Silicon enhances growth independent of silica deposition in a low-silica rice mutant, lsi1

To examine whether silica bodies are essential for silicon-enhanced growth of rice seedlings, we investigated the response of rice, Oryza sativa L., to silicon treatment. Silicic acid treatment markedly enhanced the SPAD (soil plant analytical development) values of leaf blades and the growth and development of leaves and lateral roots in cvs. Hinohikari and Oochikara, and a low-silicon mutant, lsi1. Combination of ethanol–benzene displacement and staining with crystal violet lactone enabled more detailed histochemical analysis to visualize silica bodies in the epidermis under bright-field microscopy. Supply of silicon induced the development of motor cells and silica bodies in epidermal cells in Hinohikari and Oochikara but not or marginal in lsi1. X-ray analytical microscopy detected silicon specifically in the leaf sheath, the outermost part of the stem, and the leaf blade midrib, suggesting that silicon is distributed to tissues involved in maintaining rigidity of the plant to prevent lodging, rather than being passively deposited in growing tissues. Silicon supplied at high dose accumulated in all rice seedlings and enhanced growth and SPAD values with or without silica body formation. Silicon accumulated in the cell wall may play an important physiological role different from that played by the silica deposited in the motor cell and silica bodies.     

Function of silica bodies in the epidermal system of rice (Oryza sativa L.): testing the window hypothesis

Silicon has been considered to be important for normal growthand development of the rice plant (Oryza sativa L.). To investigatethe physiological function of deposited silica in rice leaves,the hypothesis that silica bodies in the leaf epidermal systemmight act as a ‘window’ to facilitate the transmissionof light to photosynthetic mesophyll tissue was tested. Thesilica content of leaves increased with supplied silicon andwas closely correlated with the number of silica bodies perunit leaf area in the epidermal system. There was a significantdifference in silica deposition and formation of silica bodiesbetween Si-treated and non-treated leaves; silicon was polymerizedinside the silica cells and bulliform cells of the epidermis,in Si-treated leaves. Although the ‘windows’ wereonly formed in leaves with applied silicon, optical propertiesof leaf transmittance, reflectance and absorptance spectra inSi-treated and non-treated leaves were almost equal. Furthermore,light energy use efficiency and quantum yield of Si-treatedleaves were less than in leaves not containing silica bodies.Thus, silica bodies, at least based on the data, do not functionas windows in rice leaves. Key words: Silicon, window hypothesis, rice, optical property, quantum yield     

Silicon distribution and accumulation in shoot tissue of the tropical forage grass Brachiaria brizantha

Silicon (Si) accumulation in organs and cells is one of the most prominent characteristics of plants of the family Poaceae. Many species from this family are used as forage plants for animal feeding. The present study investigates in Brachiaria brizantha (Hochst. ex A. Rich.) Stapf. cv. Marandu: (1) the dry matter production and Si content in shoot due to soil Si fertilizations; (2) the Si distribution among shoot parts; and (3) the silica deposition and localization in leaves. Plants of B. brizantha cv. Marandu were grown under contrasting Si supplies in soil and nutrient solution. Silica deposition and distribution in grass leaf blades were observed using light microscope and scanning electron microscope equipped with an energy dispersive X-ray spectrometer (SEM-EDXS). Silicon concentration in the B. brizantha shoot increased according to the Si supply. Silicon in grass leaves decreased following the order: mature leaf blades > recently expanded leaf blades > non-expanded leaf blades. Silicon accumulates mainly on the upper (adaxial) epidermis of the grass leaf blades and, especially, on the bulliform cells. The Si distribution on adaxial leaf blade surface is non uniform and reflects a silica deposition exclusively on the cell wall of bulliform cells.     

Structure,morphology, and composition of silicon biocomposites in the palm tree Syagrus coronata (Mart.) Becc

Syagrus coronata is an economically important palm tree grown as an ornament, for the oil extracted from its seeds, and the wax from its leaves which has several applications in industry. Silicon biocomposites were analyzed in leaves of S. coronata. Silica bodies were found as extracellular silica masses between the hypodermal-layer cell walls and in granules present in the vacuoles of palisade cells. Scanning electron microscopy of the hypodermal layer of cells showed a collection of spherical bodies embedded in enveloping cavities that outlined the general structure of the bodies. Globular subunits with sharp edges formed the spherical bodies that ranged from 6 to 10 microm in diameter (average, 7.8 microm). X-ray microanalysis detected only silicon and oxygen homogeneously distributed throughout the bodies. Vacuoles of palisade cells contained a large number of granules ranging from 20 nm to 1.2 microm in size (average, 300 nm). Transmission electron microscopy associated with electron spectroscopic imaging and electron energy loss spectroscopy were used to determine the elemental composition of the granules. Vacuolar granules were amorphous and composed of silicon and oxygen, suggesting they consist of amorphous silica biominerals. No nitrogen, indicative of organic matter, was detected in the granules.     

