Can silicate types regulate plant defense and rhizospheric microbiome diversity differently during heat stress conditions?

Silicates (Si) improve plant growth; however, how different types of silicate sources influence plant growth and rhizosphere microbiome remains underexplored. We compare two Si types (pure and bioavailable silicic acid (Si) and mineral magnesium silicate (MgSi)) applied to the soybean (Glycine max L.) rhizosphere to determine whether two silicate types (Si-types) differently impact plant growth, defense responses, and microbiome diversity and function during heat stress. Under heat stress, Si-type treatments improved biomass (86 % with Si and 82 % with MgSi), reduced H₂O₂ (26 % phyllosphere; 33 % rhizosphere), and enhanced β-glucosidase activity (2.6-fold rhizosphere) compared to heat-only treatment and increased Proteobacteria relative abundance from ∼65 % (heat-only) to ∼74 % in Si-type-treated rhizospheric soil. Si-types showed downregulation of heat shock transcription factors, suggesting regulatory defense effects during heat stress. Metagenome-assembled genomes (MAGs) analysis revealed significant bacterial shifts across the Si-treatments, with Proteobacteria and Bacteroidetes being the dominant phyla in the rhizospheric soil. Under heat stress, the core microbiome comprised 14 rhizosphere genera (including Pelomonas, Achromobacter, Paracoccus, Nocardioides), whereas Pelomonas was the sole core root genus, and Pelomonas puraquae core species in both compartments. MAGs analysis revealed Si-based shifts in microbial metabolic pathways and enrichment of auxin biosynthesis in Si-treated roots during heat stress. Because MgSi supplies both Mg and Si, effects observed with MgSi are interpreted as combined Si + Mg effects. In conclusion, both Si-types caused shifts in microbiome diversity and function, and impacted plant growth and defense responses under heat stress, providing a foundation for improving thermotolerance in plants.