Genome-wide exploration of silicon (Si) transporter genes, Lsi1 and Lsi2 in plants; insights into Si-accumulation status/capacity of plants

The Physiological and Molecular Mechanisms of Silicon Action in Salt Stress Amelioration

Salinity is one of the most common abiotic stress factors affecting different biochemical and physiological processes in plants, inhibiting plant growth, and greatly reducing productivity. During the last decade, silicon (Si) supplementation was intensively studied and now is proposed as one of the most convincing methods to improve plant tolerance to salt stress. In this review, we discuss recent papers investigating the role of Si in modulating molecular, biochemical, and physiological processes that are negatively affected by high salinity. Although multiple reports have demonstrated the beneficial effects of Si application in mitigating salt stress, the exact molecular mechanism underlying these effects is not yet well understood. In this review, we focus on the localisation of Si transporters and the mechanism of Si uptake, accumulation, and deposition to understand the role of Si in various relevant physiological processes. Further, we discuss the role of Si supplementation in antioxidant response, maintenance of photosynthesis efficiency, and production of osmoprotectants. Additionally, we highlight crosstalk of Si with other ions, lignin, and phytohormones. Finally, we suggest some directions for future work, which could improve our understanding of the role of Si in plants under salt stress.

Genome-wide identification and functional analysis of silicon transporter family genes in moso bamboo (Phyllostachys edulis)

Silicon (Si) has crucial effects on plant development and stress resistance. Silicon transporters regulate Si absorption, transport, and distribution in plants. In this study, we identified and characterized the Si transporter gene family of moso bamboo (Phyllostachys edulis) and cloned seven putative Si transporter genes. Moso bamboo Si transporters contain conserved functional domains that mediate the accumulation of considerable amounts of Si. The analysis of gene duplication patterns and divergence times suggested that the expansion of the moso bamboo Si transporter family was mainly due to segmental duplications. The expression of moso bamboo Si transporter genes, which varied among organs, was significantly modulated by Si treatments. The subcellular localization analysis showed that Si transporters are plasma membrane proteins. The Si content increased in transgenic Arabidopsis overexpressing PeLsi1-1 or PeLsi1-2, which affected vegetative and reproductive growth. Our single-particle tracking analysis revealed the four diffusion modes of PeLsi1-1 on the plasma membrane. Moreover, the particle velocity, dwell time, and motion range of PeLsi1-1 decreased in response to Si treatments. The results of this study will further clarify the molecular mechanisms underlying Si absorption and accumulation in bamboo plants.

Structure and function of a silicic acid channel Lsi1

Silicon is a beneficial element for plant growth and production, especially in rice. Plant roots take up silicon in the form of silicic acid. Silicic acid channels, which belong to the NIP subfamily of aquaporins, are responsible for silicic acid uptake. Accumulated experimental results have deepened our understanding of the silicic acid channel for its uptake mechanism, physiological function, localization, and other aspects. However, how the silicic acid channel efficiently and selectively permeates silicic acid remains to be elucidated. Recently reported crystal structures of the silicic acid channel enabled us to discuss the mechanism of silicic acid uptake by plant roots at an atomic level. In this mini-review, we focus on the crystal structures of the silicic acid channel and provide a detailed description of the structural determinants of silicic acid permeation and its transport mechanism, which are crucial for the rational creation of secure and sustainable crops.

Genome-wide identification of 14-3-3 gene family and characterization of their expression in developmental stages of Solanum tuberosum under multiple biotic and abiotic stress conditions

GF14 proteins are a family of conserved proteins involved in many cellular processes including transport, growth, metabolism, and stress response. However, only few reports are available regarding the 14-3-3 genes in potato. In this study, twelve 14-3-3 genes were detected in the potato genome. Based on their phylogenetic relationships, the StGF14 family members were categorized into two classes. Gene expression analysis demonstrated that StGF14h, StGF14a, and StGF14k had the highest gene expression, induced by abiotic and biotic stresses in all three tissues. The number of exons in 14-3-3 genes ranged from four to seven and most of these genes in the same subfamily had similar exon-intron patterns. The results of our study showed that the conserved motifs are similar in most of the proteins in each group. The intron-exon patterns and the composition of conserved motifs validated the 14-3-3 gene phylogenetic classification. According to the genome distribution results, 14-3-3 genes were located unevenly on the 12 Solanum tuberosum chromosomes. We find out 97 orthologous gene pairs between potato and Arabidopsis as well as 15 paralogous genes among potato genomes. Our results showed that GF-14 genes have an effective role in functional and molecular mechanisms in response to environmental stresses.

