Biofortification in Millets: A Sustainable Approach for Nutritional Security

Abstract

Nutritional insecurity is a major threat to the world's population that is highly dependent on cereals-based diet, deficient in micronutrients. Next to cereals, millets are the primary sources of energy in the semi-arid tropics and drought-prone regions of Asia and Africa. Millets are nutritionally superior as their grains contain high amount of proteins, essential amino acids, minerals, and vitamins. Biofortification of staple crops is proved to be an economically feasible approach to combat micronutrient malnutrition. HarvestPlus group realized the importance of millet biofortification and released conventionally bred high iron pearl millet in India to tackle iron deficiency. Molecular basis of waxy starch has been identified in foxtail millet, proso millet, and barnyard millet to facilitate their use in infant foods. With close genetic-relatedness to cereals, comparative genomics has helped in deciphering quantitative trait loci and genes linked to protein quality in finger millet. Recently, transgenic expression of zinc transporters resulted in the development of high grain zinc while transcriptomics revealed various calcium sensor genes involved in uptake, translocation, and accumulation of calcium in finger millet. Biofortification in millets is still limited by the presence of antinutrients like phytic acid, polyphenols, and tannins. RNA interference and genome editing tools [zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR)] needs to be employed to reduce these antinutrients. In this review paper, we discuss the strategies to accelerate biofortification in millets by summarizing the opportunities and challenges to increase the bioavailability of macro and micronutrients.

Keywords: biofortification; macronutrients; micronutrients; millets; nutritional security.

FIGURE 1

Germplasm collection of millet accessions in ICRISAT genebank. The outer concentric circle represents the entire collection of millets followed by reduced subsets of core and minicore collections in the inner circles.

FIGURE 2

Molecular structure of granule-bound starch synthase 1 (GBSS 1) gene in Setaria italica and Panicum miliaceum associated with waxy phenotypes. Two polymorphisms occur in the wx allele of Setaria: insertion of transposable elements, TSI-2 (5,250 bp – Type IV) in intron 1 and TSI-7 (7,674 bp – Type V) in exon 3. Three SNPs occur in the exon sequence of waxy Panicum: 15-bp deletion [GCGCTGAACAAGGA GGCGCTG] in exon 10 (S type), insertion of an adenine residue in exon 9 leading to reading frameshift mutation (L type) and G/A substitution in exon 7 converting cysteine residue to tyrosine (L type). Exons and introns are denoted by boxes and bars, respectively.

FIGURE 3

Molecular targets for micronutrient accumulation in millets. In rice (Oryza sativa), eight ZIPs facilitate iron and zinc uptake. OsZIP1 and OsZIP3 help in zinc uptake from soil; OsZIP4, OsZIP5, and OsZIP8 translocate zinc from roots to shoots; OsZIP4 and OsZIP8 are involved in grain filling. Two Fe²⁺ transporters, OsIRT1 and OsIRT2 help in iron uptake from the soil. In finger millet (Eleusine coracana), calcium uptake involves three Ca²⁺ transporters and two calmodulin-dependent protein kinases (CaMKs). TPC1, ATPase, CAX1, CaMK1, and CaMK2 play a vital role in uptake, translocation, and accumulation of calcium. ATPase, CaM stimulated type IIB Ca²⁺ pump; CAX1, Ca²⁺/H⁺ antiporter or exchanger; TPC1, two-pore channel; ZIP, Zn-regulated transporters and Iron (Fe) regulated transporter-like protein.