Development of beta-carotene, lysine, and tryptophan-rich maize (Zea mays) inbreds through marker-assisted gene pyramiding

Maize (Zea mays L.) is the leading cereal crop and staple food in many parts of the world. This study aims to develop nutrient-rich maize genotypes by incorporating crtRB1 and o2 genes associated with increased beta-carotene, lysine, and tryptophan levels. UMI1200 and UMI1230, high quality maize inbreds, are well-adapted to tropical and semi-arid regions in India. However, they are deficient in beta-carotene, lysine, and tryptophan. We used the concurrent stepwise transfer of genes by marker-assisted backcross breeding (MABB) scheme to introgress crtRB1 and o2 genes. In each generation (from F-1, BC1F1-BC3F1, and ICF1-ICF3), foreground and background selections were carried out using gene-linked (crtRB1 3 ' TE and umc1066) and genome-wide simple sequence repeats (SSR) markers. Four independent BC3F1 lines of UMI1200 x CE477 (Cross-1), UMI1200 x VQL1 (Cross-2), UMI1230 x CE477 (Cross-3), and UMI1230 x VQL1 (Cross-4) having crtRB1 and o2 genes and 87.45-88.41% of recurrent parent genome recovery (RPGR) were intercrossed to generate the ICF1-ICF3 generations. Further, these gene pyramided lines were examined for agronomic performance and the beta-carotene, lysine, and tryptophan contents. Six ICF3 lines (DBT-IC-beta(1)sigma(4)-4-8-8, DBT-IC-beta(1)sigma(4)-9-21-21, DBT-IC-beta(1)sigma(4)-10-1-1, DBT-IC-beta(2)sigma(5)-9-51-51, DBT-IC-beta(2)sigma(5)-9-52-52 and DBT-IC-beta(2)sigma(5)-9-53-53) possessing crtRB1 and o2 genes showed better agronomic performance (77.78-99.31% for DBT-IC-beta(1)sigma(4) population and 85.71-99.51% for DBT-IC-beta(2)sigma(5) population) like the recurrent parents and beta-carotene (14.21-14.35 mu g/g for DBT-IC-beta(1)sigma(4) and 13.28-13.62 mu g/g for DBT-IC-beta(2)sigma(5)), lysine (0.31-0.33% for DBT-IC-beta(1)sigma(4) and 0.31-0.34% for DBT-IC-beta(2)sigma(5)), and tryptophan (0.079-0.082% for DBT-IC-beta(1)sigma(4) and 0.078-0.083% for DBT-IC-beta(2)sigma(5)) levels on par with that of the donor parents. In the future, these improved lines could be developed as a cultivar for various agro-climatic zones and also as good genetic materials for maize nutritional breeding programs.