The role of silicon in plant biology: a paradigm shift in research approach

Abstract

Silicon is often regarded as a plant nutritional 'non-entity'. A suite of factors associated with Si have been recently identified, relating to plant chemistry, physiology, gene regulation and interactions with other organisms. Research to date has typically focused on the impact of Si application upon plant stress responses. However, the fundamental, underlying mechanisms that account for the manifold effects of Si in plant biology remain undefined. Here, the known effects of Si in higher plants relating to alleviation of both abiotic and biotic stress are briefly reviewed and the potential importance of Si in plant primary metabolism is discussed, highlighting the need for a unifying research framework targeting common underlying mechanisms. The traditional approach of discipline-specific work on single stressors in individual plant species is currently inadequate. Thus, a holistic and comparative approach is proposed to assess the mode of action of Si between plant trait types (e.g. C3, C4 and CAM; Si accumulators and non-accumulators) and between biotic and abiotic stressors (pathogens, herbivores, drought, salt), considering potential pathways (i.e. primary metabolic processes) highlighted by recent empirical evidence. Utilizing genomic, transcriptomic, proteomic and metabolomic approaches in such comparative studies will pave the way for unification of the field and a deeper understanding of the role of Si in plants.

The controversies of silicon's role in plant biology

“…Although some of these stresses are related, this is a diverse and largely disparate set of scenarios, and therefore it stands to reason that Si is providing some fundamental protection to plants that confers a wide range of benefits. Perplexingly, a survey of the relevant literature appears to suggest otherwise, with Si seemingly involved in a plethora of processes and functions, including gene expression (Manivannan & Ahn, 2017), redox homeostasis and oxidative stress (Liang et al, 2003;Zhu et al, 2004;Farooq et al, 2016), nitrogen assimilation (Pereira et al, 2013), carbohydrate metabolism (Zhu et al, 2016), cell signaling (Detmann et al, 2012(Detmann et al, , 2013, TM ion and water fluxes (Liang et al, 2006;Liu et al, 2014), hormone regulation (Liang XL et al, 2015;Markovich et al, 2017), root exudation (Kidd et al, 2001;Wu et al, 2016), metal chelation (Wang et al, 2004;, root architecture (Gong et al, 2006;Fleck et al, 2011), transpiration (Gao et al, 2006) and photosynthesis (Shen et al, 2010;Detmann et al, 2012) (for reviews, see Epstein, 1999;Ma, 2004;Liang et al, 2007;Meharg & Meharg, 2015;Cooke & Leishman, 2016;Coskun et al, 2016;Debona et al, 2017;Frew et al, 2018). [Correction added after online publication 14 July 2018: 'heavy' has been deleted from the preceding sentence.]…”

“…Although the last 25 years have seen an unprecedented amount of research into the roles of Si in plant biology, it appears that a number of hypotheses nearly commensurate with the number of studies have been proposed, which has exacerbated the confusion. For instance, in a recent review describing the putative effects of Si, Frew et al (2018) identified an inordinate amount of reported effects under various environmental conditions, including cell signaling, amino acid metabolism, photosynthesis, cell growth and division, and transcriptomic processes, which, taken as a whole, are incongruent with what we know about the properties of Si. Thus, in trying to propose a hypothesis to define the role of Si, we have taken a holistic and parsimonious approach, encompassing the various scenarios described in the literature in line with chemical and biological realities.…”

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

“…There is emerging consensus that silicon (Si) plays an important functional role in plants, particularly in terms of mitigating the impacts of adverse environmental conditions (Cooke & Leishman, 2011;Frew, Weston, Reynolds, & Gurr, 2018). For abiotic stresses, Si can alleviate the effects of drought, salt stress, toxic metals, extreme temperatures and nutrient deficiency (Cooke & Leishman, 2016;Guntzer, Keller, & Meunier, 2012;Liang, Sun, Zhu, & Christie, 2007).…”

Combined effects of soil silicon and drought stress on host plant chemical and ultrastructural quality for leaf‐chewing and sap‐sucking insects

“…However, Si can be especially useful when plants are under environmental stresses, and the benefits of Si fertilization may be minimal unless the plant is under some form of stress (Epstein, ; Fauteux, Rémus‐Borel, Menzies, & Bélanger, ). Beneficial effects of Si fertilization on drought‐stressed plants primarily result from Si mediation of many alterations in plant biochemistry and physiology (Haynes, ; Rizwan et al, ; Sacala, ; Zhu & Gong, ), although the underlying mechanisms that account for the manifold effects of Si in plant biology remain undefined (Frew, Weston, Reynolds, & Gurr, ; Hall, Waterman, Vandegeer, Hartley, & Johnson, ). Because Si ultimately increases photosynthesis, plant growth, biomass, and crop yield and quality during drought conditions (Rizwan et al, ), the effects of drought stress (Blum, ) are mitigated by Si at the whole plant level and crop production.…”