Silicon-enhanced oxalate exudation contributes to alleviation of cadmium toxicity in wheat

• Si reduces Cd uptake by roots.
• Root-to-shoot translocation of Cd is not affected by Si.
• Subcellular distribution pattern of Cd is not affected by Si.
• Enhanced root oxalate exudation by Si decreases Cd accumulation.

Cadmium (Cd) contamination is a threat to agricultural plant production. Silicon (Si) supply to Cd-stressed plants is known to ameliorate growth by decreasing the concentrations of Cd in plant tissues. To elucidate the underlying mechanism in wheat, hydroponic experiments were conducted under Cd stressed conditions in the presence or absence of Si, with special emphasis on root morphology, root exudation of oxalate and Cd allocation. We found that Si supply decreased Cd accumulation in plant, which resulted in better plant growth. Further studies of subcellular distribution showed that Si did not accelerate Cd accumulation into cell walls or vacuoles. Since acropetal translocation of Cd was not affected regardless of Si addition as well, we reason the Si-related decrease in leaf Cd concentrations was based on exclusion at the root side. This effect can be traced back to oxalate exudation by roots. In this study, the ability to exude oxalate through the root apex in Cd-stressed plants was weakened, while Si addition recovered the ability, thus reducing Cd uptake by roots. To our knowledge, we showed the first time that Si decreases Cd uptake by the increase of root oxalate exudation as an avoidance mechanism. Meanwhile, we also found Si supply increased total number of root tips which might facilitate root oxalate exudation.