with continuous supply of 1.44 mM Si and 50 M Mn and not showing Mn toxicity symptoms was higher than that of the plants grown at 10 M Mn without Si supply which showed distinct Mn toxicity symptoms. These results show that Si supply alleviates Mn toxicity not only by decreasing the concentration of soluble apoplastic Mn through the enhanced adsorption of Mn on the cell walls. A role of the soluble Si in the apoplast in the detoxicification of apoplastic Mn is indicated.     

Abscisic acid introduced into the transpiration stream accumulates in the guard-cell apoplast and causes stomatal closure

Petioles of water‐sufficient intact Vicia faba L. plants were infused with 1 µm abscisic acid (ABA) to simulate the import of root‐source ABA. This protocol permitted quantitative ABA delivery, up to 300 pmol ABA over 60 min, to the leaf without ambiguities associated with perturbations in plant–water status. The ABA concentrations in whole‐leaf samples and in apoplastic sap increased with the amount infused; ABA degradation was not detected. The ABA concentration in apoplastic sap was consistent with uptake of imported ABA into the leaf symplast, but this interpretation is qualified. Our focus was quantitative cellular compartmentation of imported ABA in guard cells. Unlike when leaves are stressed, the guard‐cell symplast ABA content did not increase because of ABA infusion (P = 0·48; 3·0 ± 0·5 versus 4·0 ± 1·2 fg guard‐cell‐pair^?1). However, the guard‐cell apoplast ABA content increased linearly (R² = 0·98) from ?0·2 ± 0·5 to 3·1 ± 1·3 fg guard‐cell‐pair^?1 (≈ 3·1 µm ) and was inversely related to leaf conductance (R² = 0·82). Apparently, xylem ABA accumulates in the guard‐cell wall as a result of evaporation of the apoplast solution. This mechanism provides for integrating transpiration rate and ABA concentration in the xylem solution.     

Role of insulin in Cr(VI)-mediated genotoxicity in Neurospora crassa

Aims: Chromium (III) is an insulinomimetic agent whose biological and/or environmental availability is frequently in the form of Cr(VI), which is known to be toxic. Wall‐less mutant of Neurospora crassa (FGSC stock no. 4761) is known to possess insulin receptor in its cell membrane and hence is a good model for Cr toxicity studies. This study explores the toxicity of Cr(VI) and the possible consequences on simultaneous exposure to insulin in N. crassa. Methods and Results: Comet assay of N. crassa cells treated with 100 μmol l^?1 Cr(VI) showed up to 50% reduction in comet tail lengths when incubated simultaneously with 0·4 U insulin. Fluorescence measurement in Cr(VI)‐treated cells using DCFH‐DA showed six‐ to eightfold increase in free radical generation, which was reduced to fourfold by 0·4 U insulin. Annexin‐V/PI Flow cytometry analysis indicated necrotic cell death up to 28·7 ± 3·6% and 68·6 ± 2·5% on Cr(VI) exposure at concentrations 100 and 500 μmol l^?1 which was reduced by 68·3 ± 3·2% and 48·9 ± 3·6%, respectively, upon addition of insulin. Conclusion: Insulin‐mediated protection from DNA damage by Cr(VI) is because of scavenging of free radicals liberated during exposure to Cr(VI). Significance and Impact of the Study: Overall, Cr(VI) toxicity depends upon available insulin, indicating that Cr(VI) toxicity may be a serious issue in insulin‐deficient individuals with diabetes.     