Characterization of silicon transporter gene family in Saccharum and functional analysis of the ShLsi6 gene in biotic stress

Silicon, one of the most prevalent elements in the soil, is beneficial for plant growth and defense against different stresses. The silicon transporter gene (Lsi) plays an important role in the uptake and transport of silicon in higher plants. In this study, a total of 32 Lsi genes, including 20 SsLsi in sugarcane wild species Saccharum spontaneum, 5 ShLsi in Saccharum hybrid cultivar R570 and 7 SbLsi in sugarcane related species Sorghum bicolor, were identified and classified into three groups. Bioinformatics analysis showed that instability, hydrophobicity, localization of cell membranes and vacuoles were the main features of the Lsi proteins. Whole genome and segmental duplication contributed to the main expansion of Lsi gene family. Collinearity analysis of the Lsi genes showed that S. spontanum and R570 had a collinear relationship with monocotyledonous plants S. bicolor and Oryza sativa, but not with dicotyledonous plants Arabidopsis thaliana and Vitis vinifera. The replicated Lsi genes were mainly subjected to strong selection pressure for purification. The diverse cis-regulatory elements in the promoter of SsLsi, ShLsi and SbLsi genes suggested that they were widely involved in the response of plants to various stresses and the regulation of the growth and development. Transcriptome data and real time quantitative PCR analysis showed that the Lsi genes exhibited different expression profiles in sugarcane tissues and under Sporisorium scitamineum, drought and cold stresses. In addition, the cDNA and genomic DNA sequences of ShLsi6 that was homologous to SsLsi1b gene was cloned from Saccharum hybrid cultivar ROC22. Transient expression analysis showed that, compared with the control, Nicotiana benthamiana leaves which overexpressed the ShLsi6 gene showed a high sensitivity after inoculation with tobacco pathogens Ralstonia solanacearum and Fusarium solani var. coeruleum. This study provides important information for further functional analysis of Lsi genes and resistant breeding in sugarcane.

Understanding the Relationship between Water Availability and Biosilica Accumulation in Selected C4 Crop Leaves: An Experimental Approach

Biosilica accumulation in plant tissues is related to the transpiration stream, which in turn depends on water availability. Nevertheless, the debate on whether genetically and environmentally controlled mechanisms of biosilica deposition are directly connected to water availability is still open. We aim at clarifying the system which leads to the deposition of biosilica in Sorghum bicolor, Pennisetum glaucum, and Eleusine coracana, expanding our understanding of the physiological role of silicon in crops well-adapted to arid environments, and simultaneously advancing the research in archaeological and paleoenvironmental studies. We cultivated ten traditional landraces for each crop in lysimeters, simulating irrigated and rain-fed scenarios in arid contexts. The percentage of biosilica accumulated in leaves indicates that both well-watered millet species deposited more biosilica than the water-stressed ones. By contrast, sorghum accumulated more biosilica with respect to the other two species, and biosilica accumulation was independent of the water regime. The water treatment alone did not explain either the variability of the assemblage or the differences in the biosilica accumulation. Hence, we hypothesize that genetics influence the variability substantially. These results demonstrate that biosilica accumulation differs among and within C4 species and that water availability is not the only driver in this process.

