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

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