Silicon Suppresses Fusarium Wilt Development in Banana Plants

This study aimed to determine the effect of silicon (Si) in reducing the symptoms of Fusarium wilt, caused by Fusarium oxysporum f. sp. cubense (Foc), on banana plants. Banana seedlings of Grand Nain (resistant) and Maçã (susceptible) were grown in plastic trays amended with 0 (?Si) or 0.39 g Si (+Si) per kg of soil and inoculated with Foc at 60 days after transplanting. The Si concentration in the roots and rhizome‐pseudostem significantly increased by 30.26 and 58.82%, respectively, for the +Si treatment compared with ?Si treatment. The Si concentration in the roots and rhizome‐pseudostem of Grand Nain plants was, respectively, 11.57 and 37.04% greater than that found in Maçã. The +Si plants showed a reduction of 12.37, 49.81, 51.87 and 20.39%, respectively, for the area under reflex leaf symptoms progress curve, the area under root symptoms progress curve, the area under disease progress curve and the area under asymptomatic fungal colonization of tissue progress curve compared with ‐Si plants. The area under darkening of rhizome‐pseudostem progress curve (AUDRPPC) of Maçã significantly increased by 15.98% for the ?Si treatment in comparison with the +Si treatment. For the +Si treatment, the AUDRPPC of the plants from the Maçã cultivar significantly decreased by 20.59% in comparison with the plants from the Grand Nain cultivar. The area under relative lesion length progress curve (AURLLPC) of the plants from the Maçã cultivar significantly decreased by 41.54% for the +Si treatment in comparison with the ?Si treatment. There was no significant difference between the ‐Si and +Si treatments in the AUDRPPC and AURLLPC of Grand Nain. For the +Si treatment, the AURLLPC of Grand Nain significantly decreased by 9.23% in comparison with Maçã. There was no significant difference between the Grand Nain and Maçã for the AUDRPPC and AURLLPC in the ?Si treatment. The findings of this study show that supplying Si to banana plants, especially to a susceptible cultivar to Foc, had a great potential in reducing the intensity of Fusarium wilt and may play a key role in disease management when banana plants are cultivated in Si‐deficient soils infested by this pathogen.     

Manganese application alleviates the water deficit-induced decline of N2 fixation

Water deficit is a very serious constraint on N₂ fixation rates and grain yield of soybean (Glycine max Merr.). Ureides are transported from the nodules and they accumulate in the leaves during soil drying. This accumulation appears responsible for a feedback mechanism on nitrogen fixation, and it is hypothesized to result from a decreased ureide degradation in the leaf. One enzyme involved in the ureide degradation, allantoate amidohydrolase, is manganese (Mn) dependent. As Mn deficiency can occur in soils where soybean is grown, this deficiency may aggravate soybean sensitivity to water deficit. In situ ureide breakdown was measured by incubating soybean leaves in a 5 mol m ^{? 3} allantoic acid solution for 9 h before sampling leaf discs in which remnant ureide was measured over time. In situ ureide breakdown was dramatically decreased in leaves from plants grown without Mn. At the plant level, allantoic acid application in the nutrient solution of hydroponically grown soybean resulted in a higher accumulation of ureide in leaves and lower acetylene reduction activity (ARA) by plants grown with 0 mol m ^{? 3} Mn than those grown with 6·6 mol m ^{? 3} Mn. Those plants grown with 6·6 mol m ^{? 3} Mn in comparison with those grown with 52·8 mol m ^{? 3} Mn had, in turn, higher accumulated ureide and lower ARA. To determine if Mn level also influenced N₂ fixation sensitivity to water deficit, a dry‐down experiment was carried out by slowly dehydrating plants that were grown in soil under four different Mn nutritions. Plants receiving no Mn had the lowest leaf Mn concentration, 11·9 mg kg ^{? 1}, and had N₂ fixation more sensitive to water deficit than plants treated with Mn in which leaf Mn concentration was in the range of 21–33 mg kg ^{? 1}. The highest Mn treatments increased leaf Mn concentration to 37·5 mg kg ^{? 1} and above but did not delay the decline of ARA with soil drying, although these plants showed a significant increase in ARA under well‐watered conditions.     

Silicon reduces herbivore performance via different mechanisms,depending on host–plant species

There is mounting evidence silicon (Si) can alter plant nutrient dynamics and is an important functional trait in plant defence and plant–insect ecology. Despite this, there remains a paucity in our understanding of how Si‐driven changes in nutritional quality can impact herbivore performance across different plant species. We investigated how Si alters plant nutritional quality and the concomitant effects on the performance of the Australian native generalist herbivore Helicoverpa punctigera feeding on three economically significant plant species of varying Si‐uptake ability: Brassica napus (non‐Si accumulator), Cucumis sativus (intermediate Si accumulator) and Sorghum bicolor (high Si accumulator). Si supplementation reduced the nutritional quality of B. napus but increased phosphorus concentrations in S. bicolor. Si reduced herbivore performance in all host–plant species, which correlated directly with Si concentrations in Si‐accumulating host plants C. sativus and S. bicolor. However, on B. napus, Si affected herbivore performance indirectly by reducing nutritional quality (foliar carbon:nitrogen ratio and phosphorus concentration). This suggests Si availability can affect herbivore performance directly via Si concentration on Si‐accumulating hosts, and indirectly via nutritional quality in a non‐Si accumulator. The resistance‐enhancing effects of Si on multiple species offer opportunity for agriculture to utilise this abundant element in sustainable management practices.     