Impact of Silica Ions and Nano Silica on Growth and Productivity of Pea Plants under Salinity Stress

The present study was conducted to evaluate the effects of silicon (Si) and nano-silicon (NSi) on growth, yield, ions content, and antioxidant defense systems, including transcript levels of enzyme-encoding genes in Pisum sativum plants grown under salinity stress. Both Si and NSi were applied at the 3 mM level and NaCl was applied at 4 concentrations (100, 150, 200 and 250 mM). Vegetative growth, including plant height, leaf area, fresh and dry weights, and yield attributes were determined. Gene expression of antioxidant enzymes was analyzed, and their activities were determined. The results showed that salinity had deleterious effects on plant growth and yield. Salt-stressed plant leaves exhibited a greater activity of superoxide dismutase (SOD), peroxidase (POD), but a lower activity of catalase (CAT) when compared to the control. Na⁺ ions accumulated in roots and shoots of salinized plants. The application of Si and NSi significantly enhanced vegetative growth and relative water content (RWC), and caused significant increases in plant height, fresh and dry weight, total yield, and antioxidant defense systems. Si and NSi enhanced K⁺ content in roots and shoots under salinity treatment and decreased Na⁺ content in the studied tissues. It was concluded that the application of NSi was beneficial in improving the salt tolerance of Pisum sativum plants more than Si alone.

Genome-wide identification of StU-box gene family and assessment of their expression in developmental stages of Solanum tuberosum

Background

The Plant U-box (PUB), ubiquitin ligase gene, has a highly conserved domain in potato. However, little information is available about U-box genes in potato (Solanum tuberosum). In this study, 62 U-box genes were detected in the potato genome using bioinformatics methods. Further, motif analysis, gene structure, gene expression, TFBS, and synteny analysis were performed on the U-box genes.

Results

Based on in silico analysis, most of StU-boxs included a U-box domain; however, some of them lacked harbored domain the ARM, Pkinase_Tyr, and other domains. Based on their phylogenetic relationships, the StU-box family members were categorized into four classes. Analysis of transcription factor binding sites (TFBS) in the promoter region of StU-box genes revealed that StU-box genes had the highest and the lowest number of TFBS in MYB and CSD, respectively. Moreover, based on in silico and gene expression data, variable frequencies of TFBS in StU-box genes could indicate that these genes control different developmental stages and are involved in complex regulatory mechanisms. The number of exons in U-box genes ranged from one to sixteen. For most U-box genes, the exon–intron compositions and conserved motifs composition in most proteins in each group were similar. The intron–exon patterns and the composition of conserved motifs validated the U-box genes phylogenetic classification. Based on the results of genome distribution, StU-box genes were distributed unevenly on the 12 S. tuberosum chromosomes. The results showed that gene duplication may possess a significant role in genome expansion of S. tuberosum. Furthermore, genome evolution of S. tuberosum was surveyed using identification of orthologous and paralogous. We identified 40 orthologous gene pairs between S. tuberosum with Solanum lycopersicum, Oryza sativa, Triticum aestivum, Gossypium hirsutum, Zea maize, Coriaria mytifolia, and Arabidopsis thaliana as well as eight duplicated genes (paralogous) in S. tuberosum. StU-box 51 gene is one of the important gene among other StU-boxes in S. tuberosum under drought stress which was expressed in tuber and leaf under drought stress. Furthermore, StU-box 51 gene has the highest expression levels in four tissue-specific (stem, root, leaf, and tuber) in potato as well as it had the highest number of TFBS in promoter region. Based on our results, StU-box 51 can introduce to researcher to utilize in breeding program and genetic engineering in potato.

Conclusions

The results of this survey will be useful for further investigation of the probable role and molecular mechanisms of U-box genes in response to different stresses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-022-00306-7.

Exploring the significant contribution of silicon in regulation of cellular redox homeostasis for conferring stress tolerance in plants

Silicon (Si), a bioactive metalloid is beneficial for plant growth and development. It also plays a key role in the amelioration of different abiotic and biotic stresses. Extensive studies have elucidated the morpho-physiological, biochemical and molecular background of Si-mediated stress tolerance in plants. However, the mechanism acquired by Si to enhance stress tolerance in plants is still unheeded. Present review summarized the prospective mechanisms of Si in acquisition of stress tolerance with emphasis on its interactions with secondary messengers. Silicon usually modulates the different gene expressions in plants under stress conditions rather than acting as a direct signal or secondary messengers. Silicon regulates the production and accumulation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in plants under stress conditions. Furthermore, Si also activates the antioxidant defence system in plants; thereby, maintaining the cellular redox homeostasis and preventing the oxidative damage of cells. Silicon also up-regulates the synthesis of hydrogen sulfide (H₂S) or acts synergistically with nitric oxide (NO), consequently conferring stress tolerance in plants. Overall, the review may provide a progressive understanding of the role of Si in conservation of the redox homeostasis in plants.

