Consistent alleviation of abiotic stress with silicon addition: a meta‐analysis

Silicon in paddy fields: Benefits for rice production and the potential of rice phytoliths for biogeochemical carbon sequestration

Silicon (Si) biogeochemical cycling is beneficial for crop productivity and carbon (C) sequestration in agricultural ecosystem, thus offering a nonnegligible role in alleviating global warming and food crisis. Compared with other crops, rice plants have a greater quantity of phytolith production, because they are able to take up a lot of Si. However, it remains unclear on Si supply capacity of paddy soils across the world, general rice yield-increasing effect after Si fertilizer addition, and factors affecting phytolith production and potential of phytolith C sequestration in paddy fields. This study used a meta-analysis of >3500 data from 87 studies to investigate Si supply capacity of global paddy soils and elaborate the benefits of Si regarding rice productivity and phytolith C sequestration in paddy fields. Analytical results showed that the Si supply capacity of paddy soils was insufficient in the major rice producing countries/regions. Dealing with this predicament, Si fertilization was an effective strategy to supply plant-available Si to improve rice productivity. Our meta-analysis results further revealed that Si fertilization led to the average increasing rate of 36 % and 39 % in rice yield and biomass, which could reach up to 52 % and 46 % with the increasing doses of Si fertilizer, respectively. Especially, this strategy also improved the potential of phytolith C sequestration through the increased phytolith content and rice biomass, despite that this potential might have a decline in old paddy soils (≥ 7000 year) compared to in young paddy soils (≤ 1000 year) due to the slow migration and dissolution of phytoliths at millennial scale. Our findings thus indicate that a deep investigation on the benefits of Si in agroecosystem will further improve our understanding on regulating crop production and the potential of biogeochemical C sequestration within phytoliths in global cropland.

Silicon: A valuable soil element for improving plant growth and CO2 sequestration

BACKGROUND

Silicon (Si), the second most abundant and quasi-essential soil element, is locked as a recalcitrant silicate mineral in the Earth's crust. The physical abundance of silicates can play an essential role in increasing plant productivity. Plants store Si as biogenic silica (phytoliths), which is mobilized through a chemical weathering process in the soil.

AIM OF REVIEW

Although Si is a critical element for plant growth, there is still a considerable need to understand its dissolution, uptake, and translocation in agroecosystems. Here, we show recent progress in understanding the interactome of Si, CO2, the microbiome, and soil chemistry, which can sustainably govern silicate dissolution and cycling in agriculture.

KEY SCIENTIFIC CONCEPTS OF THIS REVIEW

Si cycling is directly related to carbon cycling, and the resulting climate stability can be enhanced by negative feedback between atmospheric CO2 and the silicate uptake process. Improved Si mobilization in the rhizosphere by the presence of reactive elements (for example, Ca, Na, Al, Zn, and Fe) and Si uptake through genetic transporters in plants are crucial to achieving the dual objectives of (i) enhancing crop productivity and (ii) abiotic stress tolerance. Furthermore, the microbiome is a symbiotic partner of plants. Bacterial and fungal microbiomes can solubilize silicate minerals through intriguingly complex bioweathering mechanisms by producing beneficial metabolites and enzymes. However, the interaction of Si with CO2 and the microbiome's function in mobilization have been understudied. This review shows that enhancing our understanding of Si, CO2, the microbiome, and soil chemistry can help in sustainable crop production during climatic stress events.

Silicon regulates phosphate deficiency through involvement of auxin and nitric oxide in barley roots

MAIN CONCLUSION

Silicon application mitigates phosphate deficiency in barley through an interplay with auxin and nitric oxide, enhancing growth, photosynthesis, and redox balance, highlighting the potential of silicon as a fertilizer for overcoming nutritional stresses. Silicon (Si) is reported to attenuate nutritional stresses in plants, but studies on the effect of Si application to plants grown under phosphate (Pi) deficiency are still very scarce, especially in barley. Therefore, the present work was undertaken to investigate the potential role of Si in mitigating the adverse impacts of Pi deficiency in barley Hordeum vulgare L. (var. BH902). Further, the involvement of two key regulatory signaling molecules--auxin and nitric oxide (NO)--in Si-induced tolerance against Pi deficiency in barley was tested. Morphological attributes, photosynthetic parameters, oxidative stress markers (O2·-, H2O2, and MDA), antioxidant system (enzymatic--APX, CAT, SOD, GR, DHAR, MDHAR as well as non-enzymatic--AsA and GSH), NO content, and proline metabolism were the key traits that were assessed under different treatments. The P deficiency distinctly declined growth of barley seedlings, which was due to enhancement in oxidative stress leading to inhibition of photosynthesis. These results were also in parallel with an enhancement in antioxidant activity, particularly SOD and CAT, and endogenous proline level and its biosynthetic enzyme (P5CS). The addition of Si exhibited beneficial effects on barley plants grown in Pi-deficient medium as reflected in increased growth, photosynthetic activity, and redox balance through the regulation of antioxidant machinery particularly ascorbate-glutathione cycle. We noticed that auxin and NO were also found to be independently participating in Si-mediated improvement of growth and other parameters in barley roots under Pi deficiency. Data of gene expression analysis for PHOSPHATE TRANSPORTER1 (HvPHT1) indicate that Si helps in increasing Pi uptake as per the need of Pi-deficient barley seedlings, and also auxin and NO both appear to help Si in accomplishing this task probably by inducing lateral root formation. These results are suggestive of possible application of Si as a fertilizer to correct the negative effects of nutritional stresses in plants. Further research at genetic level to understand Si-induced mechanisms for mitigating Pi deficiency can be helpful in the development of new varieties with improved tolerance against Pi deficiency, especially for cultivation in areas with Pi-deficient soils.

Silicon reduces toxicity and accumulation of arsenic and cadmium in cereal crops: A meta-analysis, mechanism, and perspective study

Arsenic (As) and cadmium (Cd) are two toxic metal(loid)s that pose significant risks to food security and human health. Silicon (Si) has attracted substantial attention because of its positive effects on alleviating the toxicity and accumulation of As and Cd in crops. However, our current knowledge of the comprehensive effects and detailed mechanisms of Si amendment is limited. In this study, a global meta-analysis of 248 original articles with over 7000 paired observations was conducted to evaluate Si-mediated effects on growth and As and Cd accumulation in rice (Oryza sativa L.), wheat (Triticum aestivum L.), and maize (Zea mays L.). Si application increases the biomass of these crops under As and/or Cd contamination. Si amendment also decreased shoot As and Cd accumulation by 24.1 % (20.6 to 27.5 %) and 31.9 % (29.0 to 31.9 %), respectively. Furthermore, the Si amendment reduced the human health risks posed by As (2.6 %) and Cd (12.9 %) in crop grains. Si-induced inhibition of Cd accumulation is associated with decreased Cd bioavailability and the downregulation of gene expression. The regulation of gene expression by Si addition was the driving factor limiting shoot As accumulation. Overall, our analysis demonstrated that Si amendment has great potential to reduce the toxicity and accumulation of As and/or Cd in crops, providing a scientific basis for promoting food safety globally.

Microbiome structure variation and soybean's defense responses during flooding stress and elevated CO2

Introduction

With current trends in global climate change, both flooding episodes and higher levels of CO2 have been key factors to impact plant growth and stress tolerance. Very little is known about how both factors can influence the microbiome diversity and function, especially in tolerant soybean cultivars. This work aims to (i) elucidate the impact of flooding stress and increased levels of CO2 on the plant defenses and (ii) understand the microbiome diversity during flooding stress and elevated CO2 (eCO2).

Methods

We used next-generation sequencing and bioinformatic methods to show the impact of natural flooding and eCO2 on the microbiome architecture of soybean plants' below- (soil) and above-ground organs (root and shoot). We used high throughput rhizospheric extra-cellular enzymes and molecular analysis of plant defense-related genes to understand microbial diversity in plant responses during eCO2 and flooding.

Results

Results revealed that bacterial and fungal diversity was substantially higher in combined flooding and eCO2 treatments than in non-flooding control. Microbial diversity was soil>root>shoot in response to flooding and eCO2. We found that sole treatment of eCO2 and flooding had significant abundances of Chitinophaga, Clostridium, and Bacillus. Whereas the combination of flooding and eCO2 conditions showed a significant abundance of Trichoderma and Gibberella. Rhizospheric extra-cellular enzyme activities were significantly higher in eCO2 than flooding or its combination with eCO2. Plant defense responses were significantly regulated by the oxidative stress enzyme activities and gene expression of Elongation factor 1 and Alcohol dehydrogenase 2 in floodings and eCO2 treatments in soybean plant root or shoot parts.