Effect of silicon on manganese tolerance of bean plants (Phaseolus vulgaris L.)

Summary The effect of silicon on manganese tolerance of bean plants (Phaseolus vulgaris L. var. ‘Red Kidney’) grown in water culture was studied at different levels of manganese supply. Without silicon, growth depression and toxicity symptoms occurred already at 5 × 10⁻⁴ mM Mn in the nutrient solution. After addition of Aerosil (0.75 ppm Si), the plants tolerated 5 × 10⁻³ mM Mn and, at a higher silicon supply of 40 ppm, as much as 10⁻² mM Mn in the nutrient solution without any growth depression. This increase in manganese tolerance was not caused by a depressing effect of silicon on uptake or translocation of manganese but rather by an increase in the manganese tolerance of the leaf tissue. In absence of silicon, 100 ppm Mn was already toxic for the leaf tissue, whereas with a supply of 40 ppm Si, this ‘critical level’ in the leaves was increased to more than 1000 ppm Mn. At lower manganese levels in the leaf tissue, a molar ratio Si/Mn of 6 within the tissue was sufficient to prevent manganese toxicity. Above 1000 ppm Mn, however, even a much wider Si/Mn ratio (> 20) could not prevent growth depression by manganese toxicity. With⁵⁴Mn and autoradiographic studies, it could be demonstrated that, in absence of silicon, even at optimal manganese supply (10⁻⁴ mM), the distribution of manganese within the leaf blades was inhomogeneous and characterized by spot-like accumulations. In presence of silicon, however, the manganese distribution was homogeneous in the lower concentration range of manganese and still fairly homogeneous in the high concentration range. This effect of silicon on manganese distribution on the tissue level was also reflected on the cellular level. In the presence of silicon, a higher proportion of the leaf manganese could be found in the press sap,i.e., had been transported into the vacuoles, than in the absence of silicon. The increase in manganese tolerance of bean leaves by silicon therefore seems to be primarily caused by the prevention of local manganese accumulation within the leaf tissue which leads to local disorders of the metabolism and, correspondingly, growth depression.     

Silicon-mediated alleviation of Mn toxicity in Cucumis sativus in relation to activities of superoxide dismutase and ascorbate peroxidase

The effects of exogenous silicon (Si) on plant growth, activities of superoxide dismutase (SOD), guaiacol peroxidase (GPX), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), glutathione reductase (GR) and catalase, and concentrations of ascorbate and glutathione were investigated in cucumber (Cucumis sativus L.) plants treated with excess manganese (Mn) (600 microM). Compared with the treatment of normal Mn (10 microM), excess Mn significantly increased H2O2 concentration and lipid peroxidation indicated by accumulation of thiobarbituric acid reactive substances. The leaves showed apparent symptoms of Mn toxicity and the plant growth was significantly inhibited by excess Mn. The addition of Si significantly decreased lipid peroxidation caused by excess Mn, inhibited the appearance of Mn toxicity symptoms, and improved plant growth. This alleviation of Mn toxicity by Si was related to a significant increase in the activities of SOD, APX, DHAR and GR and the concentrations of ascorbate and glutathione.     