Silicon modulates nitro-oxidative homeostasis along with the antioxidant metabolism to promote drought stress tolerance in lentil plants

Lentil is the fifth most important grain legume growing in arid/semi-arid regions of the world. Drought is one of the major constraints leading up to 50% of production losses just in lentil. Application of silicon (Si) has been shown to be a promising solution to improve drought tolerance; however, the biochemical mechanisms and interactions involved are not fully understood, especially in legumes. This study was designed to evaluate the effects of Si on drought stress tolerance of lentil genotypes. Seven lentil genotypes with different drought tolerance levels (tolerant, moderately tolerant and sensitive) were subjected to moderate and severe drought stress at the onset of the reproductive stage. Results showed that different drought stress treatments significantly decreased the above ground biomass, water status and the concentration of chlorophyll pigments, whereas Si supplementation of drought stressed lentil genotypes significantly improved the same traits, irrespective of their drought tolerant levels. On the other hand, Si effect on osmoregulation leads to a decline in the membrane damage and osmolytes (proline and glycine betaine) concentration in drought-stressed lentil. Application of Si to drought-stressed lentil plants significantly maintained the nitro-oxidative homeostasis by balancing the concentrations of reactive oxygen/nitrogen species, superoxide anion, hydrogen peroxide and nitrous oxide, thereby reducing the oxidative damage caused due to drought stress. Furthermore, Si supplementation also stimulated the efficiency of the glutathione (GSH)-ascorbate (ASC) cycle by increasing the concentrations of GSH and ASC as well as the activities of antioxidant enzymes like ascorbate peroxidase, guaiacol peroxidase, catalase, superoxide dismutase, glutathione reductase, dehydro-ascorbate reductase and nitrate reductase for better protection of cell membranes from reactive oxygen species. Although Si showed the same regulatory mechanisms in all the studied genotypes to protect lentil plants from moderate and severe drought stress, the defensive role of Si against drought stress was more conspicuous in drought sensitive genotypes than in the tolerant ones. Thus, this study suggests the protective role of Si on drought-stressed lentil genotypes through the modulation of nitro-oxidative homeostasis and antioxidant defence responses.

Physiological and molecular insights involved in silicon uptake and transport in ryegrass

The silicon (Si) uptake system of two ryegrass (Lolium perenne L.) cultivars was characterised by assessing the concentration- and time-dependent kinetics. Additionally, a Si transporter gene was isolated from ryegrass and their expression pattern was analysed. The concentration-dependent kinetics was examined in Jumbo and Nui cultivars supplied with 0, 0.5, 1.0, 2.0, and 4.0 mM Si and harvested at 24 h and 21 d. The time-dependent kinetics was evaluated at 0, 0.5, or 2 mM Si doses after 0, 3, 6, 9, 12, and 24 h. RACE-PCR was performed to isolate a full-length sequence codifying for a Si transporter, and semi-quantitative and quantitative RT-PCR was used to analyse its expression pattern. Differential Si uptake between ryegrass cultivars was found. Moreover, Lineweaver-Burk linearization showed similar V_max values between cultivars; however, different K_m suggested that Jumbo and Nui may have different affinities for silicic acid. The dissimilarities in K_m between cultivars might involve either the differential contribution of known proteins responsible for Si uptake and transport or the involvement of undiscovered Si transporters. We identified a putative Si transporter from ryegrass Nui (LpLsi1), which was only expressed in roots and down-regulated by Si supply. The predicted amino acid sequence of LpLsi1 did not only show a high similarity and close phylogenetic relationship with monocot Si influx transporters but also indicated that it is a membrane protein possessing a high conservation of domains essential for silicic acid selectivity. Our findings provide evidence of LpLsi1 in ryegrass, which supports its high Si accumulation capacity.

