Emerging roles of silicon in plant signaling networks and microbiome dynamics under salt stress

Silicon (Si) is widely recognized as a beneficial element that can improve plant performance under salt stress. However, a comprehensive understanding of its biological functions requires moving beyond isolated physiological responses toward an integrated view of plant signaling and rhizosphere processes. This narrative review critically synthesizes current evidence and proposes an "inside-outside" framework for Si-associated salt-stress mitigation. Internally, Si treatment has been reported to influence phytohormone homeostasis, particularly abscisic acid, jasmonic acid, and salicylic acid, while also affecting Ca²⁺-, nitric oxide-, and reactive oxygen species-related processes. These changes are associated with the regulation of stomatal behavior, root water transport, ion homeostasis, osmotic adjustment, antioxidant defense, and stress-responsive gene expression. Rather than acting as a universally established signal integrator, Si may modify the operating state, magnitude, and recovery kinetics of pre-existing stress-response networks by stabilizing membranes, restricting excessive Na⁺ accumulation, preserving K⁺ retention, and buffering cellular redox conditions. Externally, Si application can alter rhizosphere physicochemical properties, root-associated metabolites, and microbial community assembly. Si-associated enrichment of plant-beneficial microorganisms may contribute to nutrient cycling, ionic and osmotic regulation, redox protection, and plant growth, while microbial metabolites may reciprocally influence plant signaling and metabolism. Nevertheless, most microbiome functions remain inferred from community profiles and correlations, and causal validation is currently limited to a small number of experimental systems. Si uptake, transport, and spatial deposition provide the physiological basis for these interconnected responses, but their magnitude depends on plant species, genotype, Si-accumulation capacity, formulation, dose, application route, and stress intensity. This integrated framework identifies Si as a context-dependent modulator of plant-rhizosphere interactions and provides a mechanistic basis for developing precise and sustainable Si-based salinity-management strategies.