Silicon and Iron Differently Alleviate Copper Toxicity in Cucumber Leaves

Copper (Cu) toxicity in plants may lead to iron (Fe), zinc (Zn) and manganese (Mn) deficiencies. Here, we investigated the effect of Si and Fe supply on the concentrations of micronutrients and metal-chelating amino acids nicotianamine (NA) and histidine (His) in leaves of cucumber plants exposed to Cu in excess. Cucumber (Cucumis sativus L.) was treated with 10 mu M Cu, and additional 100 mu M Fe or/and 1.5 mM Si for five days. High Cu and decreased Zn, Fe and Mn concentrations were found in Cu treatment. Additional Fe supply had a more pronounced effect in decreasing Cu accumulation and improving the molar ratio between micronutrients as compared to the Si supply. However, the simultaneous supply of Fe and Si was the most effective treatment in alleviation of Cu-induced deficiency of Fe, Zn and Mn. Additional Fe supply increased the His but not NA concentration, while Si supply significantly increased both NA and His whereby the NA:Cu and His:Cu molar ratios exceeded the control values indicating that Si recruits Cu-chelation to achieve Cu tolerance. In conclusion, Si-mediated alleviation of Cu toxicity was directed toward Cu tolerance while Fe-alleviative effect was due to a dramatic decrease in Cu accumulation

Duration of priming with silicon modulates antioxidative response of wheat to salinity stress

Priming with silicon (Si) may increase plant resistance to biotic and abiotic stresses, in particular in conjunction with its subsequent application. Yet, the very effect of the duration of priming with Si is less understood. Here, we investigated the effect of the duration of priming with Si on components of the antioxidative response of wheat exposed to a gradient of salinity stress. After priming with 1.5 mM Si(OH)4 (0, 1, and 3 days), wheat seedlings were exposed to different NaCl levels (0, 30, and 60 mM) without (-Si) or with (+Si) supply of 1.5 mM Si(OH)4. The activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), and the concentration of malondialdehyde (MDA) were measured in shoots and roots after 1 and 5 days of NaCl treatments. Interaction of priming duration and addition of Si on antioxidative variables were analyzed using general regression model. Overall, priming had no influence on -Si plants under salt stress. On the day 1 of NaCl exposure, priming duration did not affect SOD and APX activities neither in roots nor in shoots. However, on the day 5 of NaCl exposure longer priming with Si significantly increased the activities of APX and SOD in both roots and shoots. The activity of CAT showed no response to priming with Si and subsequent Si supply in both plant organs irrespectively of the duration of NaCl exposure. Interestingly, in both organs the concentrations of MDA as a proxy for oxidative damage of plant membranes were very clearly and consistently lower after 3 days of priming with Si (compared to 1 day or no priming) during the whole period of NaCl exposure. This study demonstrated that longer priming with Si can enhance the ameliorative effect of Si supply on the antioxidative response of wheat plants to a gradient of salinity stress

Silicon mediates sodium transport and partitioning in maize under moderate salt stress

Silicon (Si) is known to alleviate salt stress in various crops; however, the influence of Si on sodium (Na) transport and partitioning at the tissue, cell and organelle levels is poorly understood. Maize (Zea mays L.) hybrid sensitive to salt stress was exposed to moderate salt stress (40 mM NaCl; simulating conditions in salinized agricultural soils) without or with supply of 1.5 mM Si(OH)(4). We investigated the expression of SOS genes encoding Na+ efflux transporter in various root tissues of maize, paralleled by measurements of tissue Na concentration. In addition, subcellular localization of Na (using Na fluorescent dye) within the leaf mesophyll cells was also performed. Silicon supplied plants accumulate less Na in both root apex and cortex, but allocate more Na+ to the leaves via the xylem. This was accompanied by increased expression of ZrnSOS1 and ZmSOS2 in the root apex and cortex facilitating Na+ exclusion, and in the root stele for enhanced Na+ loading into the xylem. Also, Si down-regulated the expression of ZmHKT1 in the root stele, which further decreased Na+ unloading from the xylem. Consequently, Si increased accumulation of Na in leaves, but also enhances sequestration of Na+ into the vacuoles thereby decreasing Na+ accumulation in the chloroplasts. In response to moderate salt stress in maize, Si shifts the typical glycophyte behavior of this species towards that of halophytes

Silicon Alleviates Iron Deficiency in Barley by Enhancing Expression of Strategy II Genes and Metal Redistribution