Conclusion

This work suggests that climatic-induced changes in eCO2 and submergence can reshape microbiome structure and host defenses, essential in plant breeding and developing stress-tolerant crops. This work can help in identifying core-microbiome species that are unique to flooding stress environments and increasing eCO2.

Silicon via fertigation with and without potassium application, improve physiological aspects of common beans cultivated under three water regimes in field

Frequent droughts have led to an expansion of irrigated common bean (Phaseolus vulgaris L.) cultivation areas. An effective strategy to enhance water use efficiency and optimize crop growth is the application of silicon (Si) and potassium (K). However, the interaction between Si dosage, water regimes, and plant potassium status, as well as the underlying physiological mechanisms, remains unknown. This study aimed to assess the effects of Si doses applied via fertigation under various water regimes, in the presence and absence of potassium fertilization, on gas exchange, water use efficiency, and growth of Common beans in field conditions. Two experiments were conducted, one with and one without K supply, considering that the potassium content in the soil was 6.4 mmolc dm-3 in both experiments and a replacement dose of 50 kg ha was applied in the with K treatment, with the same treatments evaluated in both potassium conditions. The treatments comprised a 3 × 4 factorial design, encompassing three water regimes: 80% (no deficit), 60% (moderate water deficit), and 40% (severe water deficit) of soil water retention capacity, and four doses of Si supplied via fertigation: 0, 4, 8, and 12 kg ha-1. Where it was evaluated, content of photosynthetic pigments, fluorescence of photosynthesis, relative water content, leaf water potential and electrolyte extravasation, dry mass of leaves, stems and total. The optimal doses of Si for fertigation application, leading to increased Si absorption in plants, varied with decreasing soil water content. The respective values were 6.6, 7.0, and 7.1 kg ha-1 for the water regimes without deficit, with moderate water deficit, and with severe water deficit. Fertigation application of Si improved plant performance, particularly under severe water deficit, regardless of potassium status. This improvement was evident in relative water content, leaf water potential, and membrane resistance, directly impacting pigment content and gas exchange rates. The physiological effects resulted in enhanced photosynthesis in water-deficient plants, mitigating dry mass production losses. This research demonstrates, for the first time in common bean, the potential of Si to enhance irrigation efficiency in areas limited by low precipitation and water scarcity.

Deciphering the mechanisms, hormonal signaling, and potential applications of endophytic microbes to mediate stress tolerance in medicinal plants

The global healthcare market in the post-pandemic era emphasizes a constant pursuit of therapeutic, adaptogenic, and immune booster drugs. Medicinal plants are the only natural resource to meet this by supplying an array of bioactive secondary metabolites in an economic, greener and sustainable manner. Driven by the thrust in demand for natural immunity imparting nutraceutical and life-saving plant-derived drugs, the acreage for commercial cultivation of medicinal plants has dramatically increased in recent years. Limited resources of land and water, low productivity, poor soil fertility coupled with climate change, and biotic (bacteria, fungi, insects, viruses, nematodes) and abiotic (temperature, drought, salinity, waterlogging, and metal toxicity) stress necessitate medicinal plant productivity enhancement through sustainable strategies. Plants evolved intricate physiological (membrane integrity, organelle structural changes, osmotic adjustments, cell and tissue survival, reclamation, increased root-shoot ratio, antibiosis, hypersensitivity, etc.), biochemical (phytohormones synthesis, proline, protein levels, antioxidant enzymes accumulation, ion exclusion, generation of heat-shock proteins, synthesis of allelochemicals. etc.), and cellular (sensing of stress signals, signaling pathways, modulating expression of stress-responsive genes and proteins, etc.) mechanisms to combat stresses. Endophytes, colonizing in different plant tissues, synthesize novel bioactive compounds that medicinal plants can harness to mitigate environmental cues, thus making the agroecosystems self-sufficient toward green and sustainable approaches. Medicinal plants with a host set of metabolites and endophytes with another set of secondary metabolites interact in a highly complex manner involving adaptive mechanisms, including appropriate cellular responses triggered by stimuli received from the sensors situated on the cytoplasm and transmitting signals to the transcriptional machinery in the nucleus to withstand a stressful environment effectively. Signaling pathways serve as a crucial nexus for sensing stress and establishing plants' proper molecular and cellular responses. However, the underlying mechanisms and critical signaling pathways triggered by endophytic microbes are meager. This review comprehends the diversity of endophytes in medicinal plants and endophyte-mediated plant-microbe interactions for biotic and abiotic stress tolerance in medicinal plants by understanding complex adaptive physiological mechanisms and signaling cascades involving defined molecular and cellular responses. Leveraging this knowledge, researchers can design specific microbial formulations that optimize plant health, increase nutrient uptake, boost crop yields, and support a resilient, sustainable agricultural system.

Novel insights into the mechanism(s) of silicon-induced drought stress tolerance in lentil plants revealed by RNA sequencing analysis

BACKGROUND

Lentil is an essential cool-season food legume that offers several benefits in human nutrition and cropping systems. Drought stress is the major environmental constraint affecting lentil plants' growth and productivity by altering various morphological, physiological, and biochemical traits. Our previous research provided physiological and biochemical evidence showing the role of silicon (Si) in alleviating drought stress in lentil plants, while the molecular mechanisms are still unidentified. Understanding the molecular mechanisms of Si-mediated drought stress tolerance can provide fundamental information to enhance our knowledge of essential gene functions and pathways modulated by Si during drought stress in plants. Thus, the present study compared the transcriptomic characteristics of two lentil genotypes (drought tolerant-ILL6002; drought sensitive-ILL7537) under drought stress and investigated the gene expression in response to Si supplementation using high-throughput RNA sequencing.

RESULTS

This study identified 7164 and 5576 differentially expressed genes (DEGs) from drought-stressed lentil genotypes (ILL 6002 and ILL 7537, respectively), with Si treatment. RNA sequencing results showed that Si supplementation could alter the expression of genes related to photosynthesis, osmoprotection, antioxidant systems and signal transduction in both genotypes under drought stress. Furthermore, these DEGs from both genotypes were found to be associated with the metabolism of carbohydrates, lipids and proteins. The identified DEGs were also linked to cell wall biosynthesis and vasculature development. Results suggested that Si modulated the dynamics of biosynthesis of alkaloids and flavonoids and their metabolism in drought-stressed lentil genotypes. Drought-recovery-related DEGs identified from both genotypes validated the role of Si as a drought stress alleviator. This study identified different possible defense-related responses mediated by Si in response to drought stress in lentil plants including cellular redox homeostasis by reactive oxygen species (ROS), cell wall reinforcement by the deposition of cellulose, lignin, xyloglucan, chitin and xylan, secondary metabolites production, osmotic adjustment and stomatal closure.

CONCLUSION

Overall, the results suggested that a coordinated interplay between various metabolic pathways is required for Si to induce drought tolerance. This study identified potential genes and different defence mechanisms involved in Si-induced drought stress tolerance in lentil plants. Si supplementation altered various metabolic functions like photosynthesis, antioxidant defence system, osmotic balance, hormonal biosynthesis, signalling, amino acid biosynthesis and metabolism of carbohydrates and lipids under drought stress. These novel findings validated the role of Si in drought stress mitigation and have also provided an opportunity to enhance our understanding at the genomic level of Si's role in alleviating drought stress in plants.

Is there silicon in flowers and what does it tell us?

The emergence of flowers marked an important development in plant evolution. Flowers in many species evolved to attract animal pollinators to increase fertilisation chances. In leaves, silicon (Si) discourages herbivores, for example by wearing down mouthparts. Flowers are essentially modified leaves and hence may also have the capacity to accumulate Si. If Si in flowers discourages animal visitors as it does in leaves, Si accumulation may be disadvantageous for pollination. Whether flowers accumulate Si, and what the implications may be, was not known for many species. We analysed leaves and flowers of different taxa, separated into their different anatomical parts. Flowers mostly have low Si concentrations in all parts (mean ± SE of BSi in mg g-1 was 0.22 ± 0.04 in petals, 0.59 ± 0.24 in sepals, 0.14 ± 0.03 in stamens, 0.15 ± 0.04 in styles and stigmas and 0.37 ± 0.19 in ovaries for a subset of 56 species). In most cases, less Si was accumulated in flowers than in leaves (mean ± SE of BSi in mg g-1 was 1.51 ± 0.55 in whole flowers vs. 2.97 ± 0.57 in leaves in 104 species) though intriguing exceptions are found, with some species accumulating more Si in flowers than leaves. The large variation in concentration among flowers across the taxa examined, with a particularly high concentration in grass inflorescences, tantalisingly suggests differences in the use of Si for flowers across plant groups. We conclude that the study of the functions of Si for flowers warrants more attention, with pollination strategy a potential contributing factor.

