Silicon alleviates iron deficiency in cucumber by promoting mobilization of iron in the root apoplast

Root responses to lack of iron (Fe) have mainly been studied in nutrient solution experiments devoid of silicon (Si). Here we investigated how Si ameliorates Fe deficiency in cucumber (Cucumis sativus) with focus on the storage and utilization of Fe in the root apoplast. A combined approach was performed including analyses of apoplastic Fe, reduction-based Fe acquisition and Fe-mobilizing compounds in roots along with the expression of related genes. Si-treated plants accumulated higher concentrations of root apoplastic Fe, which rapidly decreased when Fe was withheld from the nutrient solution. Under Fe-deficient conditions, Si also increased the accumulation of Fe-mobilizing compounds in roots. Si supply stimulated root activity of Fe acquisition at the early stage of Fe deficiency stress through regulation of gene expression levels of proteins involved in Fe acquisition. However, when the period of Fe deprivation was extended, these reactions further decreased as a consequence of Si-induced enhancement of the Fe status of the plants. This work provides new evidence for the beneficial role of Si in plant nutrition and clearly indicates that Si-mediated alleviation of Fe deficiency includes an increase of the apoplastic Fe pool in roots and an enhancement of Fe acquisition.