Marker assisted conversion of maize inbred lines to quality protein maize (GPM) adapted to temperate climate

Maize is a poor source of protein both for humans and monogastric animals due to the lack of essential amino acids, such as lysine and tryptophan. Naturally occurring opaque2 (o2) mutation increases content of these amino acids, but also confers an undesirable phenotype leading to low yields. Quality protein maize (QPM) is agronomically acceptable and nutritionally improved opaque2 maize obtained through conventional breeding. Marker assisted breeding program aimed at developing QPM genotypes for growing in temperate regions is being conducted at Maize Research Institute Zemun Polje (MRIZP). The results presented in this paper relate to foreground selection aimed to identify plants that attained homozygosity at o2 locus in BC2F2 generation in conversion of four MRIZP commercial inbred lines. Maize inbred line ZPL5 converted to its QPM counterpart and adapted to temperate climate was used as o2 donor to the four recurrent parents (RP). Foreground selection was carried out with gene-specific markers phi057 and umc1066, both segregating as per the expectation. The percentage of recessive homozygotes in BC2F2 generation was approximately 25% (24.6% in RP1, 23.3% in RP2, 25% in RP3 and 24.4% in RP4). After the self-pollination of selected recessive homozygotes, BC2F3 progenies were screened for phenotypic and biochemical characteristics to confirm their nutritional and agronomical superiority. The results of scoring endosperm modifications revealed over 95% of hard endosperm kernels. The average tryptophan content ranged from 0.070% in RP1 to 0.087% in RP3. Out of 39 derivations from four lines, 19 had tryptophan content above the QPM threshold (0.075%). A total of 16 derivations were chosen for their highest tryptophan content. Their quality index was increased by 2-46% relative to the recurrent parent. These line will serve as an important breeding material for developing QPM maize hybrids adapted to temperate regions.