Function of silica bodies in the epidermal system of rice (Oryza sativa L.): testing the window hypothesis

Silicon has been considered to be important for normal growth and development of the rice plant (Oryza sativa L.). To investigate the physiological function of deposited silica in rice leaves, the hypothesis that silica bodies in the leaf epidermal system might act as a 'window' to facilitate the transmission of light to photosynthetic mesophyll tissue was tested. The silica content of leaves increased with supplied silicon and was closely correlated with the number of silica bodies per unit leaf area in the epidermal system. There was a significant difference in silica deposition and formation of silica bodies between Si-treated and non-treated leaves; silicon was polymerized inside the silica cells and bulliform cells of the epidermis, in Si-treated leaves. Although the 'windows' were only formed in leaves with applied silicon, optical properties of leaf transmittance, reflectance and absorptance spectra in Si-treated and non-treated leaves were almost equal. Furthermore, light energy use efficiency and quantum yield of Si-treated leaves were less than in leaves not containing silica bodies. Thus, silica bodies, at least based on the data, do not function as windows in rice leaves.     

Silicon Deposition in the Roots, Culm and Leaf of Phalaris canariensis L

Silicon deposition in the roots, culm and leaf of canary grass(Phalaris canariensis L.) was investigated using light microscopy,scanning electron microscopy and electron probe microanalysis. In adventitious roots grown in solution silicon was concentratedin four endodermal walls. Silicon was not detected in the endodermisof aerial adventitious roots, but was present in the epidermisand outer cortical cell layers. Silicon deposition in the culm mainly took place in the epidermis,and particularly in epidermal papillae. The silica deposition pattern in the leaf was typical of thesub-group Festucoideae. The leaf blade showed deposits in costalprickle hairs and wavy rods, but few intercostal deposits. Inthe ligule deposition was confined to isolated groups of pricklehairs on the abaxial surface. The major sites of silica depositionin the leaf sheath were the stomatal subsidiary cells, papillaeand intercostal idioblasts. Prickle hairs were much less commonin the sheath than the blade, and costal wavy rods appearedto be absent in the sheath. Phalaris canariensis L., canary grass, silicification, root, culm, leaf, electron probe microanalysis     

Structure, morphology, and composition of silicon biocomposites in the palm tree Syagrus coronata (Mart.) Becc.

Summary.  Syagrus coronata is an economically important palm tree grown as an ornament, for the oil extracted from its seeds, and the wax from its leaves which has several applications in industry. Silicon biocomposites were analyzed in leaves of S. coronata. Silica bodies were found as extracellular silica masses between the hypodermal-layer cell walls and in granules present in the vacuoles of palisade cells. Scanning electron microscopy of the hypodermal layer of cells showed a collection of spherical bodies embedded in enveloping cavities that outlined the general structure of the bodies. Globular subunits with sharp edges formed the spherical bodies that ranged from 6 to 10 μm in diameter (average, 7.8 μm). X-ray microanalysis detected only silicon and oxygen homogeneously distributed throughout the bodies. Vacuoles of palisade cells contained a large number of granules ranging from 20 nm to 1.2 μm in size (average, 300 nm). Transmission electron microscopy associated with electron spectroscopic imaging and electron energy loss spectroscopy were used to determine the elemental composition of the granules. Vacuolar granules were amorphous and composed of silicon and oxygen, suggesting they consist of amorphous silica biominerals. No nitrogen, indicative of organic matter, was detected in the granules. Received November 26, 2001; accepted July 1, 2002; published online October 31, 2002 RID="*" ID="*" Correspondence and reprints: Departamento de Microbiologia Geral, Instituto de Microbiologia Professor Paulo de Góes, Centro de Ciências da Saude, Universidade Federal do Rio de Janeiro, 21941-590 Rio de Janeiro, RJ, Brazil.     

Do lignification and silicification of the cell wall precede silicon deposition in the silica cell of the rice (Oryza sativa L.) leaf epidermis?

