Silicon mediates ion uptake, transport and homeostasis in plants under mineral stress

Silicon is the only known mineral element that effectively alleviates multiple environmental stress in many plant species. Over the past decade rapid progress has been made in understanding the mechanisms through which Si mediates mineral excess and/or toxicity stress. It has been demonstrated that Si mediates uptake and transport of mineral elements at excess by regulating expression of various transporter genes (e.g. Kim et al., 2014; Akcay and Erkan, 2016; Che et al., 2016); however, the role of Si in nutrient uptake and transport under nutrient deficiency conditions is still insufficiently understood. In this presentation, I will talk about Si influence on (a) root P- starvation responses for rhizosphere mobilization and uptake of Pi in wheat (Triticum aestivum) and (b) acquisition and long-distance transport of Fe in cucumber (Cucumis sativus) under low Fe conditions; our recent unpublished work on (c) Si-regulated expression of the transporters involved in Na homeostasis in maize (Zea mays) subjected to NaCl stress will also be discussed

Silicon influence on plant ionome and mineral element transporters

The plant ionomics is the study of essential and nonessential mineral element composition of plants (the ionom) at cellular, tissue or organismal level. The plant ionomic profile is affected by various factors, including plant (e.g. species, genotypes, organ, developmental change) and environment (e.g. soil, fertilizers, stress conditions). Over the past decade rapid progress has been made in understanding the mechanisms through which silicon (Si) mediates mineral excess and/or toxicity stress. However, the effect of Si on the mineral element uptake and consequently the plant ionome is still unclear, in particular under conditions of limited nutrient availability. Firstly, I will present recent results of my research group demonstrating that Si application modulates the ionomic profile of various plant species (e.g. rice, barley, wheat, maize, cucumber, sunflower, soybean, grapevine and tomato) grown under both normal and stress conditions. In the second part of my talk I will review the current knowledge of Si influence on the expression of (a) root and shoot metal transporter genes under excess of cadmium (Cd), manganese (Mn) and copper (Cu) (Li et al., 2018; Kim et al. 2014; Che et al., 2016; Farooq et al., 2016); (b) transporter genes involved in the uptake, long-distance transport and homeostasis of iron (Fe) under low Fe conditions (Pavlovic et al., 2013, 2016); (c) transporter genes for inorganic phosphorus (Pi) root uptake under low P conditions (Kostic et al., manuscript submitted); and (d) transporter genes involved in shoot homeostasis of sodium (Na+) (see Bosnic et al., this proceedings) and B (Akcay & Erkan, 2016) under saline stress. In conclusion, the role of Si in modulation of plant ionome, including also nutrient and other mineral element uptake and utilization, appears to be more indirect by transcriptional regulation of genes responsible for both root acquisition and tissue homeostasis. Further understanding of how exactly Si regulates the expression of mineral element transporter genes will help to improve crop productivity, yield quality and food safety in stress conditions

Effect of N-forms on Silicon Mobilization in the Rhizosphere of White Lupin

Silicon (Si) is the major constituent of soil present in various fractions, i.e., mobile, adsorbed, occluded (in pedogenic oxides and hydroxides), amorphous (biogenic and lithogenic) and crystalline (primary and secondary silicates, and quartz). Different soil factors such as pH, temperatures, microbial activity, the presence of cations, Al/Fe oxides and hydroxides and organic compounds, influence Si transformation, thereby modifying plant availably of Si. Silicon mobility and transformation in the soil have mainly been studied in the context of pedogenesis or biogeochemical Si cycling. However, research on Si mobility, transformation, and plant availability in the rhizosphere is still lacking. Here, we investigated the root potential of white lupine (Lupinus albus L.), known as a phosphorus (P)-efficient model plant (e.g., root release of H+ and carboxylates), to mobilize Si from the soil. Plants were grown in the rhizoboxes filled with low P soil (control) and fertilized with different N-forms (NO3, NH4 and NO3NH4). The control, NO3- and NO3NH4-fertilized plants accumulated significantly lower amounts of Si than the NH4-fertilized ones. All applied N-forms influenced Si availability in the bulk soil, but Si fractions have further been modified in the rhizosphere, what was crucial for Si accumulation in plants. For instance, NO3 supply slightly decreased Si availability in the bulk soil, but lupine plants accumulated a similar amount of Si as the control plants. A strong gradient of decreasing Si concentrations between bulk and rhizosphere soils was observed in mobile, adsorbed, and amorphous biogenic Si pools in the control and in all N treatments, while occluded and lithogenic amorphous Si pools were recalcitrant. Interestingly, a gradient of increasing concentrations of the amorphous biogenic Si pool between bulk and rhizosphere soils was recorded in the NH4 treatment, concomitantly with the strongest rhizosphere acidification