Silicon supply promotes differences in growth and C:N:P stoichiometry between bamboo and tree saplings

BACKGROUND

Si can be important for the growth, functioning, and stoichiometric regulation of nutrients for high-Si-accumulating bamboo. However, other trees do not actively take up dissolved silicic acid [Si(OH)4] from the soil, likely because they have fewer or no specific Si transporters in their roots. It is unclear what causes differential growth and C:N:P stoichiometry between bamboo and other trees across levels of Si supply.

RESULTS

Si supply increased the relative growth rate of height and basal diameter of bamboo saplings, likely by increasing its net photosynthetic rate and ratios of N:P. Moreover, a high concentration of Si supply decreased the ratio of C:Si in bamboo leaves due to a partial substitution of C with Si in organic compounds. We also found that there was a positive correlation between leaf Si concentration and its transpiration rate in tree saplings.

CONCLUSIONS

We demonstrated that Si supply can decrease the ratio of C:Si in bamboo leaves and increase the ratio of N:P without altering nutrient status or the N:P ratio of tree saplings. Our findings provide experimental data to assess the different responses between bamboo and other trees in terms of growth, photosynthesis, and C:N:P stoichiometry. These results have implications for assessing the growth and competition between high-Si-accumulating bamboo and other plants when Si availability is altered in ecosystems during bamboo expansion.

The phytomicrobiome: solving plant stress tolerance under climate change

With extraordinary global climate changes, increased episodes of extreme conditions result in continuous but complex interaction of environmental variables with plant life. Exploring natural phytomicrobiome species can provide a crucial resource of beneficial microbes that can improve plant growth and productivity through nutrient uptake, secondary metabolite production, and resistance against pathogenicity and abiotic stresses. The phytomicrobiome composition, diversity, and function strongly depend on the plant's genotype and climatic conditions. Currently, most studies have focused on elucidating microbial community abundance and diversity in the phytomicrobiome, covering bacterial communities. However, least is known about understanding the holistic phytomicrobiome composition and how they interact and function in stress conditions. This review identifies several gaps and essential questions that could enhance understanding of the complex interaction of microbiome, plant, and climate change. Utilizing eco-friendly approaches of naturally occurring synthetic microbial communities that enhance plant stress tolerance and leave fewer carbon-foot prints has been emphasized. However, understanding the mechanisms involved in stress signaling and responses by phytomicrobiome species under spatial and temporal climate changes is extremely important. Furthermore, the bacterial and fungal biome have been studied extensively, but the holistic interactome with archaea, viruses, oomycetes, protozoa, algae, and nematodes has seldom been studied. The inter-kingdom diversity, function, and potential role in improving environmental stress responses of plants are considerably important. In addition, much remains to be understood across organismal and ecosystem-level responses under dynamic and complex climate change conditions.

Silicon modifies leaf nutriome and improves growth of oak seedlings exposed to phosphorus deficiency and Phytophthora plurivora infection

Beneficial effects of silicon (Si) on plants have primarily been studied in crop species under single stress. Moreover, nutrient acquisition-based responses to combination of biotic and abiotic stresses (a common situation in natural habitats) have rarely been reported, in particular in conjunction with soil amendments with Si. Pedunculate oak (Quercus robur L.), one of the ecologically and economically most important tree species in Europe, is facing a severe decline due to combined stresses, but also problems in assisted regeneration in nurseries. Here, we studied the effect of Si supply on the leaf nutriome, root traits and overall growth of 12-weeks-old oak seedlings exposed to abiotic stress [low phosphorus (P) supply], biotic stress (Phytophthora plurivora root infection), and their combination. The application of Si had the strongest ameliorative effect on growth, root health and root phenome under the most severe stress conditions (i.e., combination of P deficiency and P. plurivora root infection), where it differentially affected the uptake and leaf accumulation in 11 out of 13 analysed nutrients. Silicon supply tended to reverse the pattern of change of some, but not all, leaf nutrients affected by stresses: P, boron (B) and magnesium (Mg) under P deficiency, and P, B and sulphur (S) under pathogen attack, but also nickel (Ni) and molybdenum (Mo) under all three stresses. Surprisingly, Si affected some nutrients that were not changed by a particular stress itself and decreased leaf Mg levels under all the stresses. On the other hand, pathogen attack increased leaf accumulation of Si. This exploratory work presents the complexity of nutrient crosstalk under three stresses, and opens more questions about genetic networks that control plant physiological responses. Practically, we show a potential of Si application to improve P status and root health in oak seedlings, particularly in nurseries.

Drought stress in rice: morpho-physiological and molecular responses and marker-assisted breeding

Rice (Oryza Sativa L.) is an essential constituent of the global food chain. Drought stress significantly diminished its productivity and threatened global food security. This review concisely discussed how drought stress negatively influenced the rice's optimal growth cycle and altered its morpho-physiological, biochemical, and molecular responses. To withstand adverse drought conditions, plants activate their inherent drought resistance mechanism (escape, avoidance, tolerance, and recovery). Drought acclimation response is characterized by many notable responses, including redox homeostasis, osmotic modifications, balanced water relations, and restored metabolic activity. Drought tolerance is a complicated phenomenon, and conventional breeding strategies have only shown limited success. The application of molecular markers is a pragmatic technique to accelerate the ongoing breeding process, known as marker-assisted breeding. This review study compiled information about quantitative trait loci (QTLs) and genes associated with agronomic yield-related traits (grain size, grain yield, harvest index, etc.) under drought stress. It emphasized the significance of modern breeding techniques and marker-assisted selection (MAS) tools for introgressing the known QTLs/genes into elite rice lines to develop drought-tolerant rice varieties. Hence, this study will provide a solid foundation for understanding the complex phenomenon of drought stress and its utilization in future crop development programs. Though modern genetic markers are expensive, future crop development programs combined with conventional and MAS tools will help the breeders produce high-yielding and drought-tolerant rice varieties.

Silicon in Plants: Alleviation of Metal(loid) Toxicity and Consequential Perspectives for Phytoremediation

For the majority of higher plants, silicon (Si) is considered a beneficial element because of the various favorable effects of Si accumulation in plants that have been revealed, including the alleviation of metal(loid) toxicity. The accumulation of non-degradable metal(loid)s in the environment strongly increased in the last decades by intensified industrial and agricultural production with negative consequences for the environment and human health. Phytoremediation, i.e., the use of plants to extract and remove elemental pollutants from contaminated soils, has been commonly used for the restoration of metal(loid)-contaminated sites. In our viewpoint article, we briefly summarize the current knowledge of Si-mediated alleviation of metal(loid) toxicity in plants and the potential role of Si in the phytoremediation of soils contaminated with metal(loid)s. In this context, a special focus is on metal(loid) accumulation in (soil) phytoliths, i.e., relatively stable silica structures formed in plants. The accumulation of metal(loid)s in phytoliths might offer a promising pathway for the long-term sequestration of metal(loid)s in soils. As specific phytoliths might also represent an important carbon sink in soils, phytoliths might be a silver bullet in the mitigation of global change. Thus, the time is now to combine Si/phytolith and phytoremediation research. This will help us to merge the positive effects of Si accumulation in plants with the advantages of phytoremediation, which represents an economically feasible and environmentally friendly way to restore metal(loid)-contaminated sites.

Chitosan/silica: A hybrid formulation to mitigate phytopathogens

Long-term and indiscriminate use of synthetic pesticides to mitigate plant pathogens have created serious issues of water health, soil contamination, non-target organisms, resistant species, and unpredictable environmental and human health hazards. These constraints have forced scientists to develop alternative plant disease management strategies to reduce synthetic chemical' dependency. During the last 20 years, biological agents and resistance elicitors have been the most important used alternatives. Silica-based materials/chitosan with a dual mode of action have been proposed as promising alternatives to prevent plant diseases through direct and indirect mechanisms. Moreover, the combined application of nano-silica and chitosan, due to their controllable morphology, high loading capacity, low toxicity, and efficient encapsulation, act as suitable carriers for biological agents, pesticides, and essential oils, making them proper candidates for mitigation of phytopathogens. Based on this potential, this literature study reviewed the silica and chitosan properties and their function in the plant. It also assessed their role in the fighting against soil and aerial phytopathogens, directly and indirectly, as novel hybrid formulations in future managing platforms.