Aims

Rice is a well-known silica-accumulating plant. The dumbbell-shaped silica bodies in the silica cells in rice leaf epidermis are formed via biosilicification, but the underlying mechanisms are largely unknown.

Methods

Leaves at different developmental stages were collected to investigate silica cell differentiation by analyzing structures and silicon localization in the silica cells.

Results

Exogenous silicon application increased both shoot and root biomass. When silicon was supplied, silica cells in the leaf epidermis developed gradually into a dumbbell-shape and became increasingly silicified as leaves aged. Silicon deposition in the silica cells was not completed until the leaf was fully expanded. Multiple lines of evidence suggest that lignification of silica cell walls precedes silicon deposition in the lumen of silica cells. The organized needle-like silica microstructures were formed by moulding the inner cell walls and filling up the lumen of the silica cell following leaf maturation.

Conclusions

Two processes were involved in silicon deposition: (1) the silica cell wall was lignified and silicified, and then (2) the silicon was deposited gradually in silica cells as leaves aged. Silica body formation was not completed until the leaf was fully mature.     

SILICON IN THE ROOTS OF HIGHER PLANTS

Silicon (Si) distribution in the roots of Sorghastrum nutans (L.) Nash and Sorghum bicolor (L.) Moench. was investigated by means of the electron-probe microanalyzer and scanning electron microscope. In both species, Si was confined to the inner tangential wall of the tertiary-phase endodermal cells in the form of nodular silica aggregates of similar morphology and X-ray intensity. The results are compared to those for six closely related genera, as well as to studies of Si in the roots of species of other tribes of the family Poaceae. The various types of root deposits occurring in the family are described, and their relationships discussed. It is concluded that the type of Si distribution exhibited is determined largely by the phylogenetic status of the genus, rather than by the basic pattern of root anatomy.     

Silica Distribution in the Caryopsis and Inflorescence Bracts of Foxtail Millet [Setaria italica (L.) Beauv.] and its Possible Significance in Carcinogenesis

Silicon deposits in the caryopsis and inflorescence bracts offoxtail millet [Setaria italica (L.) Beauv.] were investigatedusing light microscopy, scanning electron microscopy and electronprobe microanalysis. The samples were obtained from Lin Xian,Henan province, northern China and CSIRO, Australia. High concentrations of silicon were observed in the papillaeon the external surfaces of the inflorescence bracts, and inthe epidermal cells. In the caryopsis silicon was depositedin the aleurone layer. Silicification was heavier in the LinXian samples. The heavy accumulation of silica in the foxtail millet and thepossibility that fragments of plant silica may be implicatedin the aetiology of oesophageal cancer in the Lin Xian regionare discussed. Setaria italica (L.), Beauv, foxtail millet, caryopsis, silica distribution, scanning electron microscopy     

MORPHOLOGY,FINE STRUCTURE,AND ONTOGENY OF THE STINGING EMERGENCE OF URTICA DIOICA

Stinging emergences in Urtica dioica L. characteristically possess an elongate stinging cell and a multicellular pedestal. The emergence is derived from the epidermal and subepidermal cell layers. The apical wall of the stinging cell is composed of silica bodies which decrease basipetally in concentration. The basal portion of the cell wall of the stinging cell is devoid of silica bodies and lacks primary pit fields or pits between it and the pedestal cells. X-ray microanalysis of electron dense particles located in the stinging cell ER-golgi complex indicate that these particles contain silicon. There is no ultrastructural evidence for the presence of a toxin synthesizing system or a toxin itself.     

The ultrastructure of Spirulina platensis in relation to temperature and light intensity

The ultrastructure of Spirulina platensis, a cyanobacterium with a helical morphology, has been studied in relation to temperature and light intensity. An increase in temperature gives rise to a more tightly coiled trichome, an increase in sheath material formation and a decrease in cyanophycin (above 17°C) and polyglucan (above 20°C) granule concentration. An increase in light intensity leads to an increase in gas vesicle concentration while the phycobilisome content decreases. Furthermore, cylindrical bodies have been observed with a somewhat different ultrastructure from those found in other species of cyanobacteria. The occurrence, size and ultrastructure of polyhedral bodies, photosynthetic lamellae, mesosomes, lipid deposits and an unknown kidney-shaped inclusion in relation to temperature and light intensity are described.     

