Silicon-augmented resistance of plants to herbivorous insects: a review
O.L. Reynolds, M.G. Keeping, J.H. Meyer
Silicon (Si) is one of the most abundant elements in the earth’s crust, although its essentiality in plant growth is not clearly established. However, the importance of Si as an element that is particularly beneficial for plants under a range of abiotic and biotic stresses is now beyond doubt. This paper reviews progress in exploring the benefits at two- and three-trophic levels and the underlying mechanism of Si in enhancing the resistance of host plants to herbivorous insects. Numerous studies have shown an enhanced resistance of plants to insect herbivores including folivores, borers, and phloem and xylem feeders. Silicon may act directly on insect herbivores leading to a reduction in insect performance and plant damage. Various indirect effects may also be caused, for example, by delaying herbivore establishment and thus an increased chance of exposure to natural enemies, adverse weather events or control measures that target exposed insects. A further indirect effect of Si may be to increase tolerance of plants to abiotic stresses, notably water stress, which can in turn lead to a reduction in insect numbers and plant damage. There are two mechanisms by which Si is likely to increase resistance to herbivore feeding. Increased physical resistance (constitutive), based on solid amorphous silica, has long been considered the major mechanism of Si-mediated defences of plants, although there is recent evidence for induced physical defence. Physical resistance involves reduced digestibility and/or increased hardness and abrasiveness of plant tissues because of silica deposition, mainly as opaline phytoliths, in various tissues, including epidermal silica cells. Further, there is now evidence that soluble Si is involved in induced chemical defences to insect herbivore attack through the enhanced production of defensive enzymes or possibly the enhanced release of plant volatiles. However, only two studies have tested for the effect of Si on an insect herbivore and third trophic level effects on the herbivore’s predators and parasitoids. One study showed no effect of Si on natural enemies, but the methods used were not favourable for the detection of semiochemical-mediated effects. Work recently commenced in Australia is methodologically and conceptually more advanced and an effect of Si on the plants’ ability to generate an induced response by acting at the third trophic level was observed. This paper provides the first overview of Si in insect herbivore resistance studies, and highlights novel, recent hypotheses and findings in this area of research. Finally, we make suggestions for future research efforts in the use of Si to enhance plant resistance to insect herbivores.