Contents Summary 67 I. Introduction 68 II. Silicon transport in plants: to absorb or not to absorb 69 III. The role of silicon in plants: not just a matter of semantics 71 IV. Silicon and biotic stress: beyond mechanical barriers and defense priming 76 V. Silicon and abiotic stress: a proliferation of proposed mechanisms 78 VI. The apoplastic obstruction hypothesis: a working model 79 VII. Perspectives and conclusions 80 Acknowledgements 81 References 81 SUMMARY: Silicon (Si) is not classified as an essential plant nutrient, and yet numerous reports have shown its beneficial effects in a variety of species and environmental circumstances. This has created much confusion in the scientific community with respect to its biological roles. Here, we link molecular and phenotypic data to better classify Si transport, and critically summarize the current state of understanding of the roles of Si in higher plants. We argue that much of the empirical evidence, in particular that derived from recent functional genomics, is at odds with many of the mechanistic assertions surrounding Si's role. In essence, these data do not support reports that Si affects a wide range of molecular-genetic, biochemical and physiological processes. A major reinterpretation of Si's role is therefore needed, which is critical to guide future studies and inform agricultural practice. We propose a working model, which we term the 'apoplastic obstruction hypothesis', which attempts to unify the various observations on Si's beneficial influences on plant growth and yield. This model argues for a fundamental role of Si as an extracellular prophylactic agent against biotic and abiotic stresses (as opposed to an active cellular agent), with important cascading effects on plant form and function.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.15343

The controversies of silicon's role in plant biology.

Salt stress is a major threat for plant growth worldwide. The regulatory mechanisms of silicon in alleviating salt stress have been widely studied using physiological, molecular genetics, and genomic approaches. Recently, progresses have been made in elucidating the alleviative effects of silicon in salt-induced osmotic stress, Na toxicity, and oxidative stress. In this review, we highlight recent development on the impact of silicon application on salt stress responses. Emphasis will be given to the following aspects. (1) Silicon transporters have been experimentally identified in different plant species and their structure feature could be an important molecular basis for silicon permeability. (2) Silicon could mediate salt-induced ion imbalance by (i) regulating Na+ uptake, transport, and distribution and (ii) regulating polyamine levels. (3) Si-mediated upregulation of aquaporin gene expression and osmotic adjustment play important roles in alleviating salinity-induced osmotic stress. (4) Silicon application direct/indirectly mitigates oxidative stress via regulating the antioxidant defense and polyamine metabolism. (5) Omics studies reveal that silicon could regulate plants' response to salt stress by modulating the expression of various genes including transcription factors and hormone-related genes. Finally, research areas that require further investigation to provide a deeper understanding of the role of silicon in plants are highlighted.

https://www.mdpi.com/2223-7747/8/6/147/pdf

Role of Silicon in Mediating Salt Tolerance in Plants: A Review.

Silicon (Si) is the most copious element of existence in the lithosphere but still it has not been added into the essential element list. The imperative role of Si in triggering growth and development of plants has been identified. It is of paramount importance in regulating overall physiological and metabolic characteristics of the plants. Being considered as a non-essential element, it has been known to occur at about 30%, majority of its presence is there in rocks as mineral salts. It has been regarded as multitalented or quasi-element on earth's crust that can be efficiently taken up by plants and translocated further towards aerial parts via transpiration phenomenon. It has also been known to mitigate different biotic and abiotic stressed conditions from plants as the need of the hour owing to its eco-friendly nature. However, the mechanisms associated with their stress attenuation are associated with Reactive Oxygen Species (ROS) scavenging, activation of antioxidative defense responses and phytohormonal signaling. Also, biotic stress factors can be ameliorated through accumulation of Si within epidermal tissues or pathogenesis-related host defense mechanisms. To explore further, omics-mediated studies have been further discussed to shed light on the stress mitigating processes. Further, to improve our understanding for Si-mediated benefits in plants we need to explore the molecular mechanisms of Si uptake, transport and gene expression studies revealing their mitigate properties. In the present review, we have evolved the Si-based studies in plants associated with its transport, uptake and accumulation. Apart from this, we have also discussed about their role in ameliorating stresses from plants by activating their defenses. Moreover, their roles in plant hormonal crosstalk have also been elucidated. Above all, we have also revealed the role of Si-Nanoparticles (SiNPs) in improving stress potential of plants along with stimulation of plant productivities via omics-based approaches.

