From Shield to Signal: A Critical Appraisal of Silicon-mediated Resistance to Fungal Pathogens in Plants

Abstract

Silicon is not classified as an essential plant nutrient, yet supplying it lowers the severity of many fungal diseases in cultivated species, and the mechanistic explanation for that effect remains contested after more than three decades of experimental work. Two traditions dominate. The older treats polymerised silica deposited beneath the cuticle and within cell walls as a mechanical shield that impedes fungal penetration. The more recent treats silicic acid as an agent that potentiates host defence chemistry and signalling, an interpretation supported by cytological, metabolomic and transcriptomic observations. A third proposal, which locates the effect in the apoplast rather than in the cytoplasm, has reframed the disagreement without settling it. This review evaluates the strength and internal consistency of the evidence for each position, examines the design features that limit inference, and asks which observations would discriminate among the competing models. Literature was identified through structured searching of biomedical, multidisciplinary and open-access scholarly sources, supplemented by citation tracking, and was appraised for design adequacy, control structure and alignment between claim and evidence rather than counted. The synthesis indicates that the barrier and signalling accounts are each supported by robust but partial evidence, and that both are compromised by a shared design weakness: transcriptional and biochemical differences between silicon-treated and untreated plants are usually measured after disease severity has already diverged, so consequence is difficult to separate from cause. Protection is nonetheless reproducible across taxonomically diverse hosts, including species with limited uptake capacity, and quantitative synthesis in grasses indicates that efficacy varies systematically with fungal infection strategy. Evidence for interference with pathogen-derived molecules, including secreted effectors and fungal ethylene, offers a route towards reconciling the traditions but rests on few studies. Research priorities include matched-inoculum designs, counter-ion-controlled treatments, spatially resolved measurement of apoplastic silicon, transport-mutant panels, and evaluation of nanoscale formulations against equimolar silicic acid rather than against untreated controls.