University of Tennessee, Knoxville From the SelectedWorks of Gregory Keith Bartley Jr. Spring April 24, 2011 Silicon - The Benefits of a Non-Essential Plant Nutrient Gregory Keith Bartley, Jr., University of Tennessee Available at: https://works.bepress.com/keith_bartley/5/
Slide 1 Silicon: Benefits of a Non-Essential Plant Nutrient G. Keith Bartley, Jr.
Slide 2 History of Use Silicon is the second most abundant element after oxygen in our soils, comprising 50-70% of soil mass (Ma & Yamaji, 2006). Now recognized as a limiting factor in the production of certain crops such as rice, oats, barley, wheat, and sugarcane, as well as many turfgrasses (Lewin & Reimann, 1969). -Silicon makes up approximately 28% of the earth s crust. -Because of the abundance of the element in nature and because visible symptoms of either Si deficiency or toxicity are not apparent, plant physiologists largely ignore it.
Slide 3 Essentiality Warm, moderately humid tropical climates with highly weathered soils generally have less Si in their soils due to desilication (Datnoff & Rodrigues, 2005). While most temperate soils will contain adequate Si, repeated cropping can significantly reduce plant available Si over time, especially in high Si uptake crops (Datnoff & Rodrigues, 2005). Friesen et al., 1994 -Soils like Ultisols and Oxisols in areas of Africa and South and Central America, experience higher degrees of Si leaching, and consequently do more poorly with growing crops like rice. -However, Si still is not recognized as an essential element for plant growth but the undeniable beneficial effects of this element on the growth, development, yield and disease resistance have been observed in a wide variety of plant species. -Silicon is an element that does not cause severe injury to plants when present in excess and can provide multiple benefits.
Slide 4 Uptake Plants vary in their ability to accumulate Si, ranging from levels as low as.1% to 10% (Ma & Yamaji, 2006). Plants uptake silicon in the form of silicic acid [Si(OH) 4 ], an uncharged monomeric molecule at a ph less than 9 (Ma & Takahashi, 2002). Typical concentrations of silicic acid in soil solution range from 0.1 to 0.6 mm (Richmond & Sussman, 2003). -Unlike monocots, most species of dicot plants are unable to accumulate high amounts of Si. These observed differences have been attributed to differences in the uptake ability of roots. -While abundant, silicon is never found in a free form and is always combined with other elements, usually forming oxides or silicates. -Upon absorption, silicon is irreversibly precipitated throughout the plant as amorphous silicon, also known as hydrated silica or opal, a substance commonly found as abrasives as toothpaste. -Amorphous silica, or opal, is also found in the fossilized remains of dead diatoms, a microscopic single celled algae, otherwise known as diatomite or diatomaceous earth, which is used in organic systems as an insect deterrent.
Slide 5 Translocation & Deposition Ma and Yamaji, 2006 -Picture showing the uptake, distribution and accumulation of silicon (Si) in rice. Si is taken up via transporters in the form of silicic acid (a) and then translocated to the shoot in the same form (b). In the shoot, Si is polymerized into silica and deposited in the bulliform cells (silica body) (c,d) and under the cuticle (e). -The transport of Si from cortical cells to the xylem (xylem loading) the Si concentration in the xylem sap is much higher in rice than it is in other low-si accumulating plants. In rice, Xylem loading is mediated by a transporter, while low-si accumulating plants use diffusion. -The distribution of Si in the shoot is controlled by transpiration. More Si accumulates in older tissues because this element is not mobile within the plants. -Silicon is deposited as a 2.5 mm layer in the space immediately beneath the thin (0.1 mm) cuticle layer, forming a cuticle Si double layer in leaf blades of rice. -There are two types of silicified cells in rice leaf blades: silica cells in vascular bundles, and silica bodies in the bulliform cells. The silicification of cells proceeds with rising concentration, from silica cells to silica bodies. -These different levels of depositions of Si protect plants from multiple abiotic and biotic stresses.
Slide 6 Translocation & Deposition Mitani et al., 2009 -In maize and barley, Si as silicic acid *Si(OH)4+ is taken up from the external solution by the influx transporter (Zm Lsi1/Hv Lsi1) localized on the distal side of cells in the epidermis and cortex layer and then transferred to the endodermis through the symplastic pathway (blue arrow). At the endodermis, the Si is released by an active Si efflux transporter (Zm Lsi2/Hv Lsi2) to the stele. In rice, Si is taken up from the external solution by Lsi1 at the distal side and released to the apoplast of aerenchyma by Lsi2 at the proximal side of the exodermal cells. Si is then transported to the stele by Lsi1 and Lsi2 at the endodermal cells. -The polarized influx and efflux carriars in rice are essential to overcoming the casparian strip barriers, releasing silicic acid into an apoplastic transport, producing a more effective and controlled flow of Silicic acid into plant.