Why do plants silicify?

Despite seminal papers that stress the significance of silicon (Si) in plant biology and ecology, most studies focus on manipulations of Si supply and mitigation of stresses. The ecological significance of Si varies with different levels of biological organization, and remains hard to capture. We show that the costs of Si accumulation are greater than is currently acknowledged, and discuss potential links between Si and fitness components (growth, survival, reproduction), environment, and ecosystem functioning. We suggest that Si is more important in trait-based ecology than is currently recognized. Si potentially plays a significant role in many aspects of plant ecology, but knowledge gaps prevent us from understanding its possible contribution to the success of some clades and the expansion of specific biomes.

Do genotypes ameliorate herbivory stress through silicon amendments differently? A case study of wheat

Agricultural productivity relies on plant resistance to insect pests, with silicon (Si) being increasingly recognized as an important anti-herbivore defense. However, the processes by which Si works to counteract the effects of insect injury are not completely understood. The role of Si in mitigating the adverse effects of herbivory has been mostly studied at the species level in various crops, ignoring the sensitivity and variability at the genotypic level. Understanding such variation across genotypes is important because Si-derived benefits are associated with the amount of Si accumulated in the plant. Therefore, the present investigation was pursued to study the effect of different Si concentrations (0, 125, and 250 mg L⁻¹) on Si accumulation and plant growth using two wheat genotypes (WW-101 and SW-2) under grasshopper herbivory for 48 h. The higher Si absorption increased the concentration of leaf chlorophyll, carotenoids, soluble sugars, and proteins. Silicon application at higher concentrations increased the dry weight, antioxidant enzyme activity, total phenolics, flavonoids and shoot Si concentration, whereas it decreased the electrolyte leakage, hydrogen peroxide (H₂O₂) and malonaldehyde (MDA) levels, thereby preventing leaf damage. We infer that the higher Si concentration alleviates the adverse effects of herbivory in wheat by improving the accumulation of secondary metabolites and enhancing the antioxidant defense system. The effects were pronounced in the genotype 'WW-101' compared to 'SW-2' for most of the studied traits, indicating overall stress response to be genotype-dependent. Thus, Si acquisition efficiency of genotypes should be considered while developing efficient crop management strategies.

Climatic Drivers of Silicon Accumulation in a Model Grass Operate in Low- but Not High-Silicon Soils

Grasses are hyper-accumulators of silicon (Si), which is known to alleviate diverse environmental stresses, prompting speculation that Si accumulation evolved in response to unfavourable climatic conditions, including seasonally arid environments. We conducted a common garden experiment using 57 accessions of the model grass Brachypodium distachyon, sourced from different Mediterranean locations, to test relationships between Si accumulation and 19 bioclimatic variables. Plants were grown in soil with either low or high (Si supplemented) levels of bioavailable Si. Si accumulation was negatively correlated with temperature variables (annual mean diurnal temperature range, temperature seasonality, annual temperature range) and precipitation seasonality. Si accumulation was positively correlated with precipitation variables (annual precipitation, precipitation of the driest month and quarter, and precipitation of the warmest quarter). These relationships, however, were only observed in low-Si soils and not in Si-supplemented soils. Our hypothesis that accessions of B. distachyon from seasonally arid conditions have higher Si accumulation was not supported. On the contrary, higher temperatures and lower precipitation regimes were associated with lower Si accumulation. These relationships were decoupled in high-Si soils. These exploratory results suggest that geographical origin and prevailing climatic conditions may play a role in predicting patterns of Si accumulation in grasses.

New approaches to the effects of Si on sugarcane ratoon under irrigation in Quartzipsamments, Eutrophic Red Oxisol, and Dystrophic Red Oxisol

BACKGROUND

C:N:P homeostasis in plants guarantees optimal levels of these nutrients in plant metabolism. H However, one of the causes to the effects of deficit irrigation is the loss of C:N:P homeostasis in leaves and stems that causes reduction in the growth of sugarcane. Being able to measure the impact of water deficit on C:N:P homeostasis in plants from the stoichiometric ratios of the concentrations of these nutrients in leaves and stems. This loss causes a decrease in nutritional efficiency, but can be mitigated with the use of silicon. Silicon favors the homeostasis of these nutrients and crop productivity. The magnitude of this benefit depends on the absorption of Si by the plant and Si availability in the soil, which varies with the type of soil used. Thus, this study aims to evaluate whether the application of Si via fertigation is efficient in increasing the absorption of Si and whether it is capable of modifying the homeostatic balance of C:N:P of the plant, causing an increase in nutritional efficiency and consequently in the production of biomass in leaves and stems of sugarcane ratoon cultivated with deficient and adequate irrigations in different tropical soils.

RESULTS

Water deficit caused biological losses in concentrations and accumulation of C, N, and P, and reduced the nutrient use efficiency and biomass production of sugarcane plants cultivated in three tropical soils due to disturbances in the stoichiometric homeostasis of C:N:P. The application of Si increased the concentration and accumulation of Si, C, N, and P and their use efficiency and reduced the biological damage caused by water deficit due to the modification of homeostatic balance of C:N:P by ensuring sustainability of the production of sugarcane biomass in tropical soils. However, the intensity of attenuation of such deleterious effects stood out in plants cultivated in Eutrophic Red Oxisols. Si contributed biologically by improving the performance of sugarcane ratoon with an adequate irrigation due to the optimization of stoichiometric ratios of C:N:P; increased the accumulation and the use efficiency of C, N, and P, and promoted production gains in biomass of sugarcane in three tropical soils.

CONCLUSION

Our study shows that fertigation with Si can mitigate the deleterious effects of deficient irrigation or potentiate the beneficial effects using an adequate irrigation system due to the induction of a new stoichiometric homeostasis of C:N:P, which in turn improves the nutritional efficiency of sugarcane cultivated in tropical soils.

New strategy for silicon supply through fertigation in sugarcane integrating the pre-sprouted seedling phase and field cultivation

Adopting a Si supply strategy can amplify the sugarcane response. Thus, this study aimed to verify whether Si supply in the pre-sprouted seedling (PSS) formation phase would have an effect after field transplanting similar to Si supply only in the field phase (via foliar spraying or fertigation). Furthermore, this study aimed to verify whether Si supply in the PSS formation phase associated with Si fertigation after transplanting can potentiate or amplify Si benefits. Two experiments were conducted. In experiment I, pre-sprouted seedlings were grown in a nursery without Si (Control) and with Si. Experiment II was conducted in the field on Eutrustox soil with the following treatments: no Si supply (Control); Si supplied during the PSS formation phase; Si supplied through foliar spraying in the field; Si supplied through fertigation in the field; Si supplied in the PSS formation phase and during field development. Silicon used in both crop phases benefited sugarcane by increasing photosynthetic pigment content and the antioxidative defense system. The innovation of Si management to be supplied via fertigation integrated with both crop phases (PSS and in the field) optimizes the element's use by increasing the crop's productivity and sustainability.

Impact of Silicon on Plant Nutrition and Significance of Silicon Mobilizing Bacteria in Agronomic Practices

Globally, rejuvenation of soil health is a major concern due to the continuous loss of soil fertility and productivity. Soil degradation decreases crop yields and threatens global food security. Improper use of chemical fertilizers coupled with intensive cultivation further reduces both soil health and crop yields. Plants require several nutrients in varying ratios that are essential for the plant to complete a healthy growth and development cycle. Soil, water, and air are the sources of these essential macro- and micro-nutrients needed to complete plant vegetative and reproductive cycles. Among the essential macro-nutrients, nitrogen (N) plays a significant in non-legume species and without sufficient plant access to N lower yields result. While silicon (Si) is the 2nd most abundant element in the Earth’s crust and is the backbone of soil silicate minerals, it is an essential micro-nutrient for some plants. Silicon is just beginning to be recognized as an important micronutrient to some plant species and, while it is quite abundant, Si is often not readily available for plant uptake. The manufacturing cost of synthetic silica-based fertilizers is high, while absorption of silica is quite slow in soil for many plants. Rhizosphere biological weathering processes includes microbial solubilization processes that increase the dissolution of minerals and increases Si availability for plant uptake. Therefore, an important strategy to improve plant silicon uptake could be field application of Si-solubilizing bacteria. In this review, we evaluate the role of Si in seed germination, growth, and morphological development and crop yield under various biotic and abiotic stresses, different pools and fluxes of silicon (Si) in soil, and the bacterial genera of the silicon solubilizing microorganisms. We also elaborate on the detailed mechanisms of Si-solubilizing/mobilizing bacteria involved in silicate dissolution and uptake by a plant in soil. Last, we discuss the potential of silicon and silicon solubilizing/mobilizing to achieve environmentally friendly and sustainable crop production.

