The ecology of herbivore‐induced silicon defences in grasses

Abstract

Summary1. Silicon as a defence against herbivory in grasses has gained increasing recognition and has now been studied in a wide range of species, at scales from individual plants in pots to plant communities in the field. The impacts of these defences have been assessed on herbivores ranging from insects to rodents to ungulates. Here, we review current knowledge of silicon mediation of plant-herbivore interactions in an ecological context. 2. The production of silicon defences by grasses is affected by both abiotic and biotic factors and by their interactions. Climate, soil type and water availability all influence levels of silicon uptake, as does plant phenology and previous herbivory. The type of defoliation matters and artificial clipping does not appear to have the same impact on silicon defence induction as herbivory which includes the presence of saliva. Induction of silicon defences has been demonstrated to require a threshold level of damage, both in the laboratory and in the field. In recent studies of vole-plant interactions, the patterns of induction were found to be quantitatively similar in glasshouse compared with field experiments, in terms of both the threshold required for induction and timing of the induction response. 3. The impacts of silicon defences differ between different classes of herbivore, possibly reflecting differences in body size, feeding behaviour and digestive physiology. General patterns are hard to discern however, and a greater number of studies on wild mammalian herbivores are required to elucidate these, particularly with an inclusion of major groups for which there are currently no data, one such example being marsupials. 4. We highlight new research areas to address what still remains unclear about the role of silicon as a plant defence, particularly in relation to plant-herbivore interactions in the field, where the effects of grazing on defence induction are harder to measure. We discuss the obstacles inherent in scaling up laboratory work to landscape-scale studies, the most ecologically relevant but most difficult to carry out, which is the next challenge in silicon ecology.

Competition between Silicifiers and Non-silicifiers in the Past and Present Ocean and Its Evolutionary Impacts

“…In embryophytes, silicification has a wide-ranging defensive role Ensikat et al, 2016;Hartley et al, 2016), from abrasive phytoliths to complex structures such as trichomes, and even by inducing anti-herbivore and anti-pathogen metabolic responses (Ye et al, 2013). These defenses lead to multiple competitive interactions, with differential effects on different types of insect feeding (Massey et al, 2006), between insect and mammalian herbivores, or between large and small mammalian herbivores (Hartley et al, 2016). There are also complex competitive interactions between plants regarding their silicified defenses, which depend on various biotic (plant species, herbivore population cycles) and abiotic factors (soil conditions, climate; Garbuzov et al, 2011;Hartley et al, 2016).…”

“…These defenses lead to multiple competitive interactions, with differential effects on different types of insect feeding (Massey et al, 2006), between insect and mammalian herbivores, or between large and small mammalian herbivores (Hartley et al, 2016). There are also complex competitive interactions between plants regarding their silicified defenses, which depend on various biotic (plant species, herbivore population cycles) and abiotic factors (soil conditions, climate; Garbuzov et al, 2011;Hartley et al, 2016). This can lead to herbivoreplant specialization and alter plant community and ecosystem structure.…”

“…With respect to biogenic silica, this can result in the production of much stronger structures, such as siliceous diatom frustules having the highest strength per unit density of any known biological material (Hamm et al, 2003;Aitken et al, 2016), or sponge spicules being many times more flexible than an equivalent structure made of pure silica (Ehrlich et al, 2008;Shimizu et al, 2015). As a result, biogenic silica structures are utilized for support (Weaver et al, 2007), feeding (Nesbit and Roer, 2016), predation defense (Pondaven et al, 2007;Friedrichs et al, 2013;Hartley et al, 2016) and environmental protection as a component of cyst walls (Preisig, 1994). Biogenic silica also has useful optical properties for light transmission and modulation in organisms as diverse as plants (Schaller et al, 2013), diatoms (Fuhrmann et al, 2004;Yamanaka et al, 2008;Romann et al, 2015), sponges (Sundar et al, 2003), and molluscs (Dougherty et al, 2014).…”

Get Tough, Get Toxic, or Get a Bodyguard: Identifying Candidate Traits Conferring Belowground Resistance to Herbivores in Grasses

“…The most obvious aboveground herbivore-resistance traits of grasses are physical, and include the deposition of silica phytoliths (Hartley and DeGabriel, 2016) and high proportions of cellulose and lignin, while chemical resistance traits are generally viewed as less significant (with notable exceptions, e.g., Vicari and Bazely, 1993). However, the apparent general lack of chemical defense in grasses may reflect a lack of investigation and a focus on a few economically important species (Kellogg, 2015).…”

“…Silica has been linked to drought resistance, structural strength, disease resistance and defense against a range of insect herbivores, the latter via reductions in digestibility and mouthpart wear (Hartley and DeGabriel, 2016). Silica is taken up by roots in the form of monosilicic acid, before being transported to the site of concentration and deposition.…”