Rice Resistance to Brown Spot Mediated by Silicon and its Interaction with Manganese

Brown spot is one of the most devasting and prevalent disease of rice and its control is mainly dependent on fungicide application. Therefore, this study aimed to examine the effects of Si and Mn on the development of brown spot on rice plants grown in hydroponic culture. Rice plants (cv. ‘Metica‐1’) were grown in plastic pots containing 0 or 2 mm Si (?Si and +Si treatments, respectively) with three Mn rates (0.5, 2.5 and 10 μm ). Plants were inoculated with B. oryzae 39 days after emergence. The following components of resistance were evaluated: incubation period (IP), number of lesions (NL) per cm² of leaf area, real disease severity (RDS) and area under brown spot progress curve (AUBSPC). The content of Si and Mn in the plant tissues was also determined. Si content was significantly higher in rice tissue of plants of the +Si treatment than of the ?Si treatment regardless of the Mn rates used. The Mn rates did not affect the Si content of the rice plants. The Mn content of the rice tissues was significantly higher in the ?Si treatment than on +Si treatment, regardless of the Mn rate used. The Mn content was significantly lower at 0.5 μm Mn in comparison to the other rates for both ?Si and +Si treatments. The IP of brown spot on rice leaves significantly increased in the +Si treatment; but the Mn rates in the presence of Si had no effect on IP. In the ?Si treatment, the IP was significantly higher only at the rate of 2.5 μm . The NL, RDS and AUBSPC were significantly reduced in the +Si treatment regardless of the Mn rates. The Mn rates in the presence of Si had no effect on these components of resistance. Overall, Si dramatically impacted the development of brown spot regardless of the presence of Mn at either low or high rates. This may be useful in regions where the soil has either toxic or deficient levels of Mn and cultivars with brown spot resistance are not commercially available.     

Antitumour and antimicrobial activities of endophytic streptomycetes from pharmaceutical plants in rainforest

Aims: The aim of this study was to screen antitumour and antimicrobial activities of endophytic actinomycetes isolated from pharmaceutical plants in rainforest in Yunnan province, China. Methods and Results: Antitumour activity was studied by the 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide assay and antimicrobial activity was determined by agar well diffusion method. The high bioactive endophytic isolates were identified and further investigated for the presence of polyketide synthases (PKS‐I, PKS‐II) and nonribosomal peptide synthetases (NRPS) sequences by specific amplification. The molecular identification confirmed that the 41 isolates showed significant activities were members of the genus Streptomyces. Among them, 31·7% of endophytic streptomycete cultures were cytotoxic against A549 cells, 29·3% against HL‐60 cells, 85·4% against BEL‐7404 cells, 90·2% against P388D1 cells, 65·9% were active against Escherichia coli, 24·4% against Staphylococcus aureus, 31·7% against Staphylococcus epidermidis, 12·2% against Candida albicans and no strain displayed antagonistic activity against Klebsiella pneumoniae. High frequencies of positive PCR amplification were obtained for PKS‐I (34·1%), PKS‐II (63·4%) and NRPS (61·0%) biosynthetic systems. Conclusions: Many endophytic streptomycetes isolated from pharmaceutical plants in rainforest possess remarkable and diverse antitumour and antimicrobial bioactivities. Significance and Impact of the Study: These endophytic streptomycetes are precious resources obtained from rainforests, and they could be a promising source for bioactive agents.     

Silicon does not always mitigate zinc toxicity in maize

We have investigated the influence of silicon on higher zinc concentration reducing the growth of aboveground parts by ca 50 % in young maize plants (hybrid Novania) grown in hydroponics. Eight different treatments were used: control, Zn (800 μM ZnSO₄·7H₂O), Si1/Si2.5/Si5 (1/2.5/5 mM Na₂SiO₇) and Zn+Si (combination of zinc and all silicon concentrations). The concentration of Zn and Si and their distribution in plants was determined. The growth parameters (length of primary seminal root, leaf area of first and second leaves, fresh and dry weight of below- and above-ground plant parts) of plants grown in various Zn+Si treatments were significantly decreased in comparison to all other treatments. Increasing concentration of Si in combination with Zn treatment and selected hybrid (Novania) resulted in increased physiological stress in comparison to Zn treatment. However, roots and shoots of all Zn+Si treated plants contained significantly lower amount of Zn than Zn treatment. The Si concentration in roots was the same in Si and Zn+Si plants. In general, higher amount of Si was observed in shoots than in roots of Si1- and Si2.5-treated plants and opposite was observed in Si5-treated plants. In spite of significantly decreased root and shoot accumulation of Zn in the presence of Si, no positive effect of Si on Zn toxicity in young maize plants under experimental conditions used in this work and used maize hybrid was observed.     

