Further characterization of a rice silicon efflux transporter, Lsi2

High silicon (Si) accumulation is required for high and sustainable production of rice. The uptake of Si by rice roots is mediated by both influx transporter Lsi1 and efflux transporter Lsi2. Here we further characterized Lsi2 in terms of localization and expression patterns. Analysis with promoter-GFP transgenic rice revealed that Lsi2 was expressed in the main and lateral roots, but not in the root hairs. In lateral roots, Lsi2 was localized at the proximal side of both exodermis and endodermis like main roots. The expression of Lsi2 gene was down-regulated by Si in the wild-type rice, but unaffected in the lsi2 mutant, suggesting that the expression of Lsi2 is regulated by Si accumulated in the shoots. Treatment with drought and abscisic acid rapidly resulted in decreased Lsi2 expression. Monitoring of Lsi2 expression at different growth stage showed a transient increase around the heading stage in rice grown in a paddy field. These expression patterns are similar to those of Lsi1, indicating that Lsi2 is co-regulated with Lsi1 and the expression of these two genes responds to both environmental condition and plant Si requirement.