Introduction

Plant tissue culture is a well-established technology used for the culture and production of plant cells, plant tissues, and whole plants under defined and controlled nutrient and environmental conditions. Plant tissue culture media contain both inorganic and organic nutrients that sustain plant growth. To improve growth and to produce the desired morphogenesis in vitro, the culture medium is often optimized and the extent of culture medium modification is largely determined by the species used (Asmar et al. 2013a, b; Reed et al. 2013). Inappropriate nutrient composition may cause physiological disorders, such as hyperhydricity, shoot tip necrosis, and upwardly curled leaves, which usually result from inappropriate inorganic nutrient concentrations in the culture medium (Reed et al. 2013). Silicon (Si) is found in the soil and is one of the most dominant mineral nutrients in plants (Epstein 1999). Silicon helps plants tolerate different environmental stresses that are present in field conditions, such as high temperature, drought, loading, UV, freezing, nutrient imbalance, salinity, and metal toxicity (Sahebi et al. 2015). A number of studies have shown that Si treatment improves crop growth and yield under abiotic and biotic stress conditions (Ahmad et al. 2013a, b; Esfahani et al. 2014; Schurt et al. 2014; Artyszak et al. 2015; Chen et al. 2015; D’Imperio et al. 2015; Hartley 2015; Hussain et al. 2015; Muneer and Jeong 2015; Noor et al. 2015; Saudy and Mubarak 2015; Wang et al. 2015; Xu et al. 2015; Yin et al. 2015).

The photosynthetic activity and nutrient uptake of plants were remarkably be enhanced by Si fertilization. The in vitro morphogenic potential of plant cells and tissues has been improved with Si (Personal communication). Silicon is shown to enhance stress tolerance by altering antioxidant enzyme activity; altering endogenous hormone levels; enhancing the production of lignin, chitinase, phenolics, phytoalexins, and glucanase; increasing nutrient uptake by plants; altering the cation binding capacity in the plant cell wall; improving plant cell strength; maintaining the stomata structure and relative water content; and reducing heavy metal uptake (Vaculík et al. 2009; Sahebi et al. 2015). In this regard, Si deposited in the plant leaf epidermis has been reported to modulate transpiration that leads to decreased water loss (Mitani and Ma 2005). This review highlights the roles of Si in the tissue culture of plants.

Importance of Silicon in Plant Tissue Culture

Mature seeds of three varieties of rice (Oryza sativa), Pajam, Kalizira, and Lucky, have been cultured to evaluate the effects of calcium silicate (CaSiO3) on callus induction and plant regeneration (Islam et al. 2005). It has been reported that MS medium containing CaSiO3 increases the frequency of callus induction. Another study examining the development of Phragmites australis also showed that the effect of Si on plant growth and root formation is dependent on the genotype used for callus induction (Mathe et al. 2012). Callus growth from the root and stem nodes of P. australis was enhanced by Si supplied as sodium silicate (Na2SiO3) to the MS medium. However, it was observed that the Si effect on somatic embryogenesis depends on explants because it is able to stimulate embryogenesis in the root callus but not in the stem nodal callus. For Cattleya loddigesii shoot multiplication, potassium metasilicate (K2SiO3) was found to be more effective than sodium silicate (Na2SiO3) (Soares et al. 2011). The molecular and biochemical bases of Si action on organogenesis and somatic embryogenesis remain unknown. It has been reported that Si accumulation in plant leaves cultured on MS medium supplemented with Si would be more increased compared to the leaves of plants cultured on MS medium without Si inclusion (Sivanesan and Jeong 2014).

