Effects of dichromate on growth and root system architecture of Arabidopsis thaliana seedlings

The hexavalent form of chromium [Cr(VI)] is toxic for most organisms; however, very little information is available regarding the effects of this metal on plant morphogenesis. In this work, we investigated the effects of Cr(VI) on the growth and development of Arabidopsis thaliana, a species widely used as a model for studying the diverse physiological and cellular processes in plants. Elongation of root hairs and biomass production were stimulated by relatively low concentrations (100 μM) of Cr(VI) as potassium dichromate. Concentrations of Cr(VI) greater than 200 μM were toxic to plants as revealed both by arrested growth of roots and shoots and the development of chlorosis in leaves. At 200 μM the primary root growth was totally inhibited but the plants continued their growth manifesting different alterations in root development. These alterations correlated with changes in mitotic activity and in cellular expansion. The analyses of A. thaliana transgenic plants that express the auxin-inducible marker DR5:uidA, and the response of the auxin-resistant mutants axr2 and aux1–7 to dichromate suggest that auxins do not participate as mediators in the cellular and physiological responses to this metal. The primary root growth inhibition by 200 μM dichromate was alleviated by more than 70% by increasing the sulfate, phosphate or nitrate concentration in the media, which suggests a relation of dichromate with these mineral nutrients.     

Hydrogen peroxide involvement in formation and development of adventitious roots in cucumber

Hydrogen peroxide (H₂O₂), an active oxygen species, is widely generated in many biological systems. The present study demonstrates that H₂O₂ was generated in seedling explants after the primary roots were removed, and it mediates the auxin response prior to adventitious root formation in cucumber (Cucumis sativus L. Ganfeng 8). When compared with the controls, treatment of cucumber seedling explants after primary roots removal with either 20–40 mM H₂O₂ or 10 μM IAA significantly increased the number of adventitious roots, and treatment with 10–50 mM H₂O₂ significantly increased the fresh weight of adventitious roots. The effects of H₂O₂ on promoting the formation and growth of adventitious roots were eliminated by 2 mM ascorbic acid, 100 U CAT or 1 μM DPI, and the effects of IAA were eliminated by 4 mM ascorbic acid, 100 U CAT or 5 μM DPI. Treatment with either 4 mM ascorbic acid or 1–5 μM DPI inhibited the formation and growth of adventitious roots, and these inhibitory effects were partly reversed by exogenous H₂O₂.Furthermore, a higher concentration of endogenous H₂O₂ was detected in seedling explants 3 h after the primary roots were removed. However, in 10 μM DPI-treated seedling explants, the concentration of endogenous H₂O₂ was markedly reduced by DPI. Results obtained suggest that H₂O₂ may function as a signaling molecule, involved in the formation and development of adventitious roots in cucumber.     

Speciation dependant antioxidative response in roots and leaves of sorghum (Sorghum bicolor (L.) Moench cv CO 27) under Cr(III) and Cr(VI) stress

Growth, lipid peroxidation, H₂O₂ produciton and the response of the antioxidant enzymes and metabolites of the ascorbate glutathione pathway to oxidative stress caused by two concentrations (50 and 100 µM) of Cr(III) and Cr(VI) was studied in 15 day old seedlings of sorghum (Sorghum bicolor (L.) Moench cv CO 27) after 10 days of treatment. Cr accumulation in sorghum plants was concentration and organ dependant. There was no significant growth retardation of plants under 50 µM Cr(III) stress. 100 µM Cr(VI) was most toxic of all the treatments in terms of root and leaf growth and oxidative stress. 50 µM Cr(VI) treated roots exhibited high significant increase in superoxide dismutase (SOD), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) (p < 0.01) and significant increases in catalse (CAT), ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR) (p < 0.05). A high increase in ascorbic acid (AA) level was seen in roots of 50 µM Cr(VI) treated plants in comparison with control. Levels of reduced glutathione (GSH) showed a varied and complex response in all the treatments in both plant parts. GSH/GSSG ratio was not affected by Cr(III) treatment in leaves, in contrast, roots exhibited significant reduction in the ratio. Results indicate that GSH depletion increased sensitivity to oxidative stress (Cr(VI) roots and leaves and Cr(III) 100 µM roots) and AA in tandem with APX compensated for GSH depletion by acting directly on H₂O₂ and the mechanism of defensive response in roots as well as leaves varied in its degree and effectiveness due to the concentration dependant differences observed in translocation of the element itself, reactive oxygen species (ROS) generation and enzyme inhibition based on the oxidation state supplied to the plants.     

