Cadmium (Cd) contamination in rice is concerned as a potential health risk for humans because rice is a major source of Cd intake from foods. Therefore, it is necessary to develop a rice cultivar with low Cd uptake. We previously found rice mutants showing low Cd uptake from a rice mutant library developed by carbon ion beam irradiation method. Previous results indicated that the defective OsNramp5 transporter greatly reduces Cd concentration in rice grains. However, because OsNramp5 is a major transporter for manganese (Mn), Mn concentration in mutants was decreased as well. On the other hand, iron (Fe) concentration in straw of the mutant was 1.5 - 2 times higher than that of the original rice cultivar (WT) when they were cultivated under continuously submerged conditions, while the Fe concentration of grains in the mutant was not significantly increased.

To investigate the molecular mechanisms on the increased Fe level in straws for rice mutants with defective OsNramp5, we performed a microarray analysis using root samples of rice seedlings grown in hydroponic culture. The analysis showed that expression levels of several genes involving in Fe transport were significantly upregulated in the mutant compared to the WT. Because the transcript levels of OsIRO2, OsNAS1, OsNAS2, OsYSL15 and OsTOM2 were increased in the mutant, it was speculated that Strategy II mechanisms of chelation-based Fe uptake were activated. Thus, OsNramp5 mutation enhanced the expression levels of genes involved in Fe transport independent of Fe deficiency. According to the study by Inoue et al. (2009), OsYSL15 gene expression is not induced by Mn deficiency. Thus, we guess that such enhanced gene expression could not be due to the reduced Mn uptake in OsNramp5 defective mutants.

Further investigation would be needed to elucidate the mechanisms on the changes in Fe transport metabolism in the OsNrmap5 defective rice mutant.