Role of Silicon as a Protective Element Against Abiotic Stresses

It has been extensively reported that Si can suppress both physical and chemical stresses. For instance, it was reported that Si treatment increases cold tolerance in Dendrobium moniliforme by increasing free proline, soluble protein, and soluble sugar contents and by reducing MDA content (XiaoYu et al. 2013). Silicon enhanced grape (Vitis vinifera × Vitis labrusca) callus survival rate at low temperature by avoiding browning (Moriguchi et al. 1988). In vitro studies on the storage of Solanum tuberosum var. gersa and Coleus hybridus indicated that silicone oil helps maintain the regenerative potential and reduces plant growth (Radovet et al. 2008; Radovet-Salinschi and Cachita-Cosma 2012). These results highlight the cryoprotective role of Si, which can be incorporated into cryoprotective mixtures. The ameliorating effect of Si on salt stress has been reported in in vitro culture of Ajuga multiflora (Sivanesan and Jeong 2014), Salvia splendens (Soundararajan et al. 2013), and S. tuberosum (Qing et al. 2005). There are reports that show Si-induced salinity tolerance as a result of reducing NaCl uptake by plants and the consequent preservation of stomatal ultrastructure, photosynthetic activity, antioxidant enzyme production, and modulation of free proline content (Qing et al. 2005; Soundararajan et al. 2013; Sivanesan and Jeong 2014). The results of another study investigating the effects of Si on aluminum (Al) tolerance of Picea abies cells indicated that Si is able to reduce the free Al concentration in the cell wall and Si is able to reduce Al toxicity (Prabagar et al. 2011). Silicon is also reported to minimize heavy metal toxicity (Cucumis sativus and Triticum spp.), improve drought tolerance (S. tuberosum L., Piperaceae, and Zea mays), improve salt stress (Glycine max, Hordeum vulgare L., O. sativa L., Brassica napus L., and Ziziphus jujuba cv.), and enhance resistance to temperature as well as radiation stresses (Z. mays and G. max) (Balakhnina and Borkowska 2013; Zhu and Gong 2014). Si is also able to induce some regulatory mechanisms in plants that may account for wide-spectrum disease resistance (Van Bockhaven et al. 2013).

In Vitro Application of Silicon

Role of silicon in plantlets grown and developed in vitro

The anatomical as well as the morphological characteristics of field-grown seedlings are different from in vitro-grown plantlets. It has been reported that the growth and development of plants are affected by Si application. For instance, Phalaenopsis hybrid plantlets cultured in Went and Vacin medium containing CaSiO3 showed increased leaf growth (Zhuo 1995). The modified Linsmaier and Skoog liquid medium used for cell suspension culture of Perilla frutescens with silicon A inclusion enhanced cell growth and anthocyanins content (Zhong et al. 1992).

It has been shown that both the root number and the growth of C. loddigesii seedlings grown in modified Knudson medium supplemented by Na2SiO3 (20.0 mgL−1) and K2SiO3 (5.0 mgL−1) were significantly increased (Soares et al. 2011). It was also reported that Si was able to increase the rooting of the Phalaenopsis hybrid (Zhuo 1995). The Na2SiO3 and GA3 in combination increased the growth, leaf number, and root number of C. loddigesii more than GA3 alone (Soares et al. 2013). The optimal concentration of Si needed for plant growth and development varies among different genotypes within the same species and between plant species (Lim et al. 2012).

The MS medium supplemented with 0.5 and 2.0 mgL−1 of CaSiO3 promotes the growth of both native “Brassavola perrinii” as well as hybrid orchid plants, respectively (Soares et al. 2012). The results of another study investigating the effects of diverse Si sources, such as Na2SiO3, K2SiO3, and CaSiO3, on the anatomical characteristics and growth of strawberry (Fragaria × ananassa) seedlings showed that the seedling’s fresh and dry weight were increased in MS medium supplemented with 1.0 g L−1 Na2SiO3 (Braga et al. 2009). Moreover, another study of banana (Maca’banana) seedlings cultured in medium containing CaSiO3 showed an increase in chlorophyll content; however, those grown in MS medium supplemented with Na2SiO3 showed an increase in the fresh, dry weight, and length of shoots (Asmar et al. 2011). It has been reported that the inclusion of Si in the rooting medium causes an increase in the thickness of the leaf tissue and a deposition of epicuticular wax in strawberry (Braga et al. 2009) and banana (Grande Naine) (Asmar et al. 2013a, b) plantlets. The modified rooting medium supplemented with K2SiO3, Na2SiO3, and CaSiO3 improved the leaf tissue anatomy of banana plantlets (Maca’banana) (Magno Queiroz Luz et al. 2012). Moreover, the inclusion of CaSiO3 in the culture medium enhanced the photosynthetic rate and, consequently, the chlorophyll content in banana (Maca’banana) plantlets (Asmar et al. 2013a, b). In contrast, the electron and light microscopic analysis of the in vitro-cultured strawberry that was cultured without Si showed deformation of the leaf epidermis and chlorenchyma of plantlets (Soares et al. 2012). Another study has reported Si deposition within the cell walls of rice (O. sativa) obtained from in vitro-cultured plantlets (He et al. 2013). This study indicated the role of Si in improving the stability of the cell wall structure. Silicon is able to enhance the stability of cell walls through elongation and subsequent division, leading to the maintenance of the shape of cells that may be vital for the function and survival of the cells. The investigation of the effects of Si on hyperhydricity in in vitro-cultured Ornithogalum dubium revealed that modified MS liquid medium supplemented with NAA, BA, and 6% sucrose using a bioreactor considerably decreases the hyperhydric shoot induction and enhances the mechanical strength and firmness of plants (Ziv 2010). It has been shown that Si treatment of the regenerated O. dubium shoots significantly decreased the hydrogen peroxide content and oxidative activity of some reductive enzymes including ascorbate oxidase, GPX, and APX in the leaves obtained from regenerated shoots compared with control plants.