Mycorrhiza and soil bacteria influence extractable iron and manganese in soil and uptake by soybean

Excess manganese (Mn) in soil is toxic to crops, but in some situations arbuscular mycorrhizal fungi (AMF) alleviate the toxic effects of Mn. Besides the increased phosphorus (P) uptake, mycorrhiza may affect the balance between Mn-reducing and Mn-oxidizing microorganisms in the mycorrhizosphere and affect the level of extractable Mn in soil. The aim of this work was to compare mycorrhizal and non-mycorrhizal plants that received extra P in relation to alleviation of Mn toxicity and the balance between Mn-oxidizing and Mn-reducing bacteria in the mycorrhizosphere. A clayey soil containing 508 mg kg⁻¹ of extractable Mn was fertilized with 30 mg kg⁻¹ (P1) or 45 mg kg⁻¹ (P2) of soluble P. Soybean (Glycine max L. Merrill, cv. IAC 8-2) plants at P1 level were non-inoculated (CP1) or inoculated with either Glomus etunicatum (GeP1) or G. macrocarpum (GmP1), while plants at P2 level were left non-inoculated (CP2). Plants were grown in a greenhouse and harvested after 80 days. In the mycorrhizosphere of the GmP1 and GeP1 plants a shift from Mn-oxidizing to Mn-reducing bacteria coincided with higher soil extractability of Mn and Fe. However, the occurrence of Mn-oxidizing/reducing bacteria in the (mycor)rhizosphere was unrelated to Mn toxicity in plants. Using 16S rDNA sequence homologies, the Mn-reducing isolates were consistent with the genus Streptomyces. The Mn-oxidizers were homologous with the genera Arthrobacter, Variovorax and Ralstonia. While CP1 plants showed Mn toxicity throughout the whole growth period, CP2 plants never did, in spite of having Fe and Mn shoot concentrations as high as in CP1 plants. Mycorrhizal plants showed Mn toxicity symptoms early in the growth period that were no longer visible in later growth stages. The shoot P concentration was almost twice as high in mycorrhizal plants compared with CP1 and CP2 plants. The shoot Mn and Fe concentrations and contents were lower in GmP1 and GeP1 plants compared with the CP2 treatment, even though levels of extractable metals increased in the soil when plants were mycorrhizal. This suggests that mycorrhiza protected its host plant from excessive uptake of Mn and Fe. In addition, higher tissue P concentrations may have facilitated internal detoxification of Mn in mycorrhizal plants. The exact mechanisms acting on alleviation of Mn toxicity in mycorrhizal plants should be further investigated.     

Early Inhibition of Photosynthesis during Development of Mn Toxicity in Tobacco

Early physiological effects of developing Mn toxicity in young leaves of burley tobacco (Nicotiana tabacum L. cv KY 14) were examined in glass-house/water cultured plants grown at high (summer) and low (winter) photon flux. Following transfer of plants to solutions containing 1 millimolar Mn²⁺, sequential samplings were made at various times for the following 9 days, during which Mn accumulation by leaves increased rapidly from ~70 on day 0 to ~1700 and ~5000 microgram per gram dry matter after 1 and 9 days, respectively. In plants grown at high photon flux, net photosynthesis declined by ~20 and ~60% after 1 and 9 days, respectively, and the onset of this decline preceded appearance (after 3 to 4 days) of visible foliar symptoms of Mn toxicity. Intercellular CO₂ concentrations and rates of transpiration were not significantly affected; moreover, the activity of the Hill and photosystem I and II partial reactions of chloroplasts remained constant despite ultimate development of severe necrosis. Though the activity of latent or activated polyphenol oxidase increased in parallel with Mn accumulation, neither leaf respiration nor the activity of catalase [EC 1.11.1.6] and peroxidase [EC 1.10.1.7] were greatly affected. These effects from Mn toxicity could not be explained by any changes in protein or chlorophyll abundance. Additionally, they were not a consequence of Mn induced Fe deficiency. Therefore, inhibition of net photosynthesis and enhancement of polyphenol oxidase activity are early indicators of excess Mn accumulation in tobacco leaves. These changes, as well as leaf visual symptoms of Mn toxicity, were less severe in plants cultured and treated at low photon flux even though the rates of leaf Mn accumulation at high and low photon flux were essentially equivalent.     