Microalgae Improve the Photosynthetic Performance of Rice Seedlings (<i>Oryza sativa</i> L.) under Physiological Conditions and Cadmium Stress

The aim of this study was to assess the impact of the microalgae Chlorella vulgaris on the rice seedlings at physiological conditions and under cadmium (Cd) stress. We examined the effects of C. vulgaris in the nutrient solution on rice seedlings grown hydroponically in the presence and the absence of 150 μM CdCl₂, using the low (77 K) temperature and pulse amplitude modulated (PAM) chlorophyll fluorescence, P700 photooxidation measurements, photochemical activities of both photosystems, kinetic parameters of oxygen evolution, oxidative stress markers (MDA, H₂O₂ and proline), pigment content, growth parameters and Cd accumulation. Data revealed that the application C. vulgaris not only stimulates growth and improves the functions of photosynthetic apparatus under physiological conditions, but also reduces the toxic effect of Cd on rice seedlings. Furthermore, the presence of the green microalgae in the nutrient solution of the rice seedlings during Cd exposure, significantly improved the growth, photochemical activities of both photosystems, the kinetic parameters of the oxygen-evolving reactions, pigment content and decreased lipid peroxidation, H₂O₂ and proline content. Data showed that the alleviation of Cd-induced effects in rice seedlings is a result of the Cd sorption by microalgae, as well as the reduced Cd accumulation in the roots and its translocation from the roots to the shoots.     

Temporal changes in the growth, saponin content and antioxidant defense in the adventitious roots of Panax ginseng subjected to nitric oxide elicitation

Nitric oxide (NO) is a diffusible, gaseous signaling molecule. In plants, NO influences growth and development, and it can also affect plant responses to various stresses. Because NO induces root differentiation and interacts with reactive oxygen species, we examined the temporal effect of NO elicitation on root growth, saponin accumulation and antioxidant defense responses in the adventitious roots of mountain ginseng (Panax ginseng). The observations revealed that NO is involved in root growth and saponin production. Elicitation with sodium nitroprusside (SNP) activated O₂ ⁻-generating NADPH oxidase (NOX) activity, which most probably subsequently enhanced growth of adventitious roots of mountain ginseng. A severe inhibition of NOX activity and decline in dry weight of SNP elicited adventitious roots in the presence of NOX inhibitor (diphenyl iodonium, DPI), which further supports involvement of NOX in root growth. Enhanced activities of antioxidant enzymes by SNP appear to be responsible for low H₂O₂, less lipid peroxidation, and modulation of ascorbate and non-protein thiol statuses in the adventitious roots of mountain ginseng. Dry mass, saponin content and NOX activity was related with NO content present in adventitious roots of mountain ginseng.     

Induction mechanism of adventitious root from leaf explants of <Emphasis Type="Italic">Morinda citrifolia</Emphasis> as affected by auxin and light quality

An efficient protocol for adventitious root induction from leaf explants of Morinda citrifolia treated with different concentrations of indole-3-butyric acid (IBA) and α-naphthaleneacetic acid (NAA) was established in relation to physiological process changes during adventitious root induction under different light sources (fluorescent, red, blue, red + blue, and far-red). Among the different concentrations of IBA and NAA, 1.0 mg l⁻¹ IBA was proven as the best auxin source for adventitious root induction under fluorescent light. Higher concentrations of IBA and NAA trigger callus formation in both light and dark conditions. Maximum numbers of adventitious roots were induced under red light (26) followed by blue light (22) and the lowest under far-red light (6). In contrast, numerous callus formations were induced by red + blue followed by red and blue, while the highest root length (1.66 cm) with negligible callusing was observed under fluorescent light. Catalase and guaicacol peroxidase activities were highest under red light followed by fluorescent light and the lowest under red + blue light, but superoxide dismutase activity was not significantly influenced by different light sources. Ascorbate peroxidase played an important role in detoxification of the harmful effects of hydrogen peroxide (H₂O₂). Under fluorescent light, significantly lower accumulation of H₂O₂ was observed. Accumulation of H₂O₂ in the induced root under different light showed a positive correlation with peroxidation of lipids and was observed higher under far-red followed by red + blue and blue light.     

