Beyond grasses: the potential benefits of studying silicon accumulation in non-grass species

INTRODUCTION Silicification in angiosperms is a phenomenon that has attracted increasing attention in recent years. It is now widely acknowledged that silicification has many benefits to angiosperms (Richmond and Sussman, 2003; Ma and Yamaji, 2008; Epstein, 2009; Guntzer et al., 2012), and that it probably plays appreciable roles at the ecosystem and landscape levels as well (Cooke and Leishman, 2011; Reynolds et al., 2012; Schoelynck et al., 2014). High silica accumulating plant species are considered a major pool in the silicon cycle, affecting silicon fluxes and turnover rates (Conley, 2002; Falkowski et al., 2004; Derry et al., 2005; Sommer et al., 2006; Li et al., 2011; Carey and Fulweiler, 2012; Struyf and Conley, 2012; Vandevenne et al., 2013; Schoelynck et al., 2014). Indeed, recent studies have demonstrated the significant roles of plant silicification on the silicon cycle in grasslands (e.g., Melzer et al., 2012), freshwater and tidal ecosystems (e.g., Jacobs et al., 2013; Schoelynck et al., 2014) and forests (e.g., Farmer, 2005; Farmer et al., 2005; Cornelis et al., 2011). Furthermore, since silicate weathering is a CO2-consuming process, the effects of silica uptake and accumulation by plants on the silicon cycle may also influence the global carbon cycle (Street-Perrott and Barker, 2008). Silica contents vary greatly among the angiosperms, with high concentrations occurring most commonly in the Poaceae (hereafter referred to as grasses) and other related monocotyledonous commelinid families (Hodson et al., 2005; Piperno, 2006) and aquatic macrophytes (Schoelynck et al., 2012). Due to the relatively high silica concentrations in grasses and to their high economic and ecological importance, silicification has been studied more frequently and more intensely in this family than in any other plant family. However, a better understanding of silicification in non-grass taxa can promote our understanding of the function and importance of silicon for plants. Unfortunately, studies of non-grass taxa are not only rarer, but also usually focus on a small number of taxa, mostly families and species that are silica-rich (e.g., Cucurbitaceae: Rogalla and Römheld, 2002), economically important (e.g., Fabaceae: Shen et al., 2010), or serve as model plant species (e.g., Arabidopsis: Fauteux et al., 2006), which are unlikely to be a fair representation of non-grass taxa. This means there is a significant gap in our knowledge of silicon processes, patterns and roles, given the fact that non-grass taxa, such as forest tree, are a major component of the silicon cycle (Farmer, 2005; Farmer et al., 2005; Cornelis et al., 2011). I shall therefore briefly discuss the variability of silicification among non-grass taxa, and how it can be used to promote a better and broader understanding of this phenomenon.