Silicon modulates root phenomics and leaf ionomics in oak under Phytophthora infection and low phosphorus conditions

Pedunculate oak (Quercus robur L.) is the most abundant deciduous tree species in Europe with high economical and ecological importance. Different species of Phytophthora are considered as one of the most important factors responsible for deterioration of oak forest, causing serious root damage not only in the forest trees, but also in the nurseries. Oak seedlings were grown in plastic pots with extremely low phosphorus (P) soil (1.5 mg kg-1 total P; no available Olsen-P detected). Silicon (Si) and P were supplied as Na2SiO3 (300 mg Si kg-1 dry soil) and KH2PO4 (180 mg P kg-1 dry soil), respectively. Four treatments (-P/-Si, -P+/Si, +P/-Si, and +P/+Si) were used in the experiment. After two months of experiment, a half of the plants in each treatment were root-inoculated with Phytophthora plurivora. After further four weeks, the first symptoms of P. plurivora infection appeared in leaves (e.g., leaf necrosis and wilting). Plants were then carefully removed from the pots, divided into roots and shoots, and the roots were scanned and analyzed by the WinRHIZO® software. Foliar concentrations of Si, P, K, Ca, Mg, B, Cu, Fe, Mn, and Zn were determined by ICP-OES, while the concentrations of N and S were determined by CHNS Analyzer. The addition of Si obviously improved root health status (e.g., decreasing de number of lesions and necrosis intensity) in the infected plants grown under -P conditions, which was followed by an increased foliar P concentration. The Si supply significantly increased the root variables (e.g., total root volume, root length, and area of thin roots) in both -P and +P plants inoculated with P. plurivora. Therefore, P. plurivora infection and supply of P and Si modulated the nutrient uptake and thereby changed the leaf ionomics, especially for infected -P plants supplied with Si (e.g., significantly increased B, Cu, and Si foliar concentrations and decreased Fe, Mn, Ca, Mg, K, and S foliar concentrations). Furthermore, Si fertilization significantly declined loses in plant dry biomass caused by P. plurivora infection and/or P deficiency, showing biomass comparable to non-infected +P plants

Macronutrient contents in the leaves and fruits of red raspberry as affected by liming in an extremely acid soil

The study evaluates the effect of liming materials application in combination with NPK fertilizer and borax on macronutrient contents (nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg)), in an extremely acid soil and raspberry leaves and fruits during a two-year period. Liming increased soil pH, N mineral content, P, Ca and Mg soil content, while K content either increased (dolomite and borax application), or decreased (lime application). The N and P contents in raspberry leaves after liming increased significantly, but P content remained below the optimal values. Some treatments with lime caused a decrease in K content in leaves, while dolomite and borax application increased K content. Initially optimal Ca content in leaves increased significantly in the treatments with lime, but decreased after dolomite application. The Mg content in leaves increased after dolomite and borax application, but mainly remained below optimal values. Liming either did not alter or only slightly altered macronutrient contents in raspberry fruits

Phosphorus Deficiency Induced Silicon Mobilization in Grapevine Rhizosphere: A Field Study

Silicon (Si) uptake by crops is well studied and Si transporters have been characterized in various crop species, including grapevine. However, information on the rhizosphere mobilization of Si is still lacking and virtually no information is available on grapevine. Our previous study showed that grapevine is a phosphorus (P)-efficient species with a high root capacity to mobilize P from the rhizosphere by the released of organic anions (mainly citrate). The field experiment was established in 12-y-old vineyard with the cultivar ‘Chardonnay’, grafted on 5BB rootstock under extremely low P conditions (Olsen P < 3 mg kg-1). Four own-designed rhizotrons (80 cm depth) were installed in a vineyard enabling easy access to the new intact roots. The following treatments were performed: –P/–Si, +P/–Si, –P/+Si (soil application) and –P/+Si (foliar application). The samples of rhizosphere and bulk soils, root exudates from intact root tips and vine tissues (root and leaves) were collected at different growth stages according to Eichhorn-Lorentz (E-L) system: flowering (E-L stage 23), berries pea-size (E-L stage 31), and veraison (E-L stage 35). In addition to Si and P concentrations in the tissues, the expressions of VvALMT, VvMATE (encoding efflux transporters for malate and citrate, respectively), and VvNIP2.1 (encoding Si influx transporter) were also determined. Phosphate fertilization decreased, while low soil P and Si fertilization increased Si availability in the rhizosphere. At the flowering stage, –P plants accumulated more Si than the P-fertilized ones and was comparable to the Si-fertilized plants. Foliar application of Si was less effective in comparison with soil application unless at the veraison stage. The leaf Si concentrations showed a clear seasonal pattern being the highest at the veraison stage. Exudation rate of citrate also showed a clear seasonal pattern and was significantly higher in the –P/–Si than in +P/–Si plants, which was followed by an increased Si availability in the vine rhizosphere. Overall, low P conditions induced Si accumulation in the leaves due to increased exudation of organic anions that can also mobilize Si in the rhizosphere, thereby increasing Si uptake by grapevine