New evidence defining the evolutionary path of aquaporins regulating silicon uptake in land plants

Understanding the evolution events defining silicon (Si) uptake in plant species is important for the efficient exploration of Si-derived benefits. In the present study, Si accumulation was studied in 456 diverse plant species grown in uniform field conditions, and in a subset of 151 species grown under greenhouse conditions, allowing efficient comparison among the species. In addition, a systematic analysis of nodulin 26-like intrinsic proteins III (NIP-III), which form Si channels, was performed in >1000 species to trace their evolutionary path and link with Si accumulation. Significant variations in Si accumulation were observed among the plant species studied. For their part, species lacking NIP-IIIs systematically showed low Si accumulation. Interestingly, seven NIP-IIIs were identified in three moss species, namely Physcomitrella patens, Andreaea rupestris, and Scouleria aquatica, indicating that the evolution of NIP-IIIs dates back as early as 515 million years ago. These results were further supported from previous reports of Si deposition in moss fossils estimated to be from around the Ordovician era. The taxonomical distribution provided in the present study will be helpful for several other disciplines, such as palaeoecology and geology, that define the biogeochemical cycling of Si. In addition to the prediction of Si uptake potential of plant species based on sequence information and taxonomical positioning, the evolutionary path of the Si uptake mechanism described here will be helpful to understand the Si environment over the different eras of land plant evolution.

In-silico Exploration of Channel Type and Efflux Silicon Transporters and Silicification Proteins in 80 Sequenced Viridiplantae Genomes

Silicon (Si) accumulation protects plants from biotic and abiotic stresses. It is transported and distributed within the plant body through a cooperative system of channel type (e.g., OsLsi1) and efflux (Lsi2s e.g., OsLsi2) Si transporters (SITs) that belong to Noduline-26 like intrinsic protein family of aquaporins and an uncharacterized anion transporter family, respectively. Si is deposited in plant tissues as phytoliths and the process is known as biosilicification but the knowledge about the proteins involved in this process is limited. In the present study, we explored channel type SITs and Lsi2s, and siliplant1 protein (Slp1) in 80 green plant species. We found 80 channel type SITs and 133 Lsi2s. The channel type SITs characterized by the presence of two NPA motifs, GSGR or STAR selectivity filter, and 108 amino acids between two NPA motifs were absent from Chlorophytes, while Streptophytes evolved two different types of channel type SITs with different selectivity filters. Both channel type SITs and Lsi2s evolved two types of gene structures each, however, Lsi2s are ancient and were also found in Chlorophyta. Homologs of Slp1 (225) were present in almost all Streptophytes regardless of their Si accumulation capacity. In Si accumulator plant species, the Slp1s were characterized by the presence of H, D-rich domain, P, K, E-rich domain, and P, T, Y-rich domain, while moderate Si accumulators lacked H, D-rich domain and P, T, Y-rich domains. The digital expression analysis and coexpression networks highlighted the role of channel type and Lsi2s, and how Slp1 homologs were ameliorating plants’ ability to withstand different stresses by co-expressing with genes related to structural integrity and signaling. Together, the in-silico exploration made in this study increases our knowledge of the process of biosilicification in plants.

Silicon flow from root to shoot in pepper: a comprehensive in silico analysis reveals a potential linkage between gene expression and hormone signaling that stimulates plant growth and metabolism

Background

Silicon (Si) is categorized as a quasi-essential element for plants thanks to the benefits on growth, development and metabolism in a hormetic manner. Si uptake is cooperatively mediated by Lsi1 and Lsi2. Nevertheless, Lsi channels have not yet been identified and characterized in pepper (Capsicum annuum), while genes involved in major physiological processes in pepper are Si-regulated. Furthermore, Si and phytohormones may act together in regulating plant growth, metabolism and tolerance against stress. Our aim was to identify potential synergies between Si and phytohormones stimulating growth and metabolism in pepper, based on in silico data.