Slide 7 Translocation & Deposition Kim et al., 2002 -Silicon deposition was not uniform in epidermal cell walls with increasing X-ray counts of silicon toward the outer regions of epidermal cell walls. Such differential deposition can be explained by silicon translocation in rice plants through cuticular transpiration. Although most water escapes through stomata, some water diffuses out through epidermal cells and cuticle. -Accordingly, silicon is translocated to the epidermis with water and constantly being deposited at the epidermis after transpiration. As rice plants become mature, silicon deposition is likely to become prevalent in the apoplast of rice plants.
Slide 8 Canopy Photosynthesis Ma and Yamaji, 2006 -Silicon has been shown to increase light interception by keeping leaves erect, which is especially important in dense growing species.
Slide 9 Light Infiltration Agarie et al., 1996 -Past hypothesis speculated that Silicon depositions on the surface of leaves could improve light transfer to the mesophyll through acting as windows to the photosynthetic tissue, however this was disproven by Agarie (et al.) in 1996. It was shown that these silicon deposits on the surface of the leaf do not influence the optical properties and light environment within the leaf. TOP Scanning Electron Micrograph of transverse fracture of Si-treated leaf BOTTOM LEFT: Deposition of Si in the bulliform cells of the leaf tissue BOTTOM-RIGHT: Non treated leaf
Slide 10 Biotic Stress Resistance Ma and Yamaji, 2006 Increase in Biotic Stress Resistance
Slide 11 Mechanisms Physical barrier to insect and pathogens Enhanced host resistance Ma and Yamaji, 2006 - Physical barriers occur through deposition of silicon below the cuticle, forming a double-layer of protection from insects and pathogens. - Enhanced host resistance has been observed in many monocots (rice and wheat) and cucumber, where the defense responses of the plants to pests where upregulated in the presence of Si. This has been observed only to occur with soluble Si in the plant, though the mechanism of these responses is still unknown. - This picture shows the effects of silicon on rice growth and yield, where low levels of silicon in the plant led to increased susceptibility to insect attack in leaves and grain discoloration owing to the infection of multiple pathogens.
Slide 12 Alleviation of Abiotic Stress Ma and Yamaji, 2006 Alleviation of Abiotic Stresses
Slide 13 Mediation of Nutrient Uptake Wang et al., 2004 -Al treatment greatly enhanced Si accumulation in the cell wall fraction, reducing the mobility of apoplastic Al. Si treatment leads to the formation of hydroxyaluminumsilicates in the apoplast of the root apex, thus detoxifying Al. -Mn toxicity is heavily correlated with Mn concentrations within intercellular fluid. In the presence of Si, Mn experiences an increased level of binding to the cell wall, thereby lowering its fluid transportation throughout the plant.
Slide 14 Mediation of Sodium Uptake Yeo et al., 1999 -Photosynthetic gas exchange in rice is known to be highly sensitive to salt accumulation in the leaves. The increased assimilation rate of salt-grown plants in the presence of silicon could broadly be accounted for by an increase in stomatal conductance. Stomatal conductance and assimilation rate have been shown to increase proportionately with the addition of silicon, resulting in more efficient photosynthesis. -This mediation was found not to occur through reduction in transpirtional volume flow, but through instead reducing the bypass flow of water, which is the volume of water which crosses the root using only apoplastic pathways. This small percentage water that leaks through the root into the xylem without encountering a membrane can contain high concentrations of sodium. -Important point being that the trade off in reducing this bypass flow (which only makes up a few percent of water uptake) is better for total transpirational volume uptake of water than if there were high sodium concentrations in the leaf if that bypass flow wasn t blocked. A=Net Assimilation rate; G=Stomatal conductance
Slide 15 UV Light Filtration in Rice Leaves Wen-Bin et al., 2004 -After 30 h exposure to UV-B radiation, silicon-deficient rice leaves (Fig. c) exhibit many brown spots and strips that are typical of UV damage. -In contrast, there is almost no visible UV damage symptoms in the silicon-fed leaves exposed to the same level of UV-B radiation (Fig. d). -It can be seen that under UV excitation, there is only faint yellow-green autofluorescence in the epidermal cell walls (blank arrow in e) and inside bulliform cells (solid arrow in e), which means that the content of UV-absorbing compoundsin silicon-deficient leaves is less. -In contrast, the epidermal cell walls (blank arrow in f) and bulliform cells (solid arrow in f) of silicon-treated leaves emitted strong yellow-green autofluorescence when excited by UV light, which indicated that there was larger quantity of UV-absorbing compounds accumulated in the epidermis of silicon-treated leaves as compared to non-treated leaves.