Manipulation of the hypocotyl sink activity by reciprocal grafting of two Raphanus sativus varieties: its effects on morphological and physiological traits of source leaves and whole‐plant growth

To reveal whether hypocotyl sink activities are regulated by the aboveground parts, and whether physiology and morphology of source leaves are affected by the hypocotyl sink activities, we conducted grafting experiments using two Raphanus sativus varieties with different hypocotyl sink activities. Comet (C) and Leafy (L) varieties with high and low hypocotyl sink activities were reciprocally grafted and resultant plants were called by their scion and stock such as CC, LC, CL and LL. Growth, leaf mass per area (LMA), total non‐structural carbohydrates (TNCs) and photosynthetic characteristics were compared among them. Comet hypocotyls in CC and LC grew well regardless of the scions, whereas Leafy hypocotyls in CL and LL did not. Relative growth rate was highest in LL and lowest in CC. Photosynthetic capacity was correlated with Rubisco (ribulose 1·5‐bisphosphate carboxylase/oxygenase) content but unaffected by TNC. High C/N ratio and accumulation of TNC led to high LMA and structural LMA. These results showed that the hypocotyl sink activity was autonomously regulated by hypocotyl and that the down‐regulation of photosynthesis was not induced by TNC. We conclude that the change in the sink activity alters whole‐plant growth through the changes in both biomass allocation and leaf morphological characteristics in R. sativus.     

Silicon amendment to rice plants contributes to reduced feeding in a phloem‐sucking insect through modulation of callose deposition

Silicon (Si) uptake by Poaceae plants has beneficial effects on herbivore defense. Increased plant physical barrier and altered herbivorous feeding behaviors are documented to reduce herbivorous arthropod feeding and contribute to enhanced plant defense. Here, we show that Si amendment to rice (Oryza sativa) plants contributes to reduced feeding in a phloem feeder, the brown planthopper (Nilaparvata lugens, BPH), through modulation of callose deposition. We associated the temporal dynamics of BPH feeding with callose deposition on sieve plates and further with callose synthase and hydrolase gene expression in plants amended with Si. Biological assays revealed that BPH feeding was lower in Si‐amended than in nonamended plants in the early stages post‐BPH infestation. Histological observation showed that BPH infestation triggered fast and strong callose deposition in Si‐amended plants compared with nonamended plants. Analysis using qRT‐PCR revealed that expression of the callose synthase gene OsGSL1 was up‐regulated more and that the callose hydrolase (β‐1,3‐glucanase) gene Gns5 was up‐regulated less in Si‐amended than in nonamended plants during the initial stages of BPH infestation. These dynamic expression levels of OsGSL1 and Gns5 in response to BPH infestation correspond to callose deposition patterns in Si‐amended versus nonamended plants. It is demonstrated here that BPH infestation triggers differential gene expression associated with callose synthesis and hydrolysis in Si‐amended and nonamended rice plants, which allows callose to be deposited more on sieve tubes and sieve tube occlusions to be maintained more thus contributing to reduced BPH feeding on Si‐amended plants.     

Manganese exposure alters the expression of N‐methyl‐d‐aspartate receptor subunit mRNAs and proteins in rat striatum

Manganese is one of the ubiquitous environmental pollutants that can induce an indirect excitotoxicity caused by altered glutamate (Glu) metabolism. The present study has been carried out to investigate the effect of Mn on the expression of N‐methyl‐d ‐aspartate receptor (NR) subunit mRNAs and proteins in rat striatum when rats were in manganism. The rats were divided randomly into four groups of six males and six females each: control group (group 1) and 8, 40, and 200 μmol/kg Mn‐treated groups (groups 2–4). The control group rats were subcutaneously (s.c.) injected with normal saline. Manganese‐treated rats were s.c. injected with respectively 8, 40, and 200 μmol/kg of MnCl₂ · 6H₂O in normal saline. The administration of MnCl₂ · 6H₂O for 4 weeks significantly increased Mn concentration in the striatum. With the increase in administered MnCl₂ dosage, Glu concentration and cell apoptosis rate increased significantly. The relative intensity of NR2A mRNA decreased significantly in 8 μmol/kg Mn‐treated rats. However, relative intensities of NR1 and NR2B mRNAs decreased significantly in 40 μmol/kg Mn‐treated rats. Similarly, the relative intensity of NR2A protein showed a significant decrease in 40 μmol/kg Mn‐treated rats whereas those of NR1 and NR2B decreased significantly in 200 μmol/kg Mn‐treated rats. Therefore, the expression of NR2A mRNA and protein were much more sensitive to Mn than that of NR1 and NR2B. In conclusion, the results suggested that Mn induced nerve cell damage by increasing extracellular Glu level and altered expression of NR subunit mRNAs and proteins in rat striatum. © 2010 Wiley Periodicals, Inc. J Biochem Mol Toxicol 24:1–9, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/jbt.20306     