Interactive effects of plant-available soil silicon and herbivory on competition between two grass species

Background and Aims

The herbivore defence system of true grasses (Poaceae) is predominantly based on silicon that is taken up from the soil and deposited in the leaves in the form of abrasive phytoliths. Silicon uptake mechanisms can be both passive and active, with the latter suggesting that there is an energetic cost to silicon uptake. This study assessed the effects of plant-available soil silicon and herbivory on the competitive interactions between the grasses Poa annua, a species that has previously been reported to accumulate only small amounts of silicon, and Lolium perenne, a high silicon accumulator.

Methods

Plants were grown in mono- and mixed cultures under greenhouse conditions. Plant-available soil silicon levels were manipulated by adding silicon to the soil in the form of sodium silicate. Subsets of mixed culture pots were exposed to above-ground herbivory by desert locusts (Schistocerca gregaria).

Key Results

In the absence of herbivory, silicon addition increased biomass of P. annua but decreased biomass of L. perenne. Silicon addition increased foliar silicon concentrations of both grass species >4-fold. Under low soil-silicon availability the herbivores removed more leaf biomass from L. perenne than from P. annua, whereas under high silicon availability the reverse was true. Consequently, herbivory shifted the competitive balance between the two grass species, with the outcome depending on the availability of soil silicon.

Conclusions

It is concluded that a complex interplay between herbivore abundance, growth–defence trade-offs and the availability of soil silicon in the grasses'' local environment affects the outcome of inter-specific competition, and so has the potential to impact on plant community structure.     

Silicification in sorghum (Sorghum bicolor) cultivars with different drought tolerance

Sorghum belongs to a group of economically important, silicon accumulating plants. X-ray microanalysis coupled with environmental scanning electron microscopy (ESEM) of fresh root endodermal and leaf epidermal samples confirms histological and cultivar specificity of silicification. In sorghum roots, silicon is accumulated mostly in endodermal cells. Specialized silica aggregates are formed predominantly in a single row in the form of wall outgrowths on the inner tangential endodermal walls. The density of silica aggregates per square mm of inner tangential endodermal cell wall is around 2700 and there is no significant difference in the cultivars with different content of silicon in roots. In the leaf epidermis, silicon deposits were present in the outer walls of all cells, with the highest concentration in specialized idioblasts termed 'silica cells'. These cells are dumb-bell shaped in sorghum. In both the root endodermis and leaf epidermis, silicification was higher in a drought tolerant cultivar Gadambalia compared with drought sensitive cultivar Tabat. Silicon content per dry mass was higher in leaves than in roots in both cultivars. The values for cv. Gadambalia in roots and leaves are 3.5 and 4.1% Si, respectively, and for cv. Tabat 2.2 and 3.3%. However, based on X-ray microanalysis the amount of Si deposited in endodermal cell walls in drought tolerant cultivar (unlike the drought susceptible cultivar) is higher than that deposited in the leaf epidermis. The high root endodermal silicification might be related to a higher drought resistance.     

Silicon nutrition potentiates the antioxidant metabolism of rice plants under iron toxicity

Iron toxicity reduces growth of rice plants in acidic lowlands. Silicon nutrition may alleviate many stresses including heavy metal toxicity in plants. In the present study, the ameliorating effects of silicon nutrition on rice (Oryza sativa L.) plants under toxic Fe levels were investigated. Plants were cultivated in greenhouse in hydroponics under different Fe treatments including 10, 50, 100, and 250 mg L^?1 as Fe-EDTA and silicon nutrition including 0 and 1.5 mM sodium silicate. Iron toxicity imposed significant reduction in plant fresh weight, tiller, and leaf number. The activity of catalase, cell wall, and soluble peroxidases, and polyphenol oxidase in shoots decreased due to moderate Fe toxicity (50 and 100 mg L^?1), but increased at greater Fe concentration. Ascorbate peroxidase activity increased in both roots and shoots of Fe-stressed plants. Iron toxicity led to increased tissue hydrogen peroxide and lipid peroxidation. Silicon nutrition improved plant growth under all Fe treatments and alleviated Fe toxicity symptoms, probably due to lower Fe concentration of Si-treated plants. Silicon application could improve the activity of antioxidant enzymes such as catalase, ascorbate peroxidase, and soluble peroxidase under moderate Fe toxicity, which resulted in greater hydrogen peroxide detoxification and declined lipid peroxidation. Thus, silicon nutrition could ameliorate harmful effects of Fe toxicity possibly through reduction of plant Fe concentration and improvement of antioxidant enzyme activity.     