Impact of Nanotechnology from Nanosilica to Mitigate N and P Deficiencies Favoring the Sustainable Cultivation of Sugar Beet

This research aimed to study the effects of the nanosilica supply on Si absorption and the physiological and nutritional aspects of beet plants with N and P deficiencies cultivated in a nutrient solution. Two experiments were performed with treatments arranged in a 2 × 2 factorial scheme in randomized blocks with five replications. The first experiment was carried out on plants under a N deficiency and complete (complete solution with all nutrients), combined with the absence of Si (0 mmol L^-1) and the presence of Si (2.0 mmol L^-1). In the other experiment, the plants were cultivated in a nutrient solution with a P deficiency and complete, combined with the absence (0 mmol L^-1) and the presence of Si (2.0 mmol L^-1). The beet crop was sensitive to the N and P deficiencies because they sustained important physiological damage. However, using nanosilica via fertigation could reverse the damage. Using nanotechnology from nanosilica constituted a sustainable strategy to mitigate the damage due to a deficiency in the beet crop of the two most limiting nutrients by optimizing the physiological processes, nutritional efficiency, and growth of the plants without environmental risks. The future perspective is the feasibility of nanotechnology for food security.

Quercetin ameliorates chromium toxicity through improvement in photosynthetic activity, antioxidative defense system; and suppressed oxidative stress in Trigonella corniculata L

Environmental stresses, including heavy metals accumulation, have posed an immense threat to the agricultural ecosystem, leading to a reduction in the yield of crucial crops. In this study, we evaluated the role of quercetin (Qu) in the alleviation of chromium (Cr) stress in Fenugreek (Trigonella corniculata L.). Different levels of Qu were prepared during the experiment, i.e., 15, 25, and 40 μM. For Cr toxification in potted soil, potassium chromate (K₂Cr₂O₇) was used. Cr toxification reduced growth of T. corniculata seedlings. Cr stress also reduced fiber, ash, moisture, carbohydrate, protein, fats, and flavonoid contents. However, seed priming with Qu improved growth and physiochemical characteristics of T. corniculata seedlings grown in normal and Cr-contaminated soil. Seed priming with Qu escalated intercellular CO₂ concentration, stomatal conductance, transpiration rate, and photosynthetic rate in T. corniculata seedlings. Application of Qu also increased the activity of antioxidative enzymes, i.e., superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POD) in T. corniculata seedlings exposed to normal and Cr-contaminated soil. Application of Qu incremented the activity of SOD, POD, CAT, and APX, which were increased by 28, 22, 29, and 33%, respectively, in T. corniculata grown in Cr-toxic soil as compared to control treatment. Chromium stress alleviation was credited to the enhanced activity of the antioxidative defensive system in T. corniculata seedlings. It is proposed that Qu supplementation can be used to mitigate other abiotic stresses in plants.

Field application of silicon alleviates drought stress and improves water use efficiency in wheat

Detrimental impacts of drought on crop yield have tripled in the last 50 years with climate models predicting that the frequency of such droughts will intensify in the future. Silicon (Si) accumulation, especially in Poaceae crops such as wheat (Triticum aestivum L.), may alleviate the adverse impacts of drought. We have very limited information, however, about whether Si supplementation could alleviate the impacts of drought under field conditions and no studies have specifically manipulated rainfall. Using field-based rain exclusion shelters, we determined whether Si supplementation (equivalent to 39, 78 and 117 kg ha^-1) affected T. aestivum growth, elemental chemistry [Si, carbon (C) and nitrogen (N)], physiology (rates of photosynthesis, transpiration, stomatal conductance, and water use efficiency) and yield (grain production) under ambient and drought (50% of ambient) rainfall scenarios. Averaged across Si treatments, drought reduced shoot mass by 21% and grain production by 18%. Si supplementation increased shoot mass by up to 43% and 73% in ambient and drought water treatments, respectively, and restored grain production in droughted plants to levels comparable with plants supplied with ambient rainfall. Si supplementation increased leaf-level water use efficiency by 32-74%, depending on Si supplementation rates. Water supply and Si supplementation did not alter concentrations of C and N, but Si supplementation increased shoot C content by 39% and 83% under ambient and drought conditions, respectively. This equates to an increase from 6.4 to 8.9 tonnes C ha^-1 and from 4.03 to 7.35 tonnes C ha^-1 under ambient and drought conditions, respectively. We conclude that Si supplementation ameliorated the negative impacts of drought on T. aestivum growth and grain yield, potentially through its beneficial impacts on water use efficiency. Moreover, the beneficial impacts of Si on plant growth and C storage may render Si supplementation a useful tool for both drought mitigation and C sequestration.

Leaf surfaces and neolithization - the case of Arundo donax L

Arundo donax L. (Arundinoideae subfamily, Poaceae family) is a sub-tropical and temperate climate reed that grows in arid and semi-arid environmental conditions, from eastern China to the Mediterranean basin, suggesting potential adaptations at the epicuticular level. A thorough physical-chemical examination of the adaxial and abaxial surfaces of A. donax leaf was performed herein in an attempt to track such chemophenetic adaptations. This sort of approach is of the utmost importance for the current debate about the hypothetical invasiveness of this species in the Mediterranean basin versus its natural colonization along the Plio-Pleistocene period. We concluded that the leaf surfaces contain, apart from stomata, prickles, and long, thin trichomes, and silicon-rich tetralobate phytolits. Chemically, the dominating elements in the leaf ashes are oxygen and potassium; minor amounts of calcium, silicon, magnesium, phosphorous, sulphur, and chlorine were also detected. In both surfaces the epicuticular waxes (whose density is higher in the adaxial surface than in the abaxial surface) form randomly orientated platelets, with irregular shape and variable size, and aggregated rodlets with variable diameter around the stomata. In the case of green mature leaves, the dominating organic compounds of the epicuticular waxes of both surfaces are triterpenoids. Both surfaces feature identical hydrophobic behaviour, and exhibit the same total transmittance, total reflectance, and absorption of incident light. The above findings suggest easy growth of the plant, remarkable epidermic robustness of the leaf, and control of water loss. These chemophenetic characteristics and human influence support a neolithization process of this species along the Mediterranean basin.

Phytolith-occluded carbon in leaves of Dendrocalamus Ronganensis influenced by drought during growing season

Being an important carbon (C) sink, phytolith-occluded carbon (PhytOC) has been investigated in various soil-plant systems. However, the effects of environmental factors (i.e., drought) on phytoliths, including altered deposition in plant tissues, morphological variation, and amounts of carbon occluded within phytoliths, are less studied. In this study, we analyzed the monthly variations of phytolith production and PhytOC in the leaves of Dendrocalamus ronganensis grown on a karst mountain in southwestern China during a drought year. This study thus sought to understand the effects of drought on phytolith formation, morphological variations and carbon sequestration within phytoliths in plants. Our results showed that the phytolith assemblages and PhytOC between new and old leaves differed significantly and varied with plant growth stages. The average PhytOC values of old leaves and tip leaves were 3.2% and 2.2%, respectively. In particular, both PhytOC and proportions of ELONGATE, BULLIFORM FLABELLATE, and STOMA phytoliths in tip leaves significantly decreased from September to January the following year because of drought effects. This study suggests that PhytOC in plants varies between phytolith morphotypes and is significantly affected by plant growth stage and hydrologic conditions. This indicates that we can improve the efficiency of phytolith carbon sequestration in plants by improving the soil water conditions required for plant growth.

The role of silicon in the mitigation of water stress in Eugenia myrcianthes Nied. seedlings

Silicon (Si) is a beneficial element that can mitigate effects of water stress on photosynthetic metabolism and plant growth. Thus, the aimed was to evaluate the effect of Si in mitigating the stressful effect of water deficit and flooding in Eugenia myrcianthes Nied. seedlings. The seedlings received three silicon doses (0, 2, and 4 mmol) and were subjected to two water regimes (I - continuous irrigation and S - water fluctuation, characterized as water stress obtained by two cycles of water regimes: irrigation suspension and flooding). Each cycle was ended when the seedlings had a photosynthetic rate close to zero (P0) when the stressful irrigation condition was normalized until the photosynthetic rate reached the values of the control seedlings (REC). The evaluations were carried out in five periods: T0 - initial seedling condition; 1st and 2nd P0; and 1st and 2nd REC. The E. myrcianthes seedlings reached P0 at 22 and 50 days under water deficit and flooding, respectively. Water stress caused damage to photochemical activities in photosystem II. E. myrcianthes is a species sensitive to water stress, but capable of adjusting to water fluctuation, and the application of 2 mmol Si contributed to the regulation of gas exchange, photochemical yields, and growth of this species at the deficit and flooding phases. We emphasize that E. myrcianthes seedlings have potential for resilience due to physiological plasticity, regardless of the silicon application.