Slide 16 Lodging Idris et al., 1975 -In the presence of added nitrogen, plants may get top heavy, causing lodging. Silicon helps reinforce the structural rigidity of the plant, decreasing the plants tendency to collapse when top heavy. -Using this experimental setup, Idris et al. (1975) found that rice plants are much more resistant to lodging when treated with silicon.
Slide 17 Summary In order to be beneficial Silicon must be taken up in large amounts. Most plants, particularly dicots, cannot accumulate Silicon in large enough amounts to be beneficial. Its benefits are associated with the high deposition of Si in plant tissues, enhancing plant strength and rigidity. -In the future, scientists hope to genetically manipulate the Silicon uptake capacity of roots in order to allow better resistance to abiotic and biotic stresses.
Slide 18 References Agarie, S., Agata, W., Uchida, H., Kubota, F., & Kaufman, P. B. (1996). Function of silica bodies in the epidermal system of rice (Oryza sativa L.): testing the window hypothesis. Journal of Experimental Botany, 47(5), 655-660. doi: 10.1093/jxb/47.5.655 Datnoff, L. E., & Rodrigues, F. Á. (2005). The role of silicon in suppressing rice diseases. APSnet Feature. URL http://www. apsnet. org/online/feature/silicon/default. asp [accessed on 14 September 2008]. Idris, M., Hossain, M., & Choudhury, F. (1975). The effect of silicon on lodging of rice in presence of added nitrogen. Plant and Soil, 43(1), 691-695. Kim, S. G., Kim, K. W., Park, E. W., & Choi, D. (2002). Silicon-induced cell wall fortification of rice leaves: a possible cellular mechanism of enhanced host resistance to blast. Phytopathology, 92(10), 1095-1103.
Slide 19 References Lewin, J., & Reimann, B. E. F. (1969). Silicon and Plant Growth. Annual Review of Plant Physiology, 20(1), 289-304. doi: doi:10.1146/annurev.pp.20.060169.001445 Ma, J. F., & Takahashi, E. (2002). Soil, fertilizer, and plant silicon research in Japan: Elsevier Science Ltd. Ma, J. F., & Yamaji, N. (2006). Silicon uptake and accumulation in higher plants. Trends in Plant Science, 11(8), 392-397. Mitani, N., Chiba, Y., Yamaji, N., & Ma, J. F. (2009). Identification and Characterization of Maize and Barley Lsi2-Like Silicon Efflux Transporters Reveals a Distinct Silicon Uptake System from That in Rice. Plant Cell, 21(7), 2133-2142. doi: 10.1105/tpc.109.067884
Slide 20 References Richmond, K. E., & Sussman, M. (2003). Got silicon? The non-essential beneficial plant nutrient. Current Opinion in Plant Biology, 6(3), 268-272. doi: Doi: 10.1016/s1369-5266(03)00041-4 Wang, Y., Stass, A., & Horst, W. J. (2004). Apoplastic Binding of Aluminum Is Involved in Silicon-Induced Amelioration of Aluminum Toxicity in Maize. Plant Physiol., 136(3), 3762-3770. doi: 10.1104/pp.104.045005 Wen-Bin, L., Xin-Hui, S., He, W., & Fu-Suo, Z. (2004). Effects of Silicon on Rice Leaves Resistance to Ultraviolet-B. 46(6), 691-697. Yeo, A. R., Flowers, S. A., Rao, G., Welfare, K., Senanayake, N., & Flowers, T. J. (1999). Silicon reduces sodium uptake in rice (Oryza sativa L.) in saline conditions and this is accounted for by a reduction in the transpirational bypass flow. Plant, Cell & Environment, 22(5), 559-565. doi: 10.1046/j.1365-3040.1999.00418.x