Genetic regulation of homeostasis, uptake, bio-fortification and efficiency enhancement of iron in rice

Soils of large areas of the globe are affected by deficiency or toxicity of iron (Fe), making it one of the major limitations to higher productivity of rice. Deficiency of Fe, an essential micronutrient for growth and development of rice, produces grain with low Fe-content. Consumption of low-Fe rice causes malnutrition affecting human health. Biofortification is an easy and low-cost way to enhance Fe content in rice, the staple food of more than half of the global population. Identification of relevant quantitative trait loci (QTLs) and genes controlling the stresses are needed for developing tolerant genotype(s). Fe deficiency is commonly observed in alkaline and aerobic soils, while toxicity is seen in low pH soils of lowland rice ecology. Rice plants cope up under deficiency or toxicity conditions through various morphological, physiological and differential gene expression strategies. Rice plant uses various transporter genes like OsNAS1, OsNAS2, OsIRT1, OsIRT2, OsNRAMP1, OsYSL15 and OsYSL16 under deficiency stress while OsIRT1, OsFRO2, OsVIT1, OsVIT2, OsNRAMP6, OsNAAT1, OsNAS3, OsNAC4, OsNAC5 and OsNAC6 under toxicity condition are involved for Fe homeostasis. Several QTLs including qFe3:1, qFe3:2, qFe7:1, qFe9:1, qFe9:2, qFe10:1, Fe11:1, qFe3.3 and qFe7.3 associated with grain-Fe content have been identified. Many Fe binding and transporters genes like OsZIP1, OsHMA4, OsACA2, OsZIP2, OsCNGC, OsZIP3, OsZIP5, OsZIP9, OsHma2, ABC transporter, OsNAS3, heavy metal transporter, Chy zinc finger and OsACA9 have been identified to improve grain-Fe content. Donor lines for grain-Fe content have been identified from rice germplasms showing even up to 147 mu g g(-1) in brown rice. Fe content in rice grain has been enhanced to many folds using ferritin genes of soybean and common bean, NAS gene and mugineic acid synthase genes (HvNAS1 and HvNAAT-A,-B or IDS3) of barley, nicotianamine transporter gene (OsYSL2) and nicotinamine synthase genes (OsNAS1, OsNAS2 and OsNAS3) through transgenic approach. The paper analyses the mechanisms of tolerance to Fe-deficiency and toxicity, identification of genes/QTLs responsible for tolerance under the stresses and helping for biofortification, assesses the stress affected symptoms, reviews the screening and summarizes the efforts for breeding programs for improving tolerance to Fe-deficiency and toxicity in rice.