Identification of Two Cucumber Putative Silicon Transporter Genes in Cucumis sativus

Silicon (Si) is known to be beneficial to plants in alleviating biotic and abiotic stresses. However, its uptake from soil and transport in plants is not well documented. In this work, we cloned two cucumber putative silicon transporter genes (CSiT-1 and CSiT-2) and characterized some of their functions in cucumber plants. The proteins coded by these two genes were almost identical to other plant Si influx transporters, containing two conserved NPAs (asparagine proline alanine) and four ar/R (aromatic/arginine) selectivity filter-forming residues [G(Gly)/C(Cys), S(Ser), G(Gly), and R(Arg)]. The two genes were expressed in all tissues and most interestingly the diurnal patterns of their expression were displaying a circadian rhythm. Compared with rice OsLsi1 or other plant Si transport genes, these two genes were highly expressed in leaves and their expression levels corresponded to Si supply in the hydroponic culture solution. Both CSiT-1 and CSiT-2 proteins were localized in the plasma membrane when they were heterogeneously expressed in tobacco leaves. Our data suggested that the two genes cloned from cucumber were likely coding silicon transporters and had a distinct diurnal expression pattern of a circadian rhythm both in leaves and roots.