Exogenous silicon alleviates the adverse effects of cinnamic acid-induced autotoxicity stress on cucumber seedling growth

Autotoxicity is a key factor that leads to obstacles in continuous cropping systems. Although Si is known to improve plant resistance to biotic and abiotic stresses, little is known about its role in regulating leaf water status, mineral nutrients, nitrogen metabolism, and root morphology of cucumber under autotoxicity stress. Here, we used cucumber seeds (Cucumis sativus L. cv. "Xinchun No. 4") to evaluate how exogenous Si (1 mmol L^-1) affected the leaf water status, mineral nutrient uptake, N metabolism-related enzyme activities, root morphology, and shoot growth of cucumber seedlings under 0.8 mmol L^-1 CA-induced autotoxicity stress. We found that CA-induced autotoxicity significantly reduced the relative water content and water potential of leaves and increase their cell sap concentration. CA-induced stress also inhibited the absorption of major (N, P, K, Ca, Mg) and trace elements (Fe, Mn, Zn). However, exogenous Si significantly improved the leaf water status (relative water content and water potential) of cucumber leaves under CA-induced stress. Exogenous Si also promoted the absorption of mineral elements by seedlings under CA-induced stress and alleviated the CA-induced inhibition of N metabolism-related enzyme activities (including nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase, glutamate dehydrogenase). Moreover, exogenous Si improved N uptake and utilization, promoted root morphogenesis, and increased the growth indexes of cucumber seedlings under CA-induced stress. Our findings have far-reaching implications for overcoming the obstacles to continuous cropping in cucumber cultivation.

The role of silicon in regulating physiological and biochemical mechanisms of contrasting bread wheat cultivars under terminal drought and heat stress environments

The individual and cumulative effects of drought stress (DS) and heat stress (HS) are the primary cause of grain yield (GY) reduction in a rainfed agricultural system. Crop failures due to DS and HS are predicted to increase in the coming years due to increasingly severe weather events. Plant available silicon (Si, H4SiO4) has been widely reported for its beneficial effects on plant development, productivity, and attenuating physiological and biochemical impairments caused by various abiotic stresses. The current study investigated the impact of pre-sowing Si treatment on six contrasting wheat cultivars (four drought and heat stress-tolerant and two drought and heat stress-susceptible) under individual and combined effects of drought and heat stress at an early grain-filling stage. DS, HS, and drought-heat combined stress (DHS) significantly (p < 0.05) altered morpho-physiological and biochemical attributes in susceptible and tolerant wheat cultivars. However, results showed that Si treatment significantly improved various stress-affected morpho-physiological and biochemical traits, including GY (>40%) and yield components. Si treatment significantly (p < 0.001) increased the reactive oxygen species (ROS) scavenging antioxidant activities at the cellular level, which is linked with higher abiotic stress tolerance in wheat. With Si treatment, osmolytes concentration increased significantly by >50% in tolerant and susceptible wheat cultivars. Similarly, computational water stress indices (canopy temperature, crop water stress index, and canopy temperature depression) also improved with Si treatment under DS, HS, and DHS in susceptible and tolerant wheat cultivars. The study concludes that Si treatment has the potential to mitigate the detrimental effects of individual and combined stress of DS, HS, and DHS at an early grain-filling stage in susceptible and tolerant wheat cultivars in a controlled environment. These findings also provide a foundation for future research to investigate Si-induced tolerance mechanisms in susceptible and tolerant wheat cultivars at the molecular level.

Alleviation of Ammonium Toxicity in Salvia splendens ‘Vista Red’ with Silicon Supplementation

Ammonium (NH₄⁺) toxicity seriously hampers the yield and quality of salvia plants because most varieties or sub-species are highly sensitive to NH₄⁺. Silicon (Si) is an alternative that is used to minimize these disturbances and maintain better growth under NH₄⁺ toxicity. Nevertheless, the mitigatory effects of Si on NH₄⁺-stressed salvia are unknown. Therefore, this study was carried out to determine how Si assists to alleviate the NH₄⁺ toxicity degree in salvia. To this end, salvia plants were cultivated in a controlled environment supplied with a constant N (nitrogen) level (13 meq·L⁻¹) in the form of three NH₄⁺:NO₃⁻ ratios (0:100, 50:50, 100:0), each with (1.0 meq·L⁻¹) or without Si. Physiological disorders and typical NH₄⁺ toxicity symptoms, as well as interrupted photosynthesis, were observed in the 100% NH₄⁺-treated plants. Furthermore, cation uptake inhibition and oxidative damage were also imposed by the 100% NH₄⁺ supply. In contrast, in the presence of Si, the NH₄⁺ toxicity degree was attenuated and plant growth was ensured. Accordingly, the NH₄⁺ toxicity appearance ratio decreased significantly. Furthermore, Si-treated plants showed an ameliorated photosynthetic ability, elevated internal K and Ca levels, and enhanced antioxidative capacity, as reflected by improved major antioxidant enzyme activities, as well as diminished accumulation of ROS (reactive oxygen species) and MDA (malondialdehyde). Our findings enlightened the agronomic importance of additional Si to nutrient solutions, especially pertaining to bedding plants at risk of NH₄⁺ toxicity.

Silicon as a Smart Fertilizer for Sustainability and Crop Improvement

Silicon (Si), despite being abundant in nature, is still not considered a necessary element for plants. Si supplementation in plants has been extensively studied over the last two decades, and the role of Si in alleviating biotic and abiotic stress has been well documented. Owing to the noncorrosive nature and sustainability of elemental Si, Si fertilization in agricultural practices has gained more attention. In this review, we provide an overview of different smart fertilizer types, application of Si fertilizers in agriculture, availability of Si fertilizers, and experiments conducted in greenhouses, growth chambers, and open fields. We also discuss the prospects of promoting Si as a smart fertilizer among farmers and the research community for sustainable agriculture and yield improvement. Literature review and empirical studies have suggested that the application of Si-based fertilizers is expected to increase in the future. With the potential of nanotechnology, new nanoSi (NSi) fertilizer applications may further increase the use and efficiency of Si fertilizers. However, the general awareness and scientific investigation of NSi need to be thoughtfully considered. Thus, we believe this review can provide insight for further research into Si fertilizers as well as promote Si as a smart fertilizer for sustainability and crop improvement.

Interactive effects of hydrogen sulphide and silicon enhance drought and heat tolerance by modulating hormones, antioxidant defence enzymes and redox status in barley (Hordeum vulgare L.)

Recent changes in climate have reduced crop productivity throughout much of the world. Drought and heat stress, particularly in arid and semi-arid regions, have seriously affected barley production. This study explored the separate and interactive effects of silicon (Si) and hydrogen sulphide (H₂ S) on plant growth and mitigation of the adverse effects of heat stress (DS) and drought stress (HS) in a barley pot experiment. The impacts of simultaneous DS + HS were more severe than individual stresses due to increased ROS production, malondialdehyde (MDA) content and higher electrolyte leakage (EL), thereby leading to reduced water, protein and photosynthetic pigment content. Exogenously applied Si and H₂ S alleviated the DS-, HS- and DS + HS-induced effects on barley by reducing ROS production, MDA and EL. A single application of H₂ S or Si + H₂ S increased plant biomass under all stress conditions, which can be ascribed to higher Si accumulation in barley shoots. A single application of Si or H₂ S significantly increased plant biomass. However, Si + H₂ S was the most effective treatment for metabolite accumulation and elevating activity of antioxidant enzymes to prevent toxicity from oxidative stress. This treatment also modulated osmolyte content, enhanced antioxidant activity and regulated the stress signalling-related endogenous hormones, abscisic acid (ABA) and indole acetic acid (IAA). Exogenous treatments regulated endogenous H₂ S and Si and resulted in higher tolerance to individual and combined drought and heat stress in barley.

