Maize silicon transporters
Maize (Zea mays L.) is a Si-accumulating species although its accumulation is not as high as rice. However, the transporters involved in Si uptake and distribution are unknown. Here, we isolated and characterized three maize silicon transporter genes ZmLsi1, ZmLsi2 and ZmLsi6, which are homologues, respectively, of rice silicon transporter genes. Each transporter shared more than 80% between maize and rice at amino acid level. Both ZmLsi1 and ZmLsi6 showed Si influx transport activity, while ZmLsi2 showed efflux activity, when they were expressed in Xenopus laevis oocytes. mRNA of ZmLsi1 and ZmLsi2 was expressed mainly in the roots, while ZmLsi6 was also in the shoots. Immunostaining showed that ZmLsi1 was localized at the distal side of plasma membrane of root epidermis, hypodermis and cortex, while ZmLsi2 was only in the endodermis without polarity. In contrast, ZmLsi6 was localized at the xylem parenchyma cells of the leaf blades. These results suggest that ZmLsi1 and ZmLsi2 are involved in the uptake of Si from the roots and ZmLsi6 is responsible for transport of Si out of the xylem, therefore regulating Si distribution in the leaves.