Novel evidence for systemic induction of silicon defences in cucumber following attack by a global insect herbivore

1. Silicon (Si) is a beneficial nutrient that has been reported to ameliorate many abiotic and biotic stresses in plants, including insect herbivory. Insect herbivory has been shown to induce Si defences in plants, although the magnitude and nature of induction remain largely ambiguous. In particular, it is unclear whether herbivore induction of Si defences is confined to attacked tissues (local) or occurs elsewhere in the plant (systemic). 2. We grew cucumber, Cucumis sativus L. plants (var. Burpless F1 and Beit Alpha), an intermediate Si accumulator, hydroponically under Si-supplemented or Si-free conditions and measured the level of Si induction caused by a polyphagous chewing insect, the cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). We also examined the impacts of Si on insect performance by conducting in vitro feeding assays on excised leaves (ex situ) and intact leaves on plants (in situ). 3. Herbivory significantly increased Si accumulation both locally in attacked leaves (21% increase in Beit Alpha and 17% in Burpless F1) and systemically in non-attacked leaves (19% increase in Beit Alpha and 10% in Burpless F1). Si supplementation significantly increased % foliar Si and C:N ratio, while significantly decreasing larval relative consumption (RC) and relative growth rate (RGR) in the in situ assays. In ex situ assays, however, Si only reduced larval RGR when fed on Beit Alpha plants. 4. Our results confirm that Si-based defences can also operate in moderate Si-accumulating plants and, for the first time, that insect herbivory induces systemic Si accumulation equivalently between plant varieties.     

Cytoplasmic streaming in the root cortex and its role in the delivery of potassium to the shoot

The influence of cytochalasin B (CB), a potent inhibitor of cytoplasmic streaming, on ⁸⁶Rb-labelled K⁺ translocation by detopped Lycopersicon esculentum Mill., Cucumis sativus L. and Zea mays L. plants was examined by measuring the radioactivity in xylem exudate before and after the addition of CB to the medium bathing the roots. CB caused complete cessation of cytoplasmic streaming in root segments within 15 min but was without effect on either total ⁸⁶Rb uptake or exudation. Thus factors other than cytoplasmic streaming limit the movement of K⁺ across the symplast of the root of higher plants.     

Effects of manganese on the microstructures of Chenopodium ambrosioides L., A manganese tolerant plant

Chenopodium ambrosioides L. can tolerate high concentrations of manganese and has potential for its use in the revegetation of manganese mine tailings. Following a hydroponic investigation, transmission electron microscopy (TEM)-energy disperse spectroscopy (EDS) was used to study microstructure changes and the possible accumulation of Mn in leaf cells of C. ambrosioides in different Mn treatments (200, 1000, 10000 μmol·L^?1). At 200 μmol·L^?1, the ultrastructure of C. ambrosioides was clearly visible without any obvious damage. At 1000 μmol·L^?1, the root, stem and leaf cells remained intact, and the organelles were clearly visible without any obvious damage. However, when the Mn concentration exceeded 1000 μmol·L^?1 the number of mitochondria in root cells decreased and the chloroplasts in stem cells showed a decrease in grana lamellae and osmiophilic granules. Compared to controls, treatment with 1000 μmol·L^?1 or 10000 μmol·L^?1 Mn over 30 days, gave rise to black agglomerations in the cells. At 10000 μmol·L^?1, Mn was observed to form acicular structures in leaf cells and intercellular spaces, which may be a form of tolerance and accumulation of Mn in C. ambrosioides. This study has furthered the understanding of Mn tolerance mechanisms in plants, and is potential for the revegetation of Mn-polluted soils.