Root exudates differently mobilize silicon from soil pools under low nutrient conditions

The availability of silicon (Si) to plants is often limited, as most soil Si is bound in primary (e.g., feldspar) and clay minerals, occluded in aluminium (Al) and iron (Fe)/manganese (Mn) oxides and hydroxides, or occurs in less soluble forms such as amorphous biogenic SiO₂ (e.g., phytoliths) and crystalline SiO₂ (e.g., quartz). A common plant response to nutrient deficiency is enhanced root exudation of low-molecular-weight (LMW) compounds. Recent evidence suggests that both rhizosphere acidification (Kostic Kravljanac et al., Plant and Soil 511:209-222, 2025) and root released carboxylates can mobilize soil Si (de Tombeur et al., Trends in Plant Science 26: 1116 1125, 2021), but the role of other LMW root exudates in Si mobilization remains unclear. To investigate this, we conducted a series of experiments using Si-accumulating crops (barley, wheat, and cucumber) subjected to phosphorus (P), iron (Fe) or zinc (Zn) deficiency to stimulate root exudation. Root exudates were collected over time and characterized: Fe-deficient cucumber exudates mainly contained carboxylates and riboflavin, while P/Zn-deficient barley root exudates mainly contained carboxylates and phytosiderophores. These exudates were used to extract Si from four soils varying in pH and clay content, with the findings further validated using individual components of the exudates. We also tested their ability to solubilize two forms of SiO₂, amorphous biogenic SiO₂ (phytoliths isolated from equisetum) and crystalline SiO₂ (quartz), which, together with mineral-bound Si, represent major soil Si pools. Our results demonstrate that root exudates can significantly mobilize Si, and that mobilization depended on soil pH and clay content. Moreover, exudates enhanced the solubility of amorphous SiO₂, particularly from phytoliths. In addition to carboxylates, both riboflavin and phytosiderophores contributed to Si mobilization, with phytosiderophores being particularly effective in mobilizing Si from quartz.