What nutrients do you consider to be important to a plant? What do plants need to be successful, to grow, photosynthesise, develop and defend themselves effectively against pests and disease? The majority of people, whether they study plants or not, would be able to mention at least a few of these: nitrogen, phosphorus . . . perhaps potassium. Probably very few, if any, would include silicon.
It has been known for many years that the application of silicon can benefit plant growth. There is a long literature associated with silicon in agriculture. The Chinese use more than 30 million tons of silicon fertiliser a year. Yet it has only been in the last few decades that our knowledge of the effects and functions of silicon in plants has significantly developed.
Silicon is the second most abundant element in the Earth’s crust, after oxygen. Most silicon in the soil, however, is not in a soluble form, and is therefore not available to plants. Plants take up silicon via their lateral roots in the form of monosilicic acid. This is then transported around the plant, and when it reaches certain concentrations, this naturally polymerises to form solid silica (SiO2), which is deposited within the plant tissue. This accumulation of silica confers numerous potential benefits to the plant, largely through mechanical or physical mechanisms. It provides structural reinforcement, increases rigidity, increases toughness, and acts as a physical barrier against some pathogens. This also reduces the digestibility of plant material for herbivores, and can wear down insect feeding mouthparts (Massey and Hartley 2009). Therefore silicon can be a key driver of plant ecological interactions.
There are now numerous studies demonstrating how silicon increases plant resistance to disease and herbivory, but also against abiotic stresses such as heat, drought and salinity (Frew et al. 2018). The vast majority of these studies focus on a cultivated crop and often only a single species, which is typically within the Poacea (grass) family. Species within this family are, broadly speaking, high silicon accumulating plants, which actively transport silicon within and around their tissues. Yet there is growing body of literature demonstrating beneficial effects of silicon in species that are not high accumulators. Indeed, silicon not only benefits plants through the deposition of silica, but also has been shown to enhance plant constitutive and induced biochemical defences, increase photosynthetic efficiency and upregulate anti-oxidation metabolism.
These data strongly suggest that silicon does more than simply augment plant physical strength and toughness. Rather, silicon may play a role in plant primary metabolism, growth and development (Frew et al. 2018). Silicon is currently deemed non-essential for plant growth, but is regarded as a beneficial nutrient. Yet it exhibits extraordinary effects on plants (some mentioned above), unrivalled by any other non-essential nutrient. There is also growing evidence suggesting that silicon may be substitutable for carbon in plants, for some plant functions (Cooke and Leishman 2012). Indeed, potential trade-offs within plants between silicon and other defences have been demonstrated (Frew et al. 2016; Johnson and Hartley 2017).
As our knowledge of silicon develops, it is becoming clear that there are gaps in our understanding of the role of silicon in plant biology. Despite the growing body of literature on the manifold effects of silicon, there has been little research to date on the underlying mechanisms by which silicon acts to enhance plant growth and resistance to stress.
The beneficial impacts of silicon hold vast potential to be exploited in plant production. Global agriculture is under intensifying pressure to meet the food demands of an increasing population. This needs to be done sustainably, while also maintaining profits. Currently available pesticides and fertilisers are ecologically unsustainable, are expensive and are undergoing increasing restrictions (the EU recently agreed to ban neonicotinoids use, the world’s most widely used insecticides). Improving our understanding of the biology and ecology of silicon in plants could lead to significant advances in our ability to meet the challenge of ecologically and economically sustainable food production.