Role of silicon in regeneration of adventitious shoots and suppression of NaCl stress

It has been shown in various studies that Si can be effective in lightening salt stress in different plants. Silicon supplementation to the culture medium increases the shoot induction frequency and average number of shoots per explant (Sivanesan and Jeong 2014). It has been shown that Si can be effective for shoot regeneration in rice (Islam et al. 2005) and reed (Máthé et al. 2012). The results of another experiment showed that, while the shoot generation of A. multiflora on the culture medium containing 100 mM NaCl is completely inhibited, supplementing this medium with Si improves the shoot generation of leaf and petiole explants (Sivanesan and Jeong 2014). They showed that the percentage of shoot induction is increased linearly by enhancing Si supplementation to the medium containing NaCl. The ion imbalance resulting from high concentrations of Na+ and Cl− ions in the culture medium reduces the number of regenerated shoots per explant, while additional Si can play key roles under nutrient imbalance (Ma 2004). It has been reported that MS media containing NaCl caused a decrease in SOD activity in the leaves and roots of A. multiflora (Sivanesan and Jeong 2014) and H. vulgare L. (Liang 1997), while both of these studies indicated that Si treatment of the culture medium containing NaCl considerably increased the SOD activity in both organs.

Silicon and cell culture in bioreactors/suspension

Correspondingly, the modified MS medium supplemented with Si as K2SiO3 led to decreased hyperhydricity in Cotoneaster wilsonii by reducing the MDA content within the regenerated shoot parts compared with the control plants (Sivanesan et al. 2011). Based on the dispersive X-ray results of one study of C. wilsonii, no trace of Si was observed in the hyperhydric leaf samples, while the presence of Si was shown in non-hyperhydric plants. Therefore, the hyperhydricity problem can be decreased through Si supplementation to the liquid as well as the solid medium (Sivanesan et al. 2011).

Tissue culture of woody plants usually includes many disorders, such as tissue browning (a bottleneck) that results from phenolic oxidative activity. It has been shown that Si is able to prevent tissue browning completely in guava (Psidium guajava) by sealing the cut ends of nodal explants with a silicon mixture (Youssef et al. 2010) without any subsequent effects for the rest of the in vitro propagation steps. Based on this result, it was suggested that Si may be utilized during the preparation of other explants to avoid phenolic tissue browning. It can be concluded that the anatomical, morphological, and physiological uniqueness of different plantlets can be improved in vitro by modifying the culture medium by supplementation with Si. Nevertheless, the precise evaluation of the effects of different Si concentrations and sources on the growth and development of various plants is necessary.

Role of Silicon on Metabolomics, Growth Traits, and Rooting Ability

The addition of the proper amount of Si to the culture medium results in an increase in A. multiflora height. For example, the inclusion of 3.6 mM Si to the MS media enhanced plant height, while 7.2 mM of Si supplementation led to shorter plants (Sivanesan and Jeong 2014). This study also showed that an increase in the Si concentration in the MS media leads to a decrease in the number of leaves grown per shoot. It has been reported that Si treatment of the culture medium causes an increase in the growth traits of plants under salinity stress (Ma 2004; Liang et al. 2007).

The maximum amount of Si supplementation to the culture media of A. multiflora is limited to 3.6 mM, which leads to an increase in the number of leaves per shoot and an increase in the fresh and dry weights of the root and shoot; however, an increase in the Si concentration to 7.2 mM leads to the greatest root length per explant (Sivanesan and Jeong 2014). This study also indicated that Si treatment of up to 3.6 mM resulted in the maximum chlorophyll level, whereas an increase in the Si concentration negatively affected the chlorophyll content. In vitro studies investigating the effects of potassium silicate on the growth characteristics of Begonia semperflorens and Pansy (Viola × wittrockiana) have indicated that Si significantly increased the fresh weight, chlorophyll content, and leaf area of Begonia and Pansy (Lim et al. 2012). Leaf anatomy studies of B. perrinii using different Si concentrations showed that treatments with silicate prevent the deformation of the chlorenchyma and the epidermis compared with non-Si treatments and can affect plant growth conditions directly or indirectly (Soares et al. 2012).