Ca<Superscript>2+</Superscript> and CaM are Involved in NO- and H<Subscript>2</Subscript>O<Subscript>2</Subscript>-Induced Adventitious Root Development in Marigold

Our previous results have demonstrated that both nitric oxide (NO) and hydrogen peroxide (H₂O₂) are involved in the promotion of adventitious root development in marigold (Tagetes erecta L.). However, not much is known about the intricate molecular network of adventitious root development triggered by NO and H₂O₂. In this study, the involvement of calcium (Ca²⁺) and calmodulin (CaM) in NO- and H₂O₂-induced adventitious rooting in marigold was investigated. Exogenous Ca²⁺ was capable of promoting adventitious rooting, with a maximal biological response at 50 μM CaCl₂. Ca²⁺ chelators and CaM antagonists prevented NO- and H₂O₂-induced adventitious rooting, indicating that both endogenous Ca²⁺ and CaM may play crucial roles in the adventitious rooting induced by NO and H₂O₂. NO and H₂O₂ treatments increased the endogenous content of Ca²⁺ and CaM, suggesting that NO and H₂O₂ enhanced adventitious rooting by stimulating the endogenous Ca²⁺ and CaM levels. Moreover, treatment with Ca²⁺ enhanced the endogenous levels of NO and H₂O₂. Additionally, Ca²⁺ might be involved as an upstream signaling molecule for CaM during NO- and H₂O₂-induced rooting. Altogether, the results suggest that both Ca²⁺ and CaM are two downstream signaling molecules in adventitious rooting induced by NO and H₂O₂.     

Mycorrhizal fungi enhance accumulation and tolerance of chromium in sunflower (Helianthus annuus)

Chromium (Cr) is a heavy metal risk to human health, and a contaminant found in agricultural soils and industrial sites. Phytoremediation, which relies on phytoextraction of Cr with biological organisms, is an important alternative to costly physical and chemical methods of treating contaminated sites. The ability of the arbuscular mycorrhizal fungus (AM),Glomus intraradices, to enhance Cr uptake and plant tolerance was tested on the growth and gas exchange of sunflower (Helianthus annuus L.). Mycorrhizal-colonized (AM) and non-inoculated (Non-AM) sunflower plants were subjected to two Cr species [trivalent cation (Cr³⁺) Cr(III) , and divalent dichromate anion (Cr₂O₇⁻) Cr(VI) ]. Both Cr species depressed plant growth, decreased net photosynthesis (A) and increased the vapor pressure difference; however, Cr(VI) was more toxic. Chromium accumulation was greatest in roots, intermediate in stems and leaves, and lowest in flowers. Greater Cr accumulation occurred with Cr(VI) than Cr(III). AM enhanced the ability of sunflower plants to tolerate and hyperaccumulate Cr. At higher Cr levels greater mycorrhizal dependency occurred, as indicated by proportionally greater growth, higherA and reduced visual symptoms of stress, compared to Non-AM plants. AM plants had greater Cr-accumulating ability than Non-AM plants at the highest concentrations of Cr(III) and Cr(VI), as indicated by the greater Cr phytoextraction coefficient. Mycorrhizal colonization (arbuscule, vesicle, and hyphae formation) was more adversely affected by Cr(VI) than Cr(III), however high levels of colonization still occurred at even the most toxic levels. Arbuscules, which play an important role in mineral ion exchange in root cortical cells, had the greatest sensitivity to Cr toxicity. Higher levels of both Cr species reduced leaf tissue phosphorus (P). While tissue P was higher in AM plants at the highest Cr(III) level, tissue P did not account for mycorrhizal benefits observed with Cr(VI) plants.     