The beneficial effects of silicon (Si) have been shown on plants using reduction-based strategy for iron (Fe) acquisition. Here we investigated the influence of Si on Fe deficiency stress alleviation in barley (Hordeum vulgare), a crop plant which uses the chelation-based strategy for Fe acquisition. Analyses of chlorophyll content, ROS accumulation, antioxidative status, concentrations of Fe and other micronutrients, along with the expression of Strategy II genes were studied in response to Si supply. Si successfully ameliorated Fe deficiency in barley, diminishing chlorophyll and biomass loss, and improving the activity of antioxidative enzymes, resulting in lowered reactive oxidative species accumulation in the youngest leaves. Alleviation of Fe deficiency stress correlated well with the Si-induced increase of Fe content in the youngest leaves, while it was decreased in root. Moreover, Si nutrition lowered accumulation of other micronutrients in the youngest leaves of Fe deprived plants, by retaining them in the root. On the transcriptional level, Si led to an expedient increase in the expression of genes involved in Strategy II Fe acquisition in roots at the early stage of Fe deficiency stress, while decreasing their expression in a prolonged stress response. Expression of Strategy II genes was remarkably upregulated in the leaves of Si supplied plants. This study broadens the perspective of mechanisms of Si action, providing evidence for ameliorative effects of Si on Strategy II plants, including its influence on accumulation and distribution of microelements, as well as on the expression of the Strategy II genes

Silicon alleviates copper (Cu) toxicity in cucumber by increased Cu-binding capacity

Aims Although silicon (Si) is known to increase plant resistance to metal toxicity stress, the mechanisms responsible for alleviation of copper (Cu) toxicity are still insufficiently clear. We investigated the role of Si on Cu-binding processes involved in buffering excessive Cu in cucumber (Cucumis sativus L.) tissues. Methods Cucumber plants were subjected to moderate Cu toxicity stress (10 mu M Cu) without (-Si) or with (+Si) supply of 1.5 mM Si. We analyzed total and cell wall concentrations of Cu and Cu-binding compounds (organic acids and Cu-proteins) along with parameters of oxidative stress (e.g. lipid peroxidation and lignification). Results Supply of Si decreased total Cu concentration in both root and leaf tissues, but increased the root cell wall Cu fraction. Also, Si increased superoxide dismutase (SOD) activity in 10 mu M Cu-treated plants. Concomitantly, protein levels of Cu/Zn SOD isoforms (CSD1 and CSD2) in root tissues also increased in +Si plants. The leaf Cu-binding compounds, such as aconitate and plastocyanin (including the expression of CsPC gene) were higher in the +Si plants. Consequently, Si supply effectively lowered lipid peroxidation in both roots and leaves of Cu-stressed plants. Conclusions Supply of Si enhanced both the accumulation of Cu-binding molecules (Zn/Cu SOD in roots; aconitate and plastocyanin in leaves), and the Cu-binding capacity of the root cell wall

Silicon increases phosphorus (P) uptake by wheat under low P acid soil conditions

Although silicon (Si) is known to improve plant growth under low phosphorus (P) conditions, the in planta mechanisms responsible for this effect are still unknown. Here, we investigated the role of Si on P uptake along with the expression of Pi transporters in wheat (Triticum aestivum L.) grown in low P acid soil in comparison with P fertilization and liming. A combined approach was performed including analyses of rhizosphere soil, tissue P content, the expression of the root Pi transporter genes (TaPHT1.1 and TaPHT1.2), and the root exudation of citrate and malate. Supply of Si in a form of Na2SiO3 increased shoot P concentration to an adequate level in the range of P-fertilized plants. Silicon ameliorated low soil pH and high Al3+ comparable to the effect of liming. The in planta effect of Si on up-regulating the expression of TaPHT1.1 and TaPHT1.2 was several fold higher and consequently P uptake doubled compared to both P fertilization and liming. In addition, Si directly stimulated root Pi acquisition by prominently increasing both malate and citrate exudation rate. Application of Si increased root exudation of organic acids that mobilize Pi in the rhizosphere and up-regulated Pi transporters in wheat roots

Selenium activates components of iron acquisition machinery in oilseed rape roots

Aims Beneficial elements may stimulate plant growth by favoring the availability of essential nutrients for metabolic processes. This study addresses the still unexplored question whether the beneficial action of selenium (Se) on plants can be due to its interaction with iron (Fe). Methods Oilseed rape (Brassica napus) plants were grown hydroponically without (-Se) or with 10 mu M Na2SeO4(+Se) and exposed to either Fe adequate (+Fe) or starvation (-Fe) treatments. Results Selenium enhanced leaf chlorophyll concentration, photochemical parameters and CO(2)fixation. Although Se did not significantly increase total leaf Fe concentration, the concentration of cytoplasmic Fe(II)pool was enhanced under -Fe + Se conditions. However, the most conspicuous Se effects occurred in Fe-deficient roots. Selenium upregulated both the root expression levels of genes encodingFERRIC-REDUCTION OXIDASE (FRO1)and the activity of FRO and exacerbated the stimulatory effect of -Fe on the exudation rate of organic acids and phenolic compounds. Both -Se and + Se plants used Fe(III)EDTA in similar way, while only +Se plants were able to take advantage of the sparingly soluble Fe(OH)(3). In order to explore the mechanism of Se action, we recorded higher endogenous level of NO and upregulation ofETHYLENE RESPONSE FACTOR 2(ERF2) by Se irrespective of Fe conditions. Conclusions Our results show that Se augments root machinery for Fe acquisition mainly through activation of Fe signaling components