Methods

We established a hydroponic system to test the effect of Si (0, 60, 125 and 250 mg L⁻¹ Si) on the concentrations of this element in different pepper plant tissues. We also performed an in silico analysis of putative Lsi genes from pepper and other species, including tomato (Solanum lycopersicum), potato (Solanum tuberosum) and Arabidopsis thaliana, to look for cis-acting elements responsive to phytohormones in their promoter regions. With the Lsi1 and Lsi2 protein sequences from various plant species, we performed a phylogenetic analysis. Taking into consideration the Lsi genes retrieved from tomato, potato and Arabidopsis, an expression profiling analysis in different plant tissues was carried out. Expression of Si-regulated genes was also analyzed in response to phytohormones and different plant tissues and developmental stages in Arabidopsis.

Results

Si concentrations in plant tissues exhibited the following gradient: roots > stems > leaves. We were able to identify 16 Lsi1 and three Lsi2 genes in silico in the pepper genome, while putative Lsi homologs were also found in other plant species. They were mainly expressed in root tissues in the genomes analyzed. Both Lsi and Si-regulated genes displayed cis-acting elements responsive to diverse phytohormones. In Arabidopsis, Si-regulated genes were transcriptionally active in most tissues analyzed, though at different expressed levels. From the set of Si-responsive genes, the NOCS2 gene was highly expressed in germinated seeds, whereas RABH1B, and RBCS-1A, were moderately expressed in developed flowers. All genes analyzed showed responsiveness to phytohormones and phytohormone precursors.

Conclusion

Pepper root cells are capable of absorbing Si, but small amounts of this element are transported to the upper parts of the plant. We could identify putative Si influx (Lsi1) and efflux (Lsi2) channels that potentially participate in the absorption and transport of Si, since they are mainly expressed in roots. Both Lsi and Si-regulated genes exhibit cis-regulatory elements in their promoter regions, which are involved in phytohormone responses, pointing to a potential connection among Si, phytohormones, plant growth, and other vital physiological processes triggered by Si in pepper.

Putative Silicon Transporters and Effect of Temperature Stresses and Silicon Supplementation on Their Expressions and Tissue Silicon Content in Poinsettia

Silicon (Si) is a beneficial element for plants. To understand Si uptake and accumulation in poinsettia, the Si transporters and their expression patterns were investigated. Nodulin 26-like intrinsic membrane proteins (NIPs) act as transporters of water and small solutes, including silicic acid. In this study, one NIP member, designated EpLsi1, was identified. Additionally, a protein from the citrate transporter family, designated EpLsi2, was identified. Sequence analyses indicated that EpLsi1 belonged to the NIP-I subgroup, which has a low Si uptake capacity. Consistently, the measured tissue Si content in the poinsettia was less than 1.73 ± 0.17 mg·g⁻¹ dry weight, which was very low when compared to that in high Si accumulators. The expressions of EpLsi1 and EpLsi2 in poinsettia cuttings treated with 0 mg·L⁻¹ Si decreased under temperature stresses. A short-term Si supplementation decreased the expressions of both EpLsi1 and EpLsi2 in the roots and leaves, while a long-term Si supplementation increased the expression of EpLsi1 in the leaves, bracts, and cyathia, and increased the expression of EpLsi2 in the roots and leaves. Tissue Si content increased in the roots of cuttings treated with 75 mg·L⁻¹ Si at both 4 and 40 °C, indicating that the transport activities of the EpLsi1 were enhanced under temperature stresses. A long-term Si supplementation increased the tissue Si content in the roots of poinsettia treated with 75 mg·L⁻¹ Si. Overall, poinsettia was a low Si accumulator, the expressions of Si transporters were down-regulated, and the tissue Si content increased with temperature stresses and Si supplementation. These results may help the breeding and commercial production of poinsettia.

