The controversies of silicon's role in plant biology

Abstract

Contents Summary 67 I. Introduction 68 II. Silicon transport in plants: to absorb or not to absorb 69 III. The role of silicon in plants: not just a matter of semantics 71 IV. Silicon and biotic stress: beyond mechanical barriers and defense priming 76 V. Silicon and abiotic stress: a proliferation of proposed mechanisms 78 VI. The apoplastic obstruction hypothesis: a working model 79 VII. Perspectives and conclusions 80 Acknowledgements 81 References 81 SUMMARY: Silicon (Si) is not classified as an essential plant nutrient, and yet numerous reports have shown its beneficial effects in a variety of species and environmental circumstances. This has created much confusion in the scientific community with respect to its biological roles. Here, we link molecular and phenotypic data to better classify Si transport, and critically summarize the current state of understanding of the roles of Si in higher plants. We argue that much of the empirical evidence, in particular that derived from recent functional genomics, is at odds with many of the mechanistic assertions surrounding Si's role. In essence, these data do not support reports that Si affects a wide range of molecular-genetic, biochemical and physiological processes. A major reinterpretation of Si's role is therefore needed, which is critical to guide future studies and inform agricultural practice. We propose a working model, which we term the 'apoplastic obstruction hypothesis', which attempts to unify the various observations on Si's beneficial influences on plant growth and yield. This model argues for a fundamental role of Si as an extracellular prophylactic agent against biotic and abiotic stresses (as opposed to an active cellular agent), with important cascading effects on plant form and function.

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

“…These Si deposits improved collard development especially when it was under stress condition. This finding supports the understanding that the benefits of Si fertilization may be minimal unless the plant is under some form of stress (Epstein, ; Fauteux et al, ), as most literature has pointed out (Coskun et al, ).…”

“…The physiological resistance hypothesis (PRH) predicts that Si enhances or induces resistance by activating plant biochemical and physiological processes (Fauteux, Rémus‐Borel, Menzies, & Bélanger, ; Reynolds et al, , ; Teixeira et al, ). A recent integrative hypothesis for Si alleviation of plant stresses has been proposed (Coskun et al, ), which states that, besides mechanical protection, the amorphous Si deposits in the apoplast interferes with and indirectly promotes the underlying processes of plant resistance. However, there is no evidence that the pathogen‐based apolastic obstruction hypothesis is relevant to herbivore defence.…”

Tomato roots have a functional silicon influx transporter but not a functional silicon efflux transporter

“…Silicon (Si) is the most abundant mineral on the earth's crust (Marschner, ). It shows various beneficial effects on plant growth and development, particularly under environmental stress conditions (Coskun et al, ; Ma, ), such as drought (Gong, Chen, Chen, Wang, & Zhang, ), salt stress (Ahmad, Zaheer, & Ismail, ), metal toxicity (Hodson & Evans, ; Neumann & zur Nieden, ), iron deficiency (Pavlovic et al, ), plant pathogen (Bélanger, Benhamou, & Menzies, ), and insect pest (Ranganathan et al, ). These beneficial roles of Si have been associated with Si accumulation in the shoots (Mitani & Ma, ).…”

Phytolith‐rich biochar: A potential Si fertilizer in desilicated soils

“…In fact, a large part of crop production is lost due to harmful pests and diseases, while the remainder is threatened by increasingly erratic weather and soil fertility decline (Sanchez, ; Vanlauwe & Giller, ). In challenging these global change issues, the accumulation of Si in cereals provides protection against pests and pathogens, and mitigates the impacts of climatic stresses such as drought and salinity (Coskun et al, ; Meunier et al, ). Si‐based plant protection could therefore open new avenues to enhance crop yields by addressing current threats and contribute to improving food security, enhancing bioenergy production, and mitigating climate change.…”