Silicon, a metalloid with bluish-grey metallic shine, is known for its semiconductor properties. Since silicon is being widely used for computers and many other semiconductor-based devices, many technological hubs worldwide received the name like Silicon Valley.
Silicon has numerous uses in industrial products, cosmetics, and medicine. Interestingly, studies published over the last couple of decades have demonstrated its benefits to crop plants (Deshmukh et al., 2017). Fundamental studies by Japanese scientists showing high levels of silicon deposition in grasses have paved the way for understanding the role of silicon in plants. Numerous recent reports have shown a link between silicon deposition and stress tolerance in plants. Silicon supplementation has been shown to improve resilience in plants under a variety of stress conditions including diseases and environmental stress like drought, cold and waterlogging. The silicon-derived benefits have been reported more frequently in plant species known to accumulate high levels of silicon. Despite thousands of research reports showing silicon-derived benefits to plants, silicon was never being considered an essential element. Since most of the plant species can complete their life cycle without silicon supplementation. But considering the benefits of silicon particularly under stress conditions, International Plant Nutrition Institute (www.ipni.net) has categorized it as a beneficial element for plant growth. Consequently, many companies have launched silicon-based fertilizers for crops like rice, wheat, maize, sugarcane, and vegetables. Silicon-based fertilizers are gaining popularity in America, Africa, and particularly in South Asian countries like India, Pakistan, and Bangladesh. Major concerns for use of silicon-based fertilizers include the availability of the source, sustainability, and most importantly the risk to the environment.
Silicon to improve resistance against plant diseases
Plant diseases cause the loss of billions of dollars worldwide and enforce a considerable threat to food security. Very frequently, farmers experience a complete loss of crop due to devastating plant diseases. A country like India which is home to over a billion people have already experienced a famine in 1943 caused by rice disease which took the lives of millions of peoples (Mukerjee, 2014). Therefore, sustainable and integrated approaches are required to ensure food security for mankind.
Any pathogen (disease causing bacteria and fungi) invades the host plant by breaking the protective barriers. The barriers include physical as well as chemical protections evolved in the plant system. Silicon uptake and deposition in various plant tissues help to strengthen such barriers against the pathogen (Figure 1). The first report showing silicon-derived improved resistance against Pyricularia oryzae bacteria which cause blight disease in rice was published over a century ago by Onodera. The study with blight disease leads to the hypothesis of mechanical barriers which suggests that the silicon deposition makes leaf surface hard to penetrate by the pathogen (Mandlik et al., 2020). Visualizing the deposition of silicon in different tissues become more convenient with recent advancements in microscopic techniques. Now it is clearer that the silicon gets deposited mostly under the leaf cuticle and several specialized cells like “silica cells” which are more prominently observed in rice leaves. Such a typical pattern of silicon deposition makes it difficult for bacterial and fungal pathogens to penetrate the plant surface. Similarly, silicon is also known to improve the host response by improving or maintaining the desired physiological processes in plant cells. Some of the notable examples of devastating plant disease where silicon supplementation is found to be beneficial include blight and blast disease of rice, soybean rust, powdery mildew in wheat, and anthracnose in sorghum (Sathe et al., 2021).
Figure 1 Images of different Date palm tissues taken with scanning electron microscopy showing a comparative view of a high-resolution image along with the distribution of Silicon (violet color). The figure is reproduced from the article by Bokor et al. (2019) published in Frontiers in Plant Science with Creative Commons Attribution License (CC BY).
Silicon to improve tolerance to abiotic stress
Abiotic stresses imposed by environmental factors cause more than 50% losses in several crops. The scenario of rapidly changing climatic conditions enforces great risk to global food production. Improving plant health through nutrition management seems to be an affordable and sustainable approach. Silicon is known to improve plant resilience under different abiotic stresses like extreme temperature and water regimes, heavy metal tolerance, lodging due to heavy wind, and scorching sunshine with UV radiations. Significant efforts have been made to understand the precise role of silicon and subsequently to know how it provides benefits under different stress. Understanding silicon’s cross-talk with reactive oxygen species (ROS), different phytohormones, and many small signaling molecules is an area where extensive efforts are being made (Deshmukh et al., 2017). Even though the molecular mechanism involved in silicon-derived stress tolerance is not yet fully understood, silicon’s proliferative role is witnessed in hundreds of studies. Silicon supplementation was shown to improve water uptake and water use efficiency under drought stress. Among the several mechanisms, silicon regulates the expression of aquaporins a class of water transporter protein. Similarly, silicon also regulates different proteins involved in the transport of osmolytes which help plants to survive and grow better under drought stress. The improved activity of aquaporin and osmolytes help the plant to increase root biomass. A high root/shoot ratio has been seen in several plant species grown with silicon supplementation. A better root system, high water conductance, and osmolytes help to maintain a high level of photosynthesis.
Silicon supplementation was found to improve seed germination under high temperatures and drought. Both of these stresses usually occur together. Poor seed germination can cause complete crop loss specifically in crops where timely sowing is important. Enhanced antioxidant defense is seen as the desired effect of silicon supplementation which helps to improve seed germination and subsequent growth of seedlings (Zargar et al., 2019). In the Indian subcontinent, drought stress become frequent and early adaptation of silicon-based fertilizers as an integrated approach with other measures will help farmers to protect their crops.
