Silicon is an inducible and effective herbivore defence againstHelicoverpa punctigera(Lepidoptera: Noctuidae) in soybean

Abstract

The role of silicon (Si) in alleviating the effects of biotic and abiotic stresses, including defence against insect herbivores, in plants is widely reported. Si defence against insect herbivores is overwhelmingly studied in grasses (especially the cereals), many of which are hyper-accumulators of Si. Despite being neglected, legumes such as soybean (Glycine max) have the capacity to control Si accumulation and benefit from increased Si supply. We tested how Si supplementation via potassium, sodium or calcium silicate affected a soybean pest, the native budworm Helicoverpa punctigera Wallengren (Lepidoptera: Noctuidae). Herbivory reduced leaf biomass similarly in Si-supplemented (+Si) and non-supplemented (–Si) plants (c. 29 and 23%, respectively) relative to herbivore-free plants. Both Si supplementation and herbivory increased leaf Si concentrations. In relative terms, herbivores induced Si uptake by c. 19% in both +Si and –Si plants. All Si treatments reduced H. punctigera relative growth rates (RGR) to a similar extent for potassium (−41%), sodium (−49%) and calcium (−48%) silicate. Moreover, there was a strong negative correlation between Si accumulation in leaves and herbivore RGR. To our knowledge, this is only the second report of Si-based herbivore defence in soybean; the rapid increase in leaf Si following herbivory being indicative of an induced defence. Taken together with the other benefits of Si supplementation of legumes, Si could prove an effective herbivore defence in legumes as well as grasses.

Is it time to include legumes in plant silicon research?

“…Ferreira, Moraes, and antunes (2011) reported that silicon supplementation in soybean had a detrimental effect on the silverleaf whitefly Bemisia tabaci . Recently, Johnson et al (2017), Johnson, Ryalls, Gherlenda, Frew, and Hartley (2018) showed that silicon supplementation in lucerne slightly benefitted the phloem‐sucking aphid Acyrthosiphon pisum but it was also reported by Johnson et al (2019) that silicon effectively reduced relative growth rate of the Australian native bollworm Helicoverpa punctigera . However, the limited number of studies thus far restricts our understanding of how silicon affects legume–herbivore interactions.…”

“…weevils) in soil (Johnson et al, 2006). However, silicon supplementation could equally reverse these negative consequences by augmenting plant resistance to herbivores (Frew, Powell, Allsopp, Sallam, & Johnson, 2017; Johnson et al, 2019).…”

“…For example, a meta‐analytic study of 145 experiments revealed that silicon benefitted plants under abiotic stresses (Cooke & Leishman, 2016). Moreover, plant silicon plays an important role in herbivore defence and is known to be an inducible defence (Johnson, Rowe, & Hall, 2019; Massey, Roland Ennos, & Hartley, 2007). For herbivore defence, at least, it is known that biophysical traits can hinder herbivory via abrasion, toughness and reductions in resource acquisition (Alhousari & Greger, 2018; Massey & Hartley, 2006, 2009; Reynolds, Padula, Zeng, & Gurr, 2016).…”

Reviews and Syntheses: Impacts of plant silica – herbivore interactions on terrestrial biogeochemical cycling

“…While researchers continue to debate the biological roles of Si and uncover the mechanisms behind them (Frew et al, 2018;Coskun et al, 2019), we conclude that, at a minimum, Si mitigates the negative impacts of various stressors, such as herbivory, which then enable plants to improve growth potential (Johnson et al, 2019).…”

“…Several studies have documented effects of soil Si addition on plant chemical defenses (e.g., Reynolds et al, 2009), anti-herbivore phytohormonal signaling (e.g., Hall et al, 2019), and changes in plant nutritional quality (e.g., Frew et al, 2018;Moise et al, 2019). For example, Si may induce indirect defense mechanisms by altering the composition of herbivore-induced plant volatiles that attract herbivore parasitoids and predators (Liu et al, 2017).…”