Interactions between iron and silicon in cucumber

Abstract

Silicon (Si) and iron (Fe) are respectively the second and the fourth most abundant minerals in the earth’s crust. While the essentiality of Fe has been discovered in the middle of the 19th century, Si is still not fully accepted as an essential element for higher plants; however, Si is proved to alleviate multiple environmental stresses in plants. So far, Fe deficiency have mainly been studied and characterized in nutrient solution experiments devoid of Si, hence information on the interactions between these two minerals in plants is still limited. Here we investigated how Si ameliorates Fe deficiency in cucumber (model of Strategy 1 and Si-accumulating species), focusing on the mechanisms involved in Fe acquisition from the rhizosphere and utilization of root apoplastic Fe, as well as on Fe distribution towards young leaves in Fe-deficient cucumber plants. Application of Si increased the root apoplastic Fe pool, together with the enhanced expression of genes involved in reduction-based Fe uptake (FRO2, IRT1 and HA1). Moreover, in Fe deficient cucumber roots, Si influenced the genes involved in the carboxylate, shikimate and phenylpropanoid metabolism, thus resulted in enhanced accumulation of Fe chelating compounds (organic acids and phenolics) for improved Fe mobilization from the rhizosphere and reutilization of root apoplastic Fe (Pavlovic et al., 2013). Application of Si also facilitated mobility and xylem traslocation of Fe towards shoot, along with the accumulation of Fe-mobilizing compounds such as citrate in xylem sap, root and shoot tissues or cathechins in roots (Pavlovic et al., 2013; Bityutskii et al., 2014). Very recently, we demonstrated that Si enhanced remobilization of labelled 57Fe from old leaves and its retranslocation to the younger leaves is accompanied by Si-induced expression of genes encoding both NA biosynthesis (SAM and NAS) and YSL transporters responsible for symplastic Fe unloading in leaves and floem transport of Fe-NA complex. In conclusion, the alleviating effect of Si seems to be more indirect, by affecting activation of Fe deficiency-related genes responsible for enhanced root acquisition and tissue mobilization of Fe. This work not only provides new evidence for the beneficial role of Si in iron nutrition, but, in perspective, can be of practical importance in the development of new sustainable measures for controlling Fe chlorosis in calcareous soils, which in general are low in available Si.