Silicon mitigates nutritional stress of nitrogen, phosphorus, and calcium deficiency in two forages plants

Forages are one of the most cultivated crops in the world. However, nutritional deficiency is common, specifically in N, P, and Ca in many forage-growing regions. Silicon (Si) can attenuate the stress caused by nutritional deficiency, but studies on Si supply's effects on forage plants are still scarce. This research was carried out to evaluate whether the Si supply can mitigate the effects of N, P, and Ca deficiencies of two forages and the physiological and nutritional mechanisms involved. Two experiments were carried out with two forage species (Urochloa brizantha cv. Marandu and Megathyrsus maximum cv. Massai). We used nutrient solution under balanced nutrition conditions and nutritional stress due to the lack of N, P, and Ca combined with the -Si and +Si. The deficiencies of N, P, and Ca in both forages' cultivation caused damage to physiological and nutritional variables, decreasing the plant dry matter. However, in both forage species, the Si addition to the nutrient solution decreased the extravasation of cellular electrolytes and increased the content of phenolic compounds, the green colour index, the quantum efficiency of photosystem II, the efficiencies of use of N, P and Ca and the production of shoot dry matter. The beneficial effects of Si were evidenced in stressed and non-stressed plants. The research emphasised the advantage of using Si to grow U. brizantha and M. maximum under N, P, and Ca deficiency, contributing to their sustainable cultivation.

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.

Drenched Silicon Suppresses Disease and Insect Pests in Coffee Plant Grown in Controlled Environment by Improving Physiology and Upregulating Defense Genes

Plant disease and insect pests are major limiting factors that reduce crop production worldwide. The ornamental indoor cultivation cash crop dwarf coffee Punica arabica ‘Pacas’ is also troubled by these issues. Silicon (Si) is one of the most abundant elements in the lithosphere and positively impacts plant health by effectively mitigating biotic and abiotic stresses. Several studies have shown that Si activates plant defense systems, although the specific nature of the involvement of Si in biochemical processes that lead to resistance is unclear. In our study, Si significantly promoted the growth and development of dwarf coffee seedlings grown in plant growth chambers. More than that, through natural infection, Si suppressed disease and insect pests by improving physiology (e.g., the strong development of the internal structures of roots, stems, and leaves; higher photosynthetic efficiency; more abundant organic matter accumulation; the promotion of root activity; the efficient absorption and transfer of mineral elements; and various activated enzymes) and up-regulating defense genes (CaERFTF11 and CaERF13). Overall, in agriculture, Si may potentially contribute to global food security and safety by assisting in the creation of enhanced crop types with optimal production as well by mitigating plant disease and insect pests. In this sense, Si is a sustainable alternative in agricultural production.

Multidimensional Role of Silicon to Activate Resilient Plant Growth and to Mitigate Abiotic Stress

Sustainable agricultural production is critically antagonistic by fluctuating unfavorable environmental conditions. The introduction of mineral elements emerged as the most exciting and magical aspect, apart from the novel intervention of traditional and applied strategies to defend the abiotic stress conditions. The silicon (Si) has ameliorating impacts by regulating diverse functionalities on enhancing the growth and development of crop plants. Si is categorized as a non-essential element since crop plants accumulate less during normal environmental conditions. Studies on the application of Si in plants highlight the beneficial role of Si during extreme stressful conditions through modulation of several metabolites during abiotic stress conditions. Phytohormones are primary plant metabolites positively regulated by Si during abiotic stress conditions. Phytohormones play a pivotal role in crop plants' broad-spectrum biochemical and physiological aspects during normal and extreme environmental conditions. Frontline phytohormones include auxin, cytokinin, ethylene, gibberellin, salicylic acid, abscisic acid, brassinosteroids, and jasmonic acid. These phytohormones are internally correlated with Si in regulating abiotic stress tolerance mechanisms. This review explores insights into the role of Si in enhancing the phytohormone metabolism and its role in maintaining the physiological and biochemical well-being of crop plants during diverse abiotic stresses. Moreover, in-depth information about Si's pivotal role in inducing abiotic stress tolerance in crop plants through metabolic and molecular modulations is elaborated. Furthermore, the potential of various high throughput technologies has also been discussed in improving Si-induced multiple stress tolerance. In addition, a special emphasis is engrossed in the role of Si in achieving sustainable agricultural growth and global food security.

Hydrogen peroxide modulates silica deposits in sorghum roots

Hydrated silica (SiO2·nH2O) aggregates in the root endodermis of grasses. Application of soluble silicates (Si) to roots is associated with variations in the balance of reactive oxygen species (ROS), increased tolerance to a broad range of stresses affecting ROS concentrations, and early lignin deposition. In sorghum (Sorghum bicolor L.), silica aggregation is patterned in an active silicification zone (ASZ) by a special type of aromatic material forming a spotted pattern. The deposition has a signature typical of lignin. Since lignin polymerization is mediated by ROS, we studied the formation of root lignin and silica controlled by ROS via modulating hydrogen peroxide (H2O2) concentrations in the growth medium. Sorghum seedlings were grown hydroponically and supplemented with Si, H2O2, and KI, an ionic compound that catalyses H2O2 decomposition. Lignin and silica deposits in the endodermis were studied by histology, scanning electron and Raman microscopies. Cell wall composition was quantified by thermal gravimetric analysis. Endodermal H2O2 concentration correlated to the extent of lignin-like deposition along the root, but did not affect its patterning in spots. Our results show that the ASZ spots were necessary for root silica aggregation, and suggest that silicification is intensified under oxidative stress as a result of increased ASZ lignin-like deposition.

The Ability of Silicon Fertilisation to Alleviate Salinity Stress in Rice is Critically Dependent on Cultivar

Silicon (Si) fertiliser can improve rice (Oryza sativa) tolerance to salinity. The rate of Si uptake and its associated benefits are known to differ between plant genotypes, but, to date, little research has been done on how the benefits, and hence the economic feasibility, of Si fertilisation varies between cultivars. In this study, a range of rice cultivars was grown both hydroponically and in soil, at different levels of Si and NaCl, to determine cultivar variation in the response to Si. There was significant variation in the effect of Si, such that Si alleviated salt-induced growth inhibition in some cultivars, while others were unaffected, or even negatively impacted. Thus, when assessing the benefits of Si supplementation in alleviating salt stress, it is essential to collect cultivar-specific data, including yield, since changes in biomass were not always correlated with those seen for yield. Root Si content was found to be more important than shoot Si in protecting rice against salinity stress, with a root Si level of 0.5-0.9% determined as having maximum stress alleviation by Si. A cost-benefit analysis indicated that Si fertilisation is beneficial in mild stress, high-yield conditions but is not cost-effective in low-yield production systems.

Does silicon really matter for the photosynthetic machinery in plants…?

Silicon (Si) is known to alleviate the adverse impact of different abiotic and biotic stresses by different mechanisms including morphological, physiological, and genetic changes. Photosynthesis, one of the most important physiological processes in the plant is sensitive to different stress factors. Several studies have shown that Si ameliorates the stress effects on photosynthesis by protecting photosynthetic machinery and its function. In stressed plants, several photosynthesis-related processes including PSII maximum photochemical quantum yield (Fv/Fm), the yield of photosystem II (φPSII), electron transport rates (ETR), and photochemical quenching (qP) were observed to be regulated when supplemented with Si, which indicates that Si effectively protects the photosynthetic machinery. In addition, studies also suggested that Si is capable enough to maintain the uneven swelling, disintegrated, and missing thylakoid membranes caused during stress. Furthermore, several photosynthesis-related genes were also regulated by Si supplementation. Taking into account the key impact of Si on the evolutionarily conserved process of photosynthesis in plants, this review article is focused on the aspects of silicon and photosynthesis interrelationships during stress and signaling pathways. The assemblages of this discussion shall fulfill the lack of constructive literature related to the influence of Si on one of the most dynamic and important processes of plant life i.e. photosynthesis.

Functions of silicon in plant drought stress responses

Silicon (Si), the second most abundant element in Earth's crust, exerts beneficial effects on the growth and productivity of a variety of plant species under various environmental conditions. However, the benefits of Si and its importance to plants are controversial due to differences among the species, genotypes, and the environmental conditions. Although Si has been widely reported to alleviate plant drought stress in both the Si-accumulating and nonaccumulating plants, the underlying mechanisms through which Si improves plant water status and maintains water balance remain unclear. The aim of this review is to summarize the morphoanatomical, physiological, biochemical, and molecular processes that are involved in plant water status that are regulated by Si in response to drought stress, especially the integrated modulation of Si-triggered drought stress responses in Si accumulators and intermediate- and excluder-type plants. The key mechanisms influencing the ability of Si to mitigate the effects of drought stress include enhancing water uptake and transport, regulating stomatal behavior and transpirational water loss, accumulating solutes and osmoregulatory substances, and inducing plant defense- associated with signaling events, consequently maintaining whole-plant water balance. This study evaluates the ability of Si to maintain water balance under drought stress conditions and suggests future research that is needed to implement the use of Si in agriculture. Considering the complex relationships between Si and different plant species, genotypes, and the environment, detailed studies are needed to understand the interactions between Si and plant responses under stress conditions.