Cadmium phytoextraction from contaminated paddy soil as influenced by EDTA and Si fertilizer

The efficiency of heavy metal (HM) phytoextraction from contaminated soil directly depends on the pollutant bioavailability, which can be increased by some soil amendments. In field test, the impacts of soil-applied ethylenediaminetetraacetic acid (EDTA) and amorphous silicon dioxide (ASD) and foliar-applied monosilicic acid (MS) on cadmium (Cd) uptake by rice plants from contaminated paddy soil were investigated. Without EDTA, the solid or liquid Si materials reduced the Cd accumulation in the aboveground part of rice by 26 to 52%. If EDTA was applied, the Cd accumulation by plants was increased by 60 to 92%; however, the biomass was reduced by 16 to 35%. The combined application of Si-rich materials and EDTA provided enhanced plant tolerance to a negative influence of EDTA, while kept high Cd content in the rice stems and leaves. As a result, the Cd amounts extracted by the stems and leaves from the unit area of contaminated paddy soil were greater by 25 and 37% in comparison with those for only EDTA treatment.

Phytolith Formation in Plants: From Soil to Cell

Silica is deposited extra- and intracellularly in plants in solid form, as phytoliths. Phytoliths have emerged as accepted taxonomic tools and proxies for reconstructing ancient flora, agricultural economies, environment, and climate. The discovery of silicon transporter genes has aided in the understanding of the mechanism of silicon transport and deposition within the plant body and reconstructing plant phylogeny that is based on the ability of plants to accumulate silica. However, a precise understanding of the process of silica deposition and the formation of phytoliths is still an enigma and the information regarding the proteins that are involved in plant biosilicification is still scarce. With the observation of various shapes and morphologies of phytoliths, it is essential to understand which factors control this mechanism. During the last two decades, significant research has been done in this regard and silicon research has expanded as an Earth-life science superdiscipline. We review and integrate the recent knowledge and concepts on the uptake and transport of silica and its deposition as phytoliths in plants. We also discuss how different factors define the shape, size, and chemistry of the phytoliths and how biosilicification evolved in plants. The role of channel-type and efflux silicon transporters, proline-rich proteins, and siliplant1 protein in transport and deposition of silica is presented. The role of phytoliths against biotic and abiotic stress, as mechanical barriers, and their use as taxonomic tools and proxies, is highlighted.

Silicon Improves the Production of High Antioxidant or Structural Phenolic Compounds in Barley Cultivars under Aluminum Stress

Aluminum (Al) toxicity is one of the main growth and yield limiting factors for barley grown on acid soils. Silicon (Si) ameliorates Al toxicity as well as it promotes the phenolic compounds production that have antioxidant or structural role. We evaluated the time-dependent kinetics of Al and Si uptake and the impact of Si on the production of antioxidant- or structural- phenols in barley cultivars at the short-term. Two barley cultivars with contrasting Al tolerance (Hordeum vulgare ‘Sebastian’, Al tolerant; and H. vulgare ‘Scarlett’, Al sensitive), exposed to either −Al (0 mM) or +Al (0.2 mM) nutrient solutions without Si (−Si) or with 2 mM (+Si) were cultured for 48 h. Aluminum and Si concentration decreased in plants at all harvest times when Al and Si were simultaneously supplied; this effect was more noticeable in ‘Scarlett’. Nevertheless, Si influenced the antioxidant system of barley irrespective of the Al tolerance of the cultivar, decreasing oxidative damage and enhancing radical scavenging activity, the production of phenolic compounds, and lignin accumulation in barley with short-term exposure to Al.

Silicon Affects Root Development, Tissue Mineral Content, and Expression of Silicon Transporter Genes in Poinsettia (Euphorbia pulcherrima Willd.) Cultivars

