Silica's role in plant abiotic stress tolerance: A redox buffer hypothesis

Supplementing plants with silica (Si) enhances growth and improves multiple stress tolerance across a large variety of plant species. Despite its importance for agriculture, this phenomenon has no satisfactory explanation. Hydrogen peroxide (H₂O₂), a common stress messenger in planta, may play a central role in Si-related plant biology. In this work, we test the hypothesis that the known in vitro chemical interaction between H₂O₂ and silica plays a role also in planta. We thus compared growth rates and response to hypoxia and salt stress in Si-accumulators (sorghum, wheat) and non-accumulators (sorghum mutant defective in Si uptake, tomato) in relation to variation in Si and H₂O₂. Si-dependent root elongation was observed in seedlings of both Si accumulators and non-accumulators. Similarly, roots elongated more rapidly in seedlings grown in media that were supplemented with 0.01-0.1 mM H₂O₂. When H₂O₂ was quenched with 20 μM KI, the Si-dependent elongation disappeared. In vitro, freshly polymerized silica gel slowed H₂O₂ decay under oxygen and in the presence of lignin. Our results indicate that Si affects the redox balance at the apoplast. We propose that cell wall-bound silica sequesters H₂O₂ and protects it from degradation, thereby acting as a buffer of apoplastic redox homeostasis. This simple physicochemical interaction may provide a unifying explanation for the agronomic benefits of Si fertilization.