Evidence for Active Uptake and Deposition of Si-based Defenses in Tall Fescue

Abstract

Silicon (Si) is taken up from the soil as monosilicic acid by plant roots, transported to leaves and deposited as phytoliths, amorphous silica (SiO2) bodies, which are a key component of anti-herbivore defense in grasses. Silicon transporters have been identified in many plant species, but the mechanisms underpinning Si transport remain poorly understood. Specifically, the extent to which Si uptake is a passive process, driven primarily by transpiration, or has both passive and active components remains disputed. Increases in foliar Si concentration following herbivory suggest plants may exercise some control over Si uptake and distribution. In order to investigate passive and active controls on Si accumulation, we examined both genetic and environmental influences on Si accumulation in the forage grass Festuca arundinacea. We studied three F. arundinacea varieties that differ in the levels of Si they accumulate. Varieties not only differed in Si concentration, but also in increases in Si accumulation in response to leaf damage. The varietal differences in Si concentration generally reflected differences in stomatal density and stomatal conductance, suggesting passive, transpiration-mediated mechanisms underpin these differences. Bagging plants after damage was employed to minimize differences in stomatal conductance between varieties and in response to damage. This treatment eliminated constitutive differences in leaf Si levels, but did not impair the damage-induced increases in Si uptake: damaged, bagged plants still had more leaf Si than undamaged, bagged plants in all three varieties. Preliminary differential gene expression analysis revealed that the active Si transporter Lsi2 was highly expressed in damaged unbagged plants compared with undamaged unbagged plants, suggesting damage-induced Si defenses are regulated at gene level. Our findings suggest that although differences in transpiration may be partially responsible for varietal differences in Si uptake, they cannot explain damage-induced increases in Si uptake and deposition, suggesting that wounding causes changes in Si uptake, distribution and deposition that likely involve active processes and changes in gene expression.

Population‐level manipulations of field vole densities induce subsequent changes in plant quality but no impacts on vole demography

“…Furthermore, the induction of silicon defenses in natural grasslands may be influenced by spatial and temporal heterogeneity in factors other than the grazing history of individual plants. These factors can affect both silicon availability in the environment and silicon uptake by plants (Hartley & DeGabriel, 2016) and include phenotypic and genotypic plasticity within a species (Hartley, Fitt, McLarnon, & Wade, 2015;McLarnon, McQueen-Mason, Lenk, & Hartley, 2017;Soininen et al, 2013), as well as abiotic factors such as temperature (Liang et al, 2006), soil type and pH (Quigley et al, 2017) , and precipitation (Quigley & Anderson, 2014).…”

Climate warming and plant biomechanical defences: Silicon addition contributes to herbivore suppression in a pasture grass

“…Some of these transporters are water channel aquaporins, which mediate passive transport, but several are active anion transporters, which plants can control to some extent (Ma, Yamaji, Tamai, & Mitani, 2007;Yamaji, Mitatni, & Ma, 2008). The extent to which plants can actively control Si accumulation relative to passive uptake that is hydraulically and osmotically driven is still debated (Kumar, Milstein, Brami, Elbaum, & Elbaum, 2017;McLarnon, McQueen-Mason, Lenk, & Hartley, 2017;Quigley & Anderson, 2014). There is, however, wider recognition that climatic factors such as water availability and temperature influence Si accumulation (Hartley, 2015;Maguire, Templer, Battles, & Fulweiler, 2017;Schoelynck et al, 2014).…”

Elevated carbon dioxide and warming impact silicon and phenolic‐based defences differently in native and exotic grasses

“…In support of the hypothesis that eCO 2 decreases silicon uptake is the fact that eCO 2 usually depresses the jasmonate signalling pathway in plants (Ode et al., ; Zavala, Nabity, & DeLucia, ) and, in rice at least, silicon uptake is stimulated by activation of the jasmonate pathway (Ye et al., ). eT often increases transpiration rates in plants and since silicon enters plants via the transpiration stream, silicon uptake may increase under eT although the link between silicon uptake and transpiration rates is subject to debate (Kumar, Milstein, Brami, Elbaum, & Elbaum, ; McLarnon, McQueen‐Mason, Lenk, & Hartley, ; Quigley & Anderson, ). Alternatively, eT could also facilitate silicon uptake by higher metabolically driven uptake of nutrients in general (Hartley & DeGabriel, ).…”