Antioxidant enzymes increase the morphogenesis and growth traits in many plants. For instance, it was shown that the antioxidant enzyme activities in Caladium bicolor (Isah and Mujib 2011), Brassica rapa var. turnip (Abbasi et al. 2011), Crocus sativus (Vatankhah et al. 2010 ), plum (Faize et al. 2013), and Piper nigrum (Ahmad et al. 2013a, b) are increased during organogenesis. It was reported that the antioxidant enzyme activities in many plants are affected by Si treatment (Ma 2004; Liang et al. 2007). The results of one study of A. multiflora indicated that the addition of Si to MS medium containing IAA and 2iP improves adventitious shoot regeneration by enhancing the activity of some antioxidant enzymes such as APX, CAT, SOD, and SOD (Sivanesan and Jeong 2014).

Gibberellins (Gas) are plant growth hormones that play critical roles in diverse developmental processes, such as dormancy, germination, stem elongation, flowering, enzyme induction, sex expression, leaf senescence, and fruit senescence. It has been reported that Si inclusion can increase the levels of both GA1 and GA20 in rice cultivars (Hwang et al. 2007). It has been indicated that shoot induction in citrus (Pérez-Tornero et al. 2010), apple (Isogai et al. 2008), and Cephaelis ipecacuanha (Kaushal et al. 2005) is affected by GA3. On the other hand, Si positively affects shoot induction; therefore, this effective role of Si may result from altering endogenous levels of Gas.

Silicon Absorption and Transportation

It has been reported that Si absorption and transportation in different plant species are dependent on both species as well as on the outer Si concentration (Sahebi et al. 2015). The role of plant species in Si absorption and transportation depends on their passive or active ability to uptake Si (Liang et al. 2007). Personal communication comparing Si accumulation between three Malaysian rice varieties indicated that calluses of the MR276 variety, with a high potential of Si absorption, were more embryogenic (Fig. 1), and the plant regeneration percentage obtained from calluses was higher than that of the other two varieties, MR219 and MR220, with less ability to accumulate Si.

Figure 1
figure 1

Scanning electron microscopy (SEM) image of the calluses of different rice varieties. (a–c) Three Malaysian rice varieties MR219, MR220, and MR276, respectively. White traces highlighted with red circle are accumulated silicon.

Full size image

It was shown that silicification occurs in the endodermis of gramineae roots during maturation. However, the cell walls of other tissues, including epidermal, vascular, and cortical tissues, may be silicified within older roots. Moreover, silicification also occurs in different parts of grasses, including roots and shoots (Sangster et al. 2001). It was shown that a layer of deposited Si of approximately 2.5 μm is formed immediately under the cuticle in rice leaf blades. It has been shown that the silicification of cells, such as “dumb-bell-shaped cells” in silica cells, vascular bundles, and silica bodies of bulliform cells, is not restricted to the leaf blades of rice because silicified cells may be found inside the epidermal layer as well as in the vascular tissues of the stem, hull, and leaf sheath (Prychid et al. 2003). Another study on Equisetum indicated that the silicified structures can be found on the epidermal surface of the cell wall (Holzhüter et al. 2003). The silicified cells produced in the different tissues including vascular tissue, leaf blade, epidermis stem, hull, and leaf sheath cooperate to protect against a wide range of stresses in plants (Hodson et al. 2005). The mechanism of bio-silica formation in plants has been reviewed in a previous study (Sahebi et al. 2015). The silica concentration in the environment is affected by many factors such as silica condensation, temperature, pH, and presence of other polymers, different ions, and small molecules. Distances of Si–O bond and the angle of Si–O–Si bond play the essential role through different silica species polymerization. Environmental reactions and OH groups may differ in diverse species due to the composition, solubility, hardness, density, and viscosity. Functional groups of amino acid residues in the structure of proteins are accessible to silica and play an important role in determining the nature and physical structure of substances which are formed during different maturing stages. Amino acids and peptides are effective in the formation of polysilicic species through interactions with different silicate species in the solution (Sahebi et al. 2015).

Future Directions

Biochemical and physiological functions of silicon at different molecular levels as well as at a single cell level should be more illustrated through utilizing two systems of the plant tissue culture and in vitro cell suspension culture. Although the beneficial role of silicon to prompt the secondary metabolites in all parts of plants has been shown recently, but there is still a big gap to understand its involved mechanisms. Hence, much work is needed to find an excel formulation of tissue culture medium not only to settle numerous micro-propagation problems but also to increase plant tissue culture success.

Conclusion

Although many beneficial effects of Si on the tissue culture of several species plants have been demonstrated recently, they need to be verified in different plant species to prove its usefulness at the field level. Plant tissue and cell suspension culture can be used as tools to study the physiological and biochemical functions of Si at single cell and plant levels. Silicon may also be used to increase the production of secondary metabolites within in vitro cultures of plant cells and tissues. It can be concluded that Si be included in the tissue culture medium as a constructive nutrient to improve micro-propagation including plant quality.