Hydrogen peroxide acts as a signal molecule in the adventitious root formation of mung bean seedlings

Hydrogen peroxide (H₂O₂), an active oxygen species, is widely generated in many biological systems and mediates various physiological and biochemical processes in plants. In this study we demonstrated that the exogenous H₂O₂ was able to promote the formation and development of adventitious roots in mung bean seedlings. Treatments with 1–100 mM H₂O₂ for 8–18 h significantly induced the formation and development of adventitious roots. Catalase (CAT) and ascorbic acid, which are H₂O₂ scavengers or inhibitors, eliminated the adventitious root-promoting effects of exogenous H₂O₂. H₂O₂ may have a downstream signaling function in the auxin signaling pathway and be involved in auxin-induced adventitious root formation. 2,3,5-Triiodobenzoic acid (TIBA), an inhibitor of auxin polar transport, strongly inhibited adventitious rooting of mung bean seedlings; however, the inhibiting effects of TIBA on adventitious rooting can be partially reversed by the exogenous IBA or H₂O₂. Diphenylene iodonium (DPI) strongly inhibits the activity of NADPH oxidase, which is one of the main sources of H₂O₂ formation in plant cells. DPI treatment strongly inhibited the formation of adventitious roots in mung bean, but the inhibitory effects of DPI on rooting can be partially reversed by the exogenous H₂O₂ or IBA. This indicates that the formation of adventitious roots was blocked once the generation of H₂O₂ through NADPH oxidase was inhibited, and H₂O₂ mediated the IBA-induced adventitious root formation. Furthermore, a rapid increase in the endogenous level of H₂O₂ was detected during incubation with water 12–36 h after the primary root removal in mung bean seedlings. Three hours after the primary root removal, the generation of endogenous H₂O₂ was markedly induced in IBA-treated seedlings in comparison with water-treated seedlings. This implies that IBA induced overproduction of H₂O₂ in mung bean seedlings, and that IBA promoted adventitious root formation via a pathway involving H₂O₂. Results obtained suggest that H₂O₂ may function as a signaling molecule involved in the formation and development of adventitious roots in mung bean seedlings.     

Podophyllotoxin production via cell and adventitious root cultures of <Emphasis Type="Italic">Podophyllum peltatum</Emphasis>

Podophyllum peltatum is an important medicinal plant that produces podophyllotoxin (PTOX) with anti-cancer properties. We established the embryogenic cell and adventitious root culture systems in P. peltatum and analyzed PTOX production. For the growth of embryogenic cell clumps in shake flask culture, the most efficient concentration of 2,4-dichloroacetic acid (2,4-D) was 6.78 μM, and the growth of embryogenic cell clumps was 15.9-fold increased in Murashige and Skoog MS liquid medium with 6.78 μM 2,4-D after 3 wk of culture. To induce adventitious roots, half-strength MS medium showed the best results for adventitious root induction compared to full strength MS medium and MS medium lacking NH₄NO₃. Optimal indole-3-butyric acid concentration for adventitious root formation was 14.78 μM. In liquid medium, the frequency of adventitious root formation from root segments was 87.7% and the number of laterally formed adventitious roots was 22.3 per segment. PTOX production in embryogenic cells and adventitious roots was confirmed by liquid chromatography and electrospray ionization–tandem mass spectrometry analysis. High-performance liquid chromatography analysis revealed that adventitious roots contained higher PTOX than embryogenic cell clumps. Elicitor treatment (20 μM methyl jasmonate) strongly enhanced the production of PTOX in both embryogenic cell clumps and adventitious roots. This observation suggests that both embryogenic cell and adventitious root culture can be adopted to produce PTOX.     

Function of nitric oxide and superoxide anion in the adventitious root development and antioxidant defence in <Emphasis Type="Italic">Panax ginseng</Emphasis>

The involvement of NO in O₂ ^·− generation, rootlet development and antioxidant defence were investigated in the adventitious root cultures of mountain ginseng. Treatments of NO producers (SNP, sodium nitroprusside; SNAP, S-nitroso-N-acetylpenicillamine; and sodium nitrite with ascorbic acid), and NO scavenger (PTIO, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide) revealed that NO is involved in the induction of new rootlets. Severe decline in number of new rootlets compared to the control under PTIO treatment indicates that NO acts downstream of auxin action in the process. NO producers (SNP, SNAP and sodium nitrite with ascorbic acid) activated NADPH oxidase activity, resulting in greater O₂ ^·− generation and higher number of new rootlets in the adventitious root explants. Moreover, treatment of diphenyliodonium chloride, a NADPH oxidase inhibitor, individually or along with SNP, inhibited root growth, NADPH oxidase activity and O₂ ^·− anion generation. NO supply also enhanced the activities of antioxidant enzymes that are likely to be responsible for reducing H₂O₂ levels and lipid peroxidation as well as modulation of ascorbate and non-protein thiol concentrations in the adventitious roots. Our results suggest that NO-induced generation of O₂ ^·− by activating NADPH oxidase activity is related to adventitious root formation in mountain ginseng.     

