The soil silicon filter: A conceptual model of how mycorrhizal fungi and their microbiome may govern biosilicification and plant-silicon availability

Silicon (Si) plays an important role in plant health and ecosystem function, yet the biological pathways controlling its cycling are often too simplified and underlying mechanisms are not clear. While the plant-centric model of Si uptake and phytolith formation is mostly used, it underestimates the complex role of the soil microbiome. This review synthesizes growing evidence on the importance of the mycorrhizosphere—the zone of interaction between roots, mycorrhizal fungi, and bacteria—as a central processing unit in the terrestrial Si cycle. We develop and evaluate the concept of a "microbial silicon filter" as a working hypothesis, where symbiotic partnerships, particularly between mycorrhizal fungi and their associated bacteria, may collectively influence the Si flux. We line out the mechanisms of mycorrhizal-mediated Si transport and review evidence for bacterial biosilicification alongside the more speculative evidence and open questions regarding fungal (particularly mycorrhizal) biosilicification. Furthermore, we examine potential synergistic microbial weathering of minerals that mobilizes Si and how biofilm matrices may enhance its retention within the hyphosphere. By integrating these processes, we present a more integrated, microbiome-inclusive model of the Si cycle that emphasizes the potential interdependencies between plants, mycorrhizal fungi, and bacteria. This perspective has profound implications, potentially influencing plant stress resilience modulated by Si supply and suggesting a possible, though not yet quantified, role in enhancing long-term carbon sequestration through phytolith formation. Finally, we outline future research directions to unravel the underlying mechanisms of this partnership of plants, mycorrhizal fungi, and bacteria and to harness it for sustainable agriculture and ecosystem restoration. A central focus of these recommendations is the critical need for advanced methodologies—particularly stable isotope tracing and nanoscale secondary ion mass spectrometry (NanoSIMS)—to move from correlative evidence to quantitative, mechanistic understanding of the microbial Si filter. • The mycorrhizosphere is a key zone for fungal-bacterial silicon cycling. • Mycorrhizal fungi transport silicon to plants, boosting uptake and stress resilience. • Microbial weathering and biofilms form a dynamic soil "silicon filter." • This process affects plant health, soil ecosystems, and carbon storage. • Stable isotope methods are needed to measure microbial silicon fluxes.