Growth promotion and an increase in cell wall extensibility by silicon in rice and some other Poaceae seedlings

The effect of silicon on organ growth and its mechanisms of action were studied in rice (Oryza sativa L. cv. Koshihikari), oat (Avena sativa L. cv. Victory), and wheat (Triticum aestivum L. cv. Daichino-Minori) seedlings grown in the dark. Applying silicon in the form of silicic acid to these seedlings via culture solution resulted in growth promotion of third (rice) or second (oat and wheat) leaves. The optimal concentration of silicon was 5–10 mM. No growth promotion was observed in early organs, such as coleoptiles or first leaves. In silicon-treated rice third leaves, the epidermal cell length increased, especially in the basal regions, without any effect on the number of cells, showing that silicon promoted cell elongation but not cell division. Silicon also increased the cell wall extensibility significantly in the basal regions of rice third leaves. These results indicate that silicon stimulates growth of rice and some other Poaceae leaves by increasing cell wall extensibility. Received: July 31, 2001 / Accepted: September 18, 2001     

Growth promotion and an increase in cell wall extensibility by silicon in rice and some other Poaceae seedlings

The effect of silicon on organ growth and its mechanisms of action were studied in rice ( Oryza sativa L. cv. Koshihikari), oat ( Avena sativa L. cv. Victory), and wheat ( Triticum aestivum L. cv. Daichino-Minori) seedlings grown in the dark. Applying silicon in the form of silicic acid to these seedlings via culture solution resulted in growth promotion of third (rice) or second (oat and wheat) leaves. The optimal concentration of silicon was 5-10 mM. No growth promotion was observed in early organs, such as coleoptiles or first leaves. In silicon-treated rice third leaves, the epidermal cell length increased, especially in the basal regions, without any effect on the number of cells, showing that silicon promoted cell elongation but not cell division. Silicon also increased the cell wall extensibility significantly in the basal regions of rice third leaves. These results indicate that silicon stimulates growth of rice and some other Poaceae leaves by increasing cell wall extensibility.     

Silicification patterns in wheat leaves related to ontogeny and soil silicon availability under field conditions

Plant and Soil - Silicon (Si) accumulation is an important strategy for plant defense against biotic and abiotic stress. Solid amorphous silica (ASi) deposits have been found to protect plants...     

Electron-probe Microanalysis Studies of Silica Distribution in Barley (Hordeum sativum L.)

Silicon occurence has been investigated by means of epidermalpeels, cryostat, and ultrathin sections of the internode, nodes,leaves, inflorescence bracts, and caryopsis of Hordeum sativumL. (cultivar Deba Abed) using the electron probe microanalyser.Analyses were made on growth stages during ear emergence andat maturity. The results indicate that silicon is present inthe internode with the highest concentration associated withthe opaline deposits. Detectable quantities are also found inthe outer tangential walls of the long cells, in the walls ofstomata, the sclerenchyma, and all vascular bundle regions.In mature upper internodes, silicifiation is confined to theupper third region, but this limit extends closer to the basalmeristem with increasing age of internode. The nodes have agreater concentration in the radial than in outer tangentialwalls. Heavy deposits are found in the leaves but with considerablevariation between blade and sheath, abaxial and adaxial surfaces,and the leaf position. The flag leaf contained the highest accumulations. In the inflorescence bracts (lemma and palea), silicon is detectableonly in the abaxial epidermis and hypodermis. Awns are alsoheavily silicified with the highest concentrations in the sclerenchymaand trichomes.     

Intranuclear silicon detection in a subcutaneous connective tissue cell by energy-dispersive x-ray miscroanalysis using fresh air-dried spread.

Silicon was detected by energy-dispersive x-ray microanalysis in the nucleus of a subcutaneous connective tissue cell of mice fed normally. To eliminate contamination, pieces of connective tissue were spread on copper grids and examined without any treatment by an energy-dispersive spectrometer with a scanning transmission apparatus attached to an electron microscope. Scanning transmission electron microscopy of the spread has demonstrated a well-preserved ultrastructure. Fibrous structures, nuclei and nucleoli of cells and mitochondrial granules were recognized. Electron probe analysis showed peaks for silicon at three spots on the nucleus of a cell in addition to those for phosphorus, sulfur, chlorine, potassium and calcium, whereas no peak of silicon could be detected at the spots on nuclei of other cells, mitochondrial granules and electron-lucent area on the same grid as the above. Silicon appears to play a significant role in the nucleus. Applicability of the technique to know the distribution of easily contaminating elements and diffusible substances is shown.