Electrospray identification of new polyoxochromate species

Electrospray ionization mass spectrometry (ESI MS) has been conducted on the ammonium and alkali metal (A=Li⁺, Na⁺ and K⁺) dichromate systems. A large number of previously unknown polyoxochromate species have been characterized. Major series that have been identified include [A_x+1H_xCr^VI_xO_4x]⁺ (Li⁺, x=1-5; Na⁺, x=1-7; K⁺, x=1-4) and [A_2x−1Cr^VI_xO_4x−1]⁺ (Li⁺, x=2, 3; Na⁺, x=2-4; K⁺, x=2, 3) in the alkali metal dichromate systems, and [HCr^VI_xO_3x+1]⁻ (x=1-5) in the ammonium dichromate system. Several series also contain mixed oxidation state species, ranging from Cr(V) to Cr(II) in conjunction with Cr(VI), which is consistent with the ease of reduction of Cr(VI). Negative ion ESI MS spectra clearly demonstrate the existence of [HCrO₄]⁻ as the most abundant ion at −20 V, suggesting that its existence in solution is not just hypothetical, as was previously thought. The polymerization units for the series observed include {AHCrO₄}, {A₂CrO₄} and {CrO₃}, with the latter prominent in the alkali metal systems. This presumably arises from the fragmentation of dichromate, A₂Cr₂O₇→{A₂CrO₄}+{CrO₃}. Moreover, the ESI MS of the dichromate compounds have illustrated that the preservation of tetrahedral stereochemistry is of paramount importance for these systems, which leads to only limited polymerization compared to the related molybdate and tungstate systems.     

The interaction between H<Subscript>2</Subscript>O<Subscript>2</Subscript> and NO,Ca<Superscript>2+</Superscript>, cGMP,and MAPKs during adventitious rooting in mung bean seedlings

Hydrogen peroxide (H₂O₂), an active oxygen species, is widely generated in many biological systems and mediates various physiological and biochemical processes in plants. In the present study, we present a signaling network involving H₂O₂, nitric oxide (NO), calcium (Ca²⁺), cyclic guanosine monophosphate (cGMP), and the mitogen-activated protein kinase (MAPK) cascade during adventitious rooting in mung bean seedlings. Both exogenous H₂O₂ and the NO donor sodium nitroprussiate were capable of promoting the formation and development of adventitious roots. H₂O₂ and NO signaling pathways were elicited in parallel in auxin-induced adventitious rooting. Cytosolic Ca²⁺ was required for adventitious rooting, and Ca²⁺ served as a downstream component of H₂O₂, as well as cGMP or MAPK, signaling cascades. cGMP and MAPK cascades function downstream of H₂O₂ signaling and depend on auxin responses in adventitious root signaling processes.     

Long-term exposure to chromium(VI) oxide leads to defects in sulfate transport system in chinese hamster ovary cells

Chromium(VI) resistant Chinese hamster ovary (CHO) cell lines were established in this study by exposing parental CHO-K1 cells to sequential increases in CrO₃ concentration. The final concentration of CrO₃ used for selection was 7 μM for Cr7 and 16 μM for Cr16 cells. Cr16-1 was a subclone derived from Cr16 cells. Next, these resistant cells were cultured in media without CrO₃ for more than 6 months. The resistance of these cells to CrO₃ was determined by colony-forming ability following a 24-h treatment. The LD₅₀ of CrO₃ for chromium(VI) resistant cells was at least 25-fold higher than that of the parental cells. The cellular growth rate, chromosome number, and the hprt mutation frequency of these chromium(VI) resistant cells were quite similar to their parental cells. The glutathione level, glutathione S-transferase, catalase activity, and metallothionine mRNA level in Cr7 and Cr16-1 cells were not significantly different from their parental cells. Furthermore, Cr16-1 cells were as sensitive as CHO-K1 cells to free-radical generating agents, including hydrogen peroxide, nickel chloride, and methanesulfonate methyl ester, and emetine, i.e., a protein synthesis inhibitor. The uptake of chromium(VI) and the remaining amount of this metal in these resistant and the parental cell lines were assayed by atomic absorption spectrophotometry. Experimental results indicated that a vastly smaller amount of CrO₃ entered the resistant cell lines than their parental cells did. A comparison was made of the sulfate uptake abilities of CHO-K1 and chromium(VI) resistant cell lines. These results revealed that the uptake of sulfate anion was substantially reduced in Cr7 and Cr16-1 cells. Extracellular chloride reduced sulfate uptake in CHO-K1 but not in Cr16-1 cells. Therefore, the major causative for chromium(VI) resistance in these resistant cells could possibly be due to the defects in SO₄^2-/C1^? transport system for uptake chromium(VI).     