Benefits of silicon under contrasting stress
In the climate change scenario, prediction of potential environmental stress for cropping becomes difficult. The measures taken to reduce crop loss due to drought can become a disaster if there is a flood situation. Similar issues are there with other contrasting stress conditions like low and high temperatures. Therefore, finding a solution that can deal with contrasting stress conditions is important for climate-smart cropping. In the case of extreme water regimes like flood and drought, an improved root system helps the plant to uptake more water while under flooding conditions it helps to anchor down and avoid hypoxia (Thakral et al., 2021). As described in the previous section, silicon improves root growth and therefore help plant under extreme water regimes. In addition, silicon improves the oxidative defense system of plants under flood conditions.
Silicon supplementation was also found to be beneficial under extreme temperature regimes (Thakral et al., 2021). High temperatures cause structural deformities to protein and lipids which ultimately affect the cell membrane. Silicon supplementation help to reduce electrolyte leakage and maintain cell functionality under high temperature (Mandlik et al., 2020). Several efforts are being made to explore possibilities of silicon-based sprays which can provide instant relief under stress situations.
Use of silicon-based fertilizers and environmental concerns
Silicon-based fertilizers are already getting popularity in the global market. The application of any fertilizer has a great environmental concern. The adverse effect of silicon-based fertilizers is largely associated with chemical compositions. Some silicon sources like still slag are being used as a fertilizer. However, slag may have heavy metal contamination which can be hazardous for plants and also for the entire food chain. Chemical compounds like sodium, calcium, and potassium silicates are the most widely used fertilizers (Figure 2). In the case of sodium silicate and calcium silicate, they bring in unwanted sodium and calcium to the soil. Compared to both of these, potassium silicate has several benefits since potassium is one of the essential elements for plant growth. The use of potassium silicate also helps to cut down the separate cost of potassium fertilizers. Results have shown that silicon fertilizer improves soil microbiota and overall soil health. Therefore, the use of appropriate silicon fertilizer is beneficial for soil health. But certainly, it will affect the natural composition of the microbiota. Another major concern is the runoff of silicate fertilizer with irrigated or rainwater. Since silicate fertilizers are highly soluble they can easily runoff. Such runoff will lead not only to loss of fertilizer but also impose a threat to water bodies. In this regard, the use of silicate solubilizing bacteria seems to be the most affordable and sustainable approach. Silicon is highly abundant in the soil as insoluble silicate forms that plants cannot uptake. However, the application of silicon solubilizing bacteria as a biofertilizer will slowly convert the abundant insoluble silicate to soluble which plants can easily uptake.
Figure 2 Different silicon-rich compounds among which calcium silicate and diatomaceous earth are widely used as silicon fertilizer.
Benefits of silicon-rich food to human health
Silicon is reported to have numerous health benefits for humans. In traditional medicine systems plant extract of Horsetail, a hyper silicon accumulator species have been used as a cure for several diseases (Jugdaohsingh, 2007). The most prominent effect of silicon tonics has been reported for bone health. In European countries, Silicon-based tonics are being prescribed to patients suffering from osteoporosis. The osteoporosis condition is caused by several factors including genetic, hormonal, and nutritional. In osteoporosis, bone density rapidly decreases leading to bone fractures, and in elderly people, it may cause mortality. Therefore, a silicon-rich diet is advised for elderly people. The general dietary intake of silicon is around 20 to 50 mg per day. Seminal studies performed with chickens and rat has also confirmed the silicon-derived benefits for bone health. Over the last three decades, plenty of studies describing the beneficial effect of silicon on bone health has been published. Several silicon-based compositions enhance silicon absorption in the body. Interestingly, silicon-based bone implants and cementing agents were found to bind to the bone more efficiently as compared to the non-silicon material.
Silicon based life: science fiction seems to be a reality
Silicon has four valences and like carbon, it can form a covalent bond with four atoms. Silicon is highly abundant in the universe. For instance, about 30% of the earth's crust is made up of silicon which is about 150 times more abundant than carbon. Therefore, possibilities for the evolution of silicon-based life have been in discussion for many years. Many researchers speculate about alien life based on silicon, where silicon serves as a backbone for the organic compound. Although silicon-based life was long been science fiction for the scientific community, a recent study from Dr. Jennifer Kan, Dr. Russell Lewis, Dr. Kai Chen, and Dr. Frances Arnold showed the possibility of such evolution. Dr. Arnold’s research team working at California Institute of Technology, Pasadena, USA used directed evolution of cytochrome c enzyme isolated from Rhodothermus marinus bacterium to create carbon-silicon bond which is not present in any natural compound. Rather than the fascinating research discovery, work by Dr. Arnold’s team is very important for a variety of industrial products like polymers and semiconductors. The biogeochemical silicon cycle is interesting area of research since it covers the different interconnecting aspcets related to role of silicon (Figure 3).
Figure 3 Graphical illustration of the biogeochemical cycle of silicon and its components having significant influence. The figure is reproduced with permission from Raturi et al. (2021) published in Plant Physiology and Biochemistry.
Silicon provides numerous benefits to crop plants, especially under stress conditions. The ability of silicon to furnish benefits under contrasting stresses like extreme water regimes and extreme temperature regimes makes it a choice of fertilizer for climate-smart cropping. Silicon is also beneficial for soil health which is an important aspect for sustainable use. The environmental concerns of the large-scale application of silicon fertilizers can be bypassed through the use of biofertilizers formulated using silicate solubilizing bacteria. More notably, silicon is beneficial for human health and is widely being regarded for Biofortification. The area of silicon biology is fascinating and essential for today's needs.
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