Supplying silicon alters microbial community and reduces soil cadmium bioavailability to promote health wheat growth and yield

Soil amendments of black bone (BB), biochar (BC), silicon fertilizer (SI), and leaf fertilizer (LF) play vital roles in decreasing cadmium (Cd) availability, thereby supporting healthy plant growth and food security in agroecosystems. However, the effect of their additions on soil microbial community and the resulting soil Cd bioavailability, plant Cd uptake and health growth are still unknown. Therefore, in this study, BB, BC, SI, and LF were selected to evaluate Cd amelioration in wheat grown in Cd-contaminated soils. The results showed that relative to the control, all amendments significantly decreased both soil Cd bioavailability and its uptake in plant tissues, promoting healthy wheat growth and yield. This induced-decrease effect in seeds was the most obvious, wherein the effect was the highest in SI (52.54%), followed by LF (43.31%), and lowest in BC (35.24%) and BB (31.98%). Moreover, the induced decrease in soil Cd bioavailability was the highest in SI (29.56%), followed by BC (28.85%), lowest in LF (17.55%), and BB (15.30%). The significant effect in SI likely resulted from a significant increase in both the soil bioavailable Si and microbial community (Acidobacteria and Thaumarchaeota), which significantly decreased soil Cd bioavailability towards plant roots. In particular, a co-occurrence network analysis indicated that soil microbes played a substantial role in wheat yield under Si amendment. Therefore, supplying Si alters the soil microbial community, positively and significantly interacting with soil bioavailable Si and decreasing Cd bioavailability in soils, thereby sustaining healthy crop development and food quality.

Influence of Silicon on Biocontrol Strategies to Manage Biotic Stress for Crop Protection, Performance, and Improvement

Silicon (Si) has never been acknowledged as a vital nutrient though it confers a crucial role in a variety of plants. Si may usually be expressed more clearly in Si-accumulating plants subjected to biotic stress. It safeguards several plant species from disease. It is considered as a common element in the lithosphere of up to 30% of soils, with most minerals and rocks containing silicon, and is classified as a "significant non-essential" element for plants. Plant roots absorb Si, which is subsequently transferred to the aboveground parts through transpiration stream. The soluble Si in cytosol activates metabolic processes that create jasmonic acid and herbivore-induced organic compounds in plants to extend their defense against biotic stressors. The soluble Si in the plant tissues also attracts natural predators and parasitoids during pest infestation to boost biological control, and it acts as a natural insect repellent. However, so far scientists, policymakers, and farmers have paid little attention to its usage as a pesticide. The recent developments in the era of genomics and metabolomics have opened a new window of knowledge in designing molecular strategies integrated with the role of Si in stress mitigation in plants. Accordingly, the present review summarizes the current status of Si-mediated plant defense against insect, fungal, and bacterial attacks. It was noted that the Si-application quenches biotic stress on a long-term basis, which could be beneficial for ecologically integrated strategy instead of using pesticides in the near future for crop improvement and to enhance productivity.

Silicate fertilization improves microbial functional potentials for stress tolerance in arsenic-enriched rice cropping systems

The host plant and its rhizosphere microbiome are similarly exposed to abiotic stresses under arsenic (As)-enriched cropping systems. Since silicon (Si) fertilization is effective in alleviating As-induced stresses in plants, and plant-microbe interactions are tightly coupled, we hypothesized that Si-fertilization would improve soil microbial functional potentials to environmental stress tolerance, which was not yet studied. With the help of high throughput metagenome, microarray and analyzing plant impacts on soil microbiome and the environment, we tested the hypothesis in two geographically different rice (i.e., Japonica and Indica) grown on As-enriched soils. Silicate fertilization in rice grown on As-enriched soils altered rhizosphere bacterial communities and increased several commensal microorganisms and their genetic potential to tolerate oxidative stress, osmotic stress, oxygen limitation, nitrogen and phosphate limitation, heat and cold shock, and radiation stress. The stress resistant microbial communities shifted with the changes in rhizosphere nutrient flows and cumulative plant impacts on the soil environment. The study highlights a thus-far unexplored behavior of Si-fertilization to improve microbial stress resilience under As-laden cropping systems and opens up a promising avenue to further study how commonalities in plant-microbe signaling in response to Si-fertilization alleviates As-induced stresses in agro-systems.

Leaf silicon accumulation rates in relation to light environment and shoot growth rates in paper mulberry (Broussonetia papyrifera, Moraceae)

While increasing numbers of studies report wide variations of leaf silicon (Si) accumulation among plant species, within-species variations of leaf Si accumulation have scarcely been examined for tree species. As in crop plants, environmental factors that affect transpiration rates may influence passive transpiration-dependent transport of Si uptake in trees. Here, we tested a hypothesis that leaf Si accumulation rate should be higher in shoots that receive more light and thus achieve faster growth, using Broussonetia papyrifera, a pioneer tree species with successive leaf production and Si accumulation with leaf age. We marked individual leaves weekly throughout the growing season (June-September), and measured Si concentration and light availability in relation to the chronosequence of leaf age in September. In shoots that continued growing and successively produced leaves throughout the growing season, leaf Si content increased linearly with leaf age. In support of our hypothesis, leaf Si accumulation rate varied widely among shoots with positive correlations with shoot growth and light availability. In conclusion, both leaf age and microenvironment affect within-species variations in leaf Si concentration of this species, a moderate Si accumulator.

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.

Exploration of silicate solubilizing bacteria for sustainable agriculture and silicon biogeochemical cycle

Silicon (Si), a quasi-essential element for plants, is abundant in the soil typically as insoluble silicate forms. However, plants can uptake Si only in the soluble form of monosilicic acid. Production of monosilicic acid by rock-weathering mostly depends on temperature, pH, redox-potential, water-content, and microbial activities. In the present review, approaches involved in the efficient exploration of silicate solubilizing bacteria (SSB), its potential applications, and available technological advances are discussed. Present understanding of Si uptake, deposition, and subsequent benefits to plants has also been discussed. In agricultural soils, pH is found to be one of the most critical factors deciding silicate solubilization and the formation of different Si compounds. Numerous studies have predicted the role of Indole-3-Acetic Acid (IAA) and organic acids produced by SSB in silicate solubilization. In this regard, approaches for the isolation and characterization of SSB, quantification of IAA, and subsequent Si solubilization mechanisms are addressed. Phylogenetic evaluation of previously reported SSB showed a highly diverse origin which provides an opportunity to study different mechanisms involved in Si solubilization. Soil biochemistry in concern of silicon availability, microbial activity and silicon mediated changes in plant physiology are addressed. In addition, SSB's role in Si-biogeochemical cycling is summarized. The information presented here will be helpful to explore the potential of SSB more efficiently to promote sustainable agriculture.

Population Genomics Reveals Gene Flow and Adaptive Signature in Invasive Weed Mikania micrantha

A long-standing and unresolved issue in invasion biology concerns the rapid adaptation of invaders to nonindigenous environments. Mikania micrantha is a notorious invasive weed that causes substantial economic losses and negative ecological consequences in southern China. However, the contributions of gene flow, environmental variables, and functional genes, all generally recognized as important factors driving invasive success, to its successful invasion of southern China are not fully understood. Here, we utilized a genotyping-by-sequencing approach to sequence 306 M. micrantha individuals from 21 invasive populations. Based on the obtained genome-wide single nucleotide polymorphism (SNP) data, we observed that all the populations possessed similar high levels of genetic diversity that were not constrained by longitude and latitude. Mikania micrantha was introduced multiple times and subsequently experienced rapid-range expansion with recurrent high gene flow. Using FST outliers, a latent factor mixed model, and the Bayesian method, we identified 38 outlier SNPs associated with environmental variables. The analysis of these outlier SNPs revealed that soil composition, temperature, precipitation, and ecological variables were important determinants affecting the invasive adaptation of M. micrantha. Candidate genes with outlier signatures were related to abiotic stress response. Gene family clustering analysis revealed 683 gene families unique to M. micrantha which may have significant implications for the growth, metabolism, and defense responses of M. micrantha. Forty-one genes showing significant positive selection signatures were identified. These genes mainly function in binding, DNA replication and repair, signature transduction, transcription, and cellular components. Collectively, these findings highlight the contribution of gene flow to the invasion and spread of M. micrantha and indicate the roles of adaptive loci and functional genes in invasive adaptation.