The effects of silicon (Si) on root development, mineral content, and expression of Si transporter genes in Euphorbia pulcherrima Willd. ‘Flame’, ‘Mable Bell’, ‘Green Star’, ‘Pink Bell’, and ‘Peach Bowl’ cultivars were investigated in this study. Stem cuttings in a propagation bench were drenched regularly with a solution containing either 0 (control) or 50 ppm of silicon (Si treatment) from potassium silicate (K₂SiO₃), with a 25 °C mean air temperature and 80% relative humidity (RH) under 70% shading. The results showed that the ‘Flame’ treated with Si had a significantly higher survival ratio as compared with that of the control (P ≤ 0.05) and that the Si treatment improved number of roots, length of longest root, fresh root weight, and dry root weight in all cultivars except ‘Mable Bell’. Supplementary Si increased the content of magnesium (Mg) and decreased the content of boron (B) and zinc (Zn) in the roots. The content of sulfur (S) in the shoots was increased by supplementary Si. The relative expression of Lsi1 and Lsi2 was higher in ‘Peach Bowl’, while it was lower in ‘Mable Bell’ and ‘Green Star’, which may be caused by the differing accumulation of Si in the shoot. Overall, supplementary Si had beneficial effects during cutting propagation of poinsettia cultivars, although these effects were cultivar-dependent.

Silicon and Nitrate Differentially Modulate the Symbiotic Performances of Healthy and Virus-Infected Bradyrhizobium-nodulated Cowpea (Vigna unguiculata), Yardlong Bean (V. unguiculata subsp. sesquipedalis) and Mung Bean (V. radiata)

The effects of 2 mM silicon (Si) and 10 mM KNO₃ (N)-prime signals for plant resistance to pathogens-were analyzed in healthy and Cowpea chlorotic mottle virus (CCMV) or Cowpea mild mottle virus (CMMV)-infected Bradyrhizobium-nodulated cowpea, yardlong bean and mung bean plants. In healthy plants of the three Vigna taxa, nodulation and growth were promoted in the order of Si + N > N > Si > controls. In the case of healthy cowpea and yardlong bean, the addition of Si and N decreased ureide and α-amino acids (AA) contents in the nodules and leaves in the order of Si + N> N > Si > controls. On the other hand, the addition of N arrested the deleterious effects of CCMV or CMMV infections on growth and nodulation in the three Vigna taxa. However, the addition of Si or Si + N hindered growth and nodulation in the CCMV- or CMMV-infected cowpea and yardlong bean, causing a massive accumulation of ureides in the leaves and nodules. Nevertheless, the AA content in leaves and nodules of CCMV- or CMMV-infected cowpea and yardlong bean was promoted by Si but reduced to minimum by Si + N. These results contrasted to the counteracting effects of Si or Si + N in the CCMV- and CMMV-infected mung bean via enhanced growth, nodulation and levels of ureide and AA in the leaves and nodules. Together, these observations suggest the fertilization with Si + N exclusively in virus-free cowpea and yardlong bean crops. However, Si + N fertilization must be encouraged in virus-endangered mung bean crops to enhance growth, nodulation and N-metabolism. It is noteworthy to see the enhanced nodulation of the three Vigna taxa in the presence of 10 mM KNO₃.

Silicon Regulates Antioxidant Activities of Crop Plants under Abiotic-Induced Oxidative Stress: A Review

Silicon (Si) is the second most abundant element in soil, where its availability to plants can exhilarate to 10% of total dry weight of the plant. Si accumulation/transport occurs in the upward direction, and has been identified in several crop plants. Si application has been known to ameliorate plant growth and development during normal and stressful conditions over past two-decades. During abiotic (salinity, drought, thermal, and heavy metal etc) stress, one of the immediate responses by plant is the generation of reactive oxygen species (ROS), such as singlet oxygen (¹O₂), superoxide ([Formula: see text]), hydrogen peroxide (H₂O₂), and hydroxyl radicals (OH), which cause severe damage to the cell structure, organelles, and functions. To alleviate and repair this damage, plants have developed a complex antioxidant system to maintain homeostasis through non-enzymatic (carotenoids, tocopherols, ascorbate, and glutathione) and enzymatic antioxidants [superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX)]. To this end, the exogenous application of Si has been found to induce stress tolerance by regulating the generation of ROS, reducing electrolytic leakage, and malondialdehyde (MDA) contents, and immobilizing and reducing the uptake of toxic ions like Na, under stressful conditions. However, the interaction of Si and plant antioxidant enzyme system remains poorly understood, and further in-depth analyses at the transcriptomic level are needed to understand the mechanisms responsible for the Si-mediated regulation of stress responses.