Silicon and Plants: Current Knowledge and Future Prospects

Salinity affects around 20% of all arable land while an even larger area suffers from recurrent drought. Together these stresses suppress global crop production by as much as 50% and their impacts are predicted to be exacerbated by climate change. Infrastructure and management practices can mitigate these detrimental impacts, but are costly. Crop breeding for improved tolerance has had some success but is progressing slowly and is not keeping pace with climate change. In contrast, Silicon (Si) is known to improve plant tolerance to a range of stresses and could provide a sustainable, rapid and cost-effective mitigation method. The exact mechanisms are still under debate but it appears Si can relieve salt stress via accumulation in the root apoplast where it reduces "bypass flow of ions to the shoot. Si-dependent drought relief has been linked to lowered root hydraulic conductance and reduction of water loss through transpiration. However, many alternative mechanisms may play a role such as altered gene expression and increased accumulation of compatible solutes. Oxidative damage that occurs under stress conditions can be reduced by Si through increased antioxidative enzymes while Si-improved photosynthesis has also been reported. Si fertilizer can be produced relatively cheaply and to assess its economic viability to improve crop stress tolerance we present a cost-benefit analysis. It suggests that Si fertilization may be beneficial in many agronomic settings but may be beyond the means of smallholder farmers in developing countries. Si application may also have disadvantages, such as increased soil pH, less efficient conversion of crops into biofuel and reduced digestibility of animal fodder. These issues may hamper uptake of Si fertilization as a routine agronomic practice. Here, we critically evaluate recent literature, quantifying the most significant physiological changes associated with Si in plants under drought and salinity stress. Analyses show that metrics associated with photosynthesis, water balance and oxidative stress all improve when Si is present during plant exposure to salinity and drought. We further conclude that most of these changes can be explained by apoplastic roles of Si while there is as yet little evidence to support biochemical roles of this element.

/pdf/is-silicon-a-panacea-for-alleviating-drought-and-salt-stress-3ca6zp9jhs.pdf

Is Silicon a Panacea for Alleviating Drought and Salt Stress in Crops

As sessile organisms, plants are unable to avoid being subjected to environmental stresses that negatively affect their growth and productivity Instead, they utilize various mechanisms at the morphological, physiological, and biochemical levels to alleviate the deleterious effects of such stresses Amongst these, secondary metabolites produced by plants represent an important component of the defense system Secondary metabolites, namely phenolics, terpenes, and nitrogen-containing compounds, have been extensively demonstrated to protect plants against multiple stresses, both biotic (herbivores and pathogenic microorganisms) and abiotic (eg drought, salinity, and heavy metals) The regulation of secondary metabolism by beneficial elements such as silicon (Si) is an important topic Silicon-mediated alleviation of both biotic and abiotic stresses has been well documented in numerous plant species Recently, many studies have demonstrated the involvement of Si in strengthening stress tolerance through the modulation of secondary metabolism In this review, we discuss Si-mediated regulation of the synthesis, metabolism, and modification of secondary metabolites that lead to enhanced stress tolerance, with a focus on physiological, biochemical, and molecular aspects Whilst mechanisms involved in Si-mediated regulation of pathogen resistance via secondary metabolism have been established in plants, they are largely unknown in the case of abiotic stresses, thus leaving an important gap in our current knowledge

Integration of silicon and secondary metabolites in plants: a significant association in stress tolerance.

Silicon (Si), the quasi-essential element occurs as the second most abundant element in the earth’s crust. Biological importance of Si in plant kingdom has become inevitable particularly under stressed environment. In general, plants are classified as high, medium, and low silicon accumulators based on the ability of roots to absorb Si. The uptake of Si directly influence the positive effects attributed to the plant but Si supplementation proves to mitigate stress and recover plant growth even in low accumulating plants like tomato. The application of Si in soil as well as soil-less cultivation systems have resulted in the enhancement of quantitative and qualitative traits of plants even under stressed environment. Silicon possesses several mechanisms to regulate the physiological, biochemical, and antioxidant metabolism in plants to combat abiotic and biotic stresses. Nevertheless, very few reports are available on the aspect of Si-mediated molecular regulation of genes with potential role in stress tolerance. The recent advancements in the era of genomics and transcriptomics have opened an avenue for the determination of molecular rationale associated with the Si amendment to the stress alleviation in plants. Therefore, the present endeavor has attempted to describe the recent discoveries related to the regulation of vital genes involved in photosynthesis, transcription regulation, defense, water transport, polyamine synthesis, and housekeeping genes during abiotic and biotic stress alleviation by Si. Furthermore, an overview of Si-mediated modulation of multiple genes involved in stress response pathways such as phenylpropanoid pathway, jasmonic acid pathway, ABA-dependent or independent regulatory pathway have been discussed in this review.

Yul-Kuyn Ahn

Papers

Silicon Regulates Potential Genes Involved in Major Physiological Processes in Plants to Combat Stress