Silicon enhanced plant resistance to insect herbivory: a paradigm shift

The metalloid element silicon (Si) is taken up by plants, accumulates in plant tissues and is irreversibly deposited as solid phytoliths (SiO2) that increase plant rigidity, toughness, and resistance to pathogens and insect herbivores. Indeed, applying Si to plants has been shown to increase plant resistance to numerous abiotic and biotic stresses. Si has been shown to reduce the damage by, and performance of, herbivorous insect pests both aboveground and belowground. However, in addition to the physical defences conferred by Si accumulation in plant tissue, Si has also increased plant resistance to insects via other direct and indirect mechanisms including increased accumulation of defence compounds, altered oxidation and anti-oxidant metabolism, and increased attraction of insect natural enemies, amongst others. More recent studies have begun to increase our understanding of how Si can impact gene regulation, particularly in high Si accumulating plant species. Despite these advances, the fundamental mechanisms underpinning Si enhanced plant resistance to insect herbivores remain unknown. Here we review the current understanding of the mechanisms by which Si increases plant resistance to highlight the fractional nature of research to date and to propose a paradigm shift in approach that recognises the potentially fundamental role of Si in plants. We propose an integrated transcriptomic and metabolomic approach with multidisciplinary interpretation and a bioinformatics approach to data analysis, thus generating much needed information on genes and pathways impacted by Si application and uptake in higher plants. As such, we look to understand the effects of Si from the perspectives of entomology, plant pathology, plant physiology, biochemistry and molecular biology. By using a multidisciplinary approach to study the impacts of Si in selected model systems we can begin to understand the central role of Si in plant biology, and therefore fully exploit its ability to enhance plant resistance to insect herbivores.