Functional characterization of the SbNrat1 gene in sorghum

The Natural Resistance Associated Macrophage Protein (Nramp) members play diverse roles in metal transport in plants. Recent studies have showed that OsNrat1 (OsNramp4) encodes an Al transporter, which is required for rice Al tolerance. In this study, we functionally characterized a Nramp member in sorghum, SbNrat1, which is homologous to OsNrat1 with 88% identity. SbNrat1 was expressed in both roots and shoots, and its expression was not induced by Al treatment. When expressed in yeast, SbNrat1 transports trivalent Al ion, but not Mn and Cd. Furthermore, introduction of SbNrat1 into the rice mutant osnrat1 can rescue its sensitivity to Al. However, no correlation between Al tolerance and the expression level of SbNrat1 was found in thirteen sorghum cultivars tested. These results indicate that SbNrat1 functions as an Al transporter that is possibly involved in basic Al tolerance in sorghum.