Silicon Differently Affects Apoplastic Binding of Excess Boron in Wheat and Sunflower Leaves

Monocots and dicots differ in their boron (B) requirement, but also in their capacity to accumulate silicon (Si). Although an ameliorative effect of Si on B toxicity has been reported in various crops, differences among monocots and dicots are not clear, in particular in light of their ability to retain B in the leaf apoplast. In hydroponic experiments under controlled conditions, we studied the role of Si in the compartmentation of B within the leaves of wheat (Triticum vulgare L.) as a model of a high-Si monocot and sunflower (Helianthus annuus L.) as a model of a low-Si dicot, with the focus on the leaf apoplast. The stable isotopes 10B and 11B were used to investigate the dynamics of cell wall B binding capacity. In both crops, the application of Si did not affect B concentration in the root, but significantly decreased the B concentration in the leaves. However, the application of Si differently influenced the binding capacity of the leaf apoplast for excess B in wheat and sunflower. In wheat, whose capacity to retain B in the leaf cell walls is lower than in sunflower, the continuous supply of Si is crucial for an enhancement of high B tolerance in the shoot. On the other hand, the supply of Si did not contribute significantly in the extension of the B binding sites in sunflower leaves

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

Seed priming with zinc improves field performance of maize hybrids grown on calcareous chernozem

Delivery of micronutrients to plants through seed priming improves seedling vigour and increases crops yields. Two-year filed trial was conducted in Pančevo, Serbia, with aim to study the effect of seed priming with zinc (Zn) on field performance of three maize hybrids on calcareous chernozem deficient in plant available Zn. Seed priming treatments were: control (without priming), water priming and priming with 4 mM zinc sulphate water solu-tion. Seed priming had significant effect on early plant growth, plant height, yield components, grain yield and grain Zn concentra-tion. Zn-priming promoted plant growth and increased final plant height. Across two growing seasons with contrasting precipitation and three tested maize hybrids, Zn-priming resulted in an average increase of grain yield by about 18% compared to control, and by about 8.4% compared to water priming. A significant relationship between plant growth parameters, grain yield components and grain yield was detected. Grain Zn concentration was increased by Zn-priming in two hybrids in the season with less precipitation and in one hybrid in the second season. The results imply that using the seeds with elevated Zn content can improve overall field performance of maize grown on calcareous chernozem

Methods for the assessment of background limits of Cd and Cr in the soil of Moravički district

This paper presents different approaches to background limits assessment of Cr and Cd content in the soil of the Moravički district. The investigated elements have right-skewed asymmetrical distribution, with high dispersion, especially Cr. By using graphical methods (cumulative curve - CDF and boxplot ) the obtained background limits of the natural contents were Cd 1.40 mg kg-1 and Cr 230 mg kg-1. Natural and log-transformed data were used for empirical methods, and significantly higher values were obtained than in the natural simulation. The background limits obtained by empirical methods were different. The maps show that the largest part of the territory has a relatively low concentration of investigated elements, whose values are the most similar to the background limits calculated by [Median+2MAD] methods and below. In parts of the territory with an increased content the most suitable computational methods are [Mean ± 2SD] and boxplot - calculation of the upper threshold. Background limits are given for certain homogenous geochemical wholes. [Projekat Ministarstva nauke Republike Srbije, br. TP 037006: Proučavanje uticaja kvaliteta zemljišta i voda za navodnjavanje na efikasniju proizvodnju poljoprivrednih kultura i očuvanje životne sredine