Breeding for low phytic acid maize – status and prospects

Abstract

Seed biomineralization is an important biochemical process by which the mature seeds reserve some of the essential macro􀀐 and micronutrients required during germination. One of the most widely studied seed biomineralization process is the storage of seed phosphorous (P) and other important cations namely calcium (Ca), potassium (K), Magnesium (Mg), Manganese (Mn), Iron (Fe)and Zinc (Zn) in phytate or phytic acid (PA) salts. The seed biomineralization is provide essential micro- and macronutrients to germinating seeds through their mobilization; most the above mentioned mineral elements act as an important co factor in several metabolic processes; either directly or indirectly determine the function or efficiency of several enzymes which catalyze several biochemical processes. However, sometimes the level of such reserves in some of the crops like maize, soybean, barley, wheat and many other crops are so high that the level is not only well above the requirement by the germinating seeds but also affects the bioavailability of such nutrients in animals which depend on these crops for their dietary nutritional requirement. In this context, PA has been termed rightly as an anti-nutritional factor and it is an important challenge to reduce its level without compromising the overall germination and vigour of the seedlings in different crop species. The distribution of the phytate and other essential mineral elements in different parts of the seeds especially in cereals were studies (O’Dell et al. 1972). The results suggested that >80% of the total phosphorous is stored in the form of PA. The distribution of PA among different parts of the seeds has shown that approximately 80􀀐90% of the PA is stored in germ (maize) or pericarp (rice) or aleurone layer (wheat); differential storage organ depending upon the type of grain or crop. Raboy et al (1990) for the first time in maize surveyed the level of PA in different maize mutants. The study on different mutations has gave an opportunity to explore for breeding for low phytic acid in maize as the study has indicated that the mutants which affect the embryo reduced the PA substantially without reducing the total P; the corresponding increase in the inorganic phosphorous (Pi) was observed. In maize, several mutant alleles in three different gene(s) namely lpa1, lpa2and lpa3 which affect three important critical steps in PA biosynthesis namely PA transportation, insositol phosphate kinases (IPK) or myo􀀐insositol kinases (MIK) have been identified (Raboy et al. 2000 and Shi et al. 2005). The first effort to transfer of LPA mutant lpa1-1 into different genetic background mainly the elite lines of hybrids was attempted as early as 1998. Several near-isogenic lines (NILs) by transferring lpa1-1 have been developed and isohybrids using such NILSs have also been developed. The performances of such resultant isohybrid were evaluated for various agronomic and different yield component traits across multiple locations. The results had shown that the hybrids have showed normal growth and development from germination to harvesting. The hybrids have shown good stalk strength/ standability and also comparable traits for flowering and other yield contributing traits. (Ertl et al. 1998, Raboy, 2002). In recent years, the availability of molecular markers linked to gene(s) determining low phytic acid traits has facilitated breeders to mobilize such mutant alleles across different genetic background through marker-assisted selection (MAS). In case of maize, few elite inbred lines are converted by transferring LPA mutants like lpa1􀀐1 (Naidoo et al. 2012), lpa2􀀐2 (Tamilkumar et al. 2014, Sureshkumar et al. 2014a, 2014b). However, the low phytic hybrids have as yet not released in India commercial cultivation. Presently in India, several hybrids are under pipeline with low􀀐phytic acid mutant alleles which are at various developmental stages. The hybrids are being developed through marker assisted backcross breeding (MABB).