Rice is a staple crop for more than 50% of the world population. Insufficient levels of Fe and Zn in the seed cause deficiency diseases in populations relying on rice as the main source of calories. More than 2 billion people suffer from Fe deficiency anemia and Zn deficiency. Rice grown on contaminated soils also causes cadmium (Cd) toxicity. Therefore, biofortification strategies that enhance Fe and Zn levels in rice endosperm should do so without also encouraging the accumulation of Cd. The maximum levels of Fe and Zn reported in seeds of biofortified rice are well below the Recommended Daily Intake. This suggests an upper limit for Fe and Zn accumulation in the seed. Metal homeostasis acts to prevent the seed becoming overloaded with these metals, but the underlying mechanisms are largely unknown. Nicotianamine (NA) and deoxymugenic acid (DMA) are endogenous metal chelating molecules; increasing the amount of NA or DMA in the plant increases the accumulation of Fe and Zn in the seeds. NA and DMA are synthesized from S-adenosylmethionine in three steps involving nicotianamine synthase (NAS), nicotianamine aminotransferase (NAAT) and DMA synthase (DMAS).
We generated transgenic rice lines co-expressing NAS and NAAT. These plants accumulated higher levels of NA and DMA and promoted the accumulation of Fe and Zn in seeds Increasing the external supply of Fe affected the uptake of Fe and Zn into the roots and the mobilization of these metals in the aboveground organs, but compensatory mechanisms involving sequestration in leaves have a buffering effect and impose strict limits on the accumulation of metals in the seed. Surprisingly, the preferential retention of Fe and Zn in the seed led to the competitive exclusion of Cd, halving the amount of Cd. This can provide a useful strategy to increase the abundance of metal nutrients in rice while ensuring that toxic metals are exported to the bran. Such strategies could help simultaneously to address micronutrient deficiency and heavy metal toxicity in communities that rely predominantly on cereal-based diets.