Hydrogen peroxide is involved in abscisic acid-induced adventitious rooting in cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.) under drought stress

Abscisic acid (ABA) and hydrogen peroxide (H₂O₂) are important regulatory factors involved in plant development under adversity stress. Here, the involvement of H₂O₂ in ABA-induced adventitious root formation in cucumber (Cucumis sativus L.) under drought stress was determined. The results indicated that exogenous ABA or H₂O₂ promoted adventitious rooting under drought stress, with a maximal biological response at 0.5 μM ABA or 800 μM H₂O₂. The promotive effects of ABA-induced adventitious rooting under drought stress were suppressed by CAT or DPI, suggesting that endogenous H₂O₂ might be involved in ABA-induced adventitious rooting. ABA increased relative water content (RWC), leaf chlorophyll content, chlorophyll fluorescence parameters (Fv/Fm, ΦPS II and qP), water soluble carbohydrate (WSC) and soluble protein content, and peroxidase (POD), polyphenol oxidase (PPO) and indoleacetate oxidase (IAAO) activities, while decreasing transpiration rate. However, the effects of ABA were inhibited by H₂O₂ scavenger CAT. Therefore, H₂O₂ may be involved in ABA-induced adventitious root development under drought stress by stimulating water and chlorophyll content, chlorophyll fluorescence, carbohydrate and nitrogen content, as well as some enzyme activities.     

Trivalent chromium pretreatment alleviates the toxicity of oxidative damage in wheat plants exposed to hexavalent chromium

Treating plants with abiotic or biotic factors can lead to the establishment of a unique primed state of defense. Primed plants display enhanced defense reactions upon further challenge with environmental stressors. Here, we report that trivalent chromium (Cr(III)) pretreatment can alleviate hexavalent chromium (Cr(VI)) toxicity in 2-week-old wheat plants. The data indicate that Cr(III)-pretreated wheat displayed longer survival times and less heavy metal toxicity symptoms under Cr(VI) exposure than the control. To investigate the possible mechanism from an antioxidant defense perspective, we determined the H₂O₂ and lipid peroxide content (TBARS), the activities of antioxidant enzymes (SOD, CAT, APX and GR) and the antioxidant metabolite content (ascorbate and glutathione content, AsA/DHA and GSH/GSSG ratios) in pretreated wheat roots. The results showed that 0.5 μM Cr(III) pretreatment can alleviate oxidative damage, such as H₂O₂ and TBARS accumulation, in root tissues compared to the control during the first 3 days of Cr(VI) exposure. Furthermore, we determined that this pretreatment can significantly increase the antioxidant enzyme activities and total ascorbate and glutathione contents compared to the control treatment. In addition, redox homeostasis declined slightly in pretreated wheat compared to the control in the presence of Cr(VI). We discuss a possible mechanism for Cr(III)-mediated protection of wheat.     

Bacterial Cr(VI) Reduction Concurrently Improves Sunflower (<Emphasis Type="Italic">Helianthus Annuus</Emphasis> L.) Growth

Four Cr(VI)-reducing bacterial strains (Ochrobactrum intermedium, CrT-2, CrT-3 and CrT-4) previously isolated from chromium-contaminated sites were inoculated on to seeds of sunflower (Helianthus annuus var SF-187), which were germinated and grown along with non-inoculated controls with chromate salts (300 μg CrCl₃ or K₂CrO₄ ml⁻¹). Severe reduction (20%) in seed germination was observed in Cr(VI) stress. Plant height decreased (36%) with Cr(VI) when compared with chromium-free control, while O. intermedium inoculation resulted a 20% increment in this parameter as compared to non-inoculated chromium-free control. CrT-3 inoculation resulted a 69% increment in auxin content as compared to non-inoculated control. O. intermedium caused 30% decrease in chromium uptake in sunflower plant roots under Cr(VI) stress as compared to chromium-free control plants.     

Nickel-induced Inhibition of Wheat Root Growth is Related to H2O2 Production, but not to Lipid Peroxidation

Effects of exogenous nickel (Ni: 10 and 200 μM) on growth, mitotic activity, Ni accumulation, H₂O₂ content and lipid peroxidation as well as the activities of various antioxidative enzymes, such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione peroxidase (GSH-Px) were investigated in wheat roots. A considerable Ni accumulation in the roots occurred at both the concentrations. Although Ni at 10 μM did not have any significant effect on root growth, it strongly inhibited the root growth at 200 μM. Mitotic activity in the root tips was not significantly affected by exposure of the seedlings to 10 μM Ni; however, it was almost completely inhibited at 200 μM treatment. Ni stress did not result in any significant changes in CAT and APX activities as well as lipid peroxidation. However, H₂O₂ concentration increased up to 82% over the control in the roots of seedlings exposed to 200 μM Ni. There was a significant decline in both SOD (50%) and GSH-Px (20–30%) activities in the roots when the seedlings were treated with 200 μM Ni. The results indicated that a strong inhibition of wheat root growth caused by Ni stress was not due to enhanced lipid peroxidation, but might be related to the accumulation of H₂O₂ in root tissue.     

Involvement of nitric oxide-induced NADPH oxidase in adventitious root growth and antioxidant defense in Panax ginseng

Nitric oxide (NO) affects the growth and development of plants and also affects plant responses to various stresses. Because NO induces root differentiation, we examined whether or not it is involved in increased ROS generation. Treatments with sodium nitroprusside (SNP), an NO donor, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO), a specific NO scavenger, and Nω-nitro-l-arginine methyl ester hydrochloride (l-NAME), an NO synthase (NOS) inhibitor, revealed that NO is involved in the adventitious root growth of mountain ginseng. Supply of an NO donor, SNP, activates NADPH oxidase activity, resulting in increased generation of O₂ ^·−, which subsequently induces growth of adventitious roots. Moreover, treatment with diphenyliodonium chloride (DPI), an NADPH oxidase inhibitor, individually or with SNP, inhibited root growth, NADPH oxidase activity, and O₂ ^·− anion generation. Supply of the NO donor, SNP, did not induce any notable isoforms of enzymes; it did, however, increase the activity of pre-existing bands of NADPH oxidase, superoxide dismutase, catalase, peroxidase, ascorbate peroxidase, and glutathione reductase. Enhanced activity of antioxidant enzymes induced by SNP supply seems to be responsible for a low level of H₂O₂ in the adventitious roots of mountain ginseng. It was therefore concluded that NO-induced generation of O₂ ^·− by NADPH oxidase seems to have a role in adventitious root growth of mountain ginseng. The possible mechanism of NO involvement in O₂ ^·− generation through NADPH oxidase and subsequent root growth is discussed.     

Efficient Evergreen Plant Regeneration of <i>Cinnamomum japonicum</i> Sieb. through <i>in vitro</i> Organogenesis

Cinnamomum japonicum Sieb. is an excellent roadside tree and medicinal tree species with considerable ornamental and economic value. In this study, we successfully developed a large-scale micropropagation protocol for C. japonicum for the first time. Sterilized shoots were excised and used as explants for shoot induction on several basal media, supplemented with different concentrations of plant growth regulators (PGRs), such as Thidiazuron (TDZ), N⁶ -Benzyladenine (6-benzylaminopurine) (BA), α-naphthaleneacetic acid (NAA) and Gibberellic acid (GA₃). After comparison, the most efficient medium for shoot regeneration was 1/2 Murashige and Skoog (MS) medium containing 0.5 mg L^–1 BA, 0.05 mg L^–1 NAA and 0.2 mg L^–1 GA₃, which resulted in an average number of induced shoots per explant and shoot length of 5.2 and 1.62 cm at 28 d, respectively. Then, elongated adventitious shoots were transferred to induce roots. 86.7% of shoots was able to root on 1/2 MS medium supplemented with 0.5 mg L^–1 NAA and 0.1 mg L^–1 BA. The earliest rooting time observed was after 21 d and the average root length was up to 3.3 cm after 28 d. Our study shows that C. japonicum can be successfully regenerated through de novo organogenesis, which lays a foundation for future transformation research on this tree.