Submitted to: Maydica
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/14/2004
Publication Date: 11/4/2004
Citation: Scott, M.P., Bhatnagar, S., Betran, J. 2004. Tryptophan and methionine levels in quality protein maize breeding germplasm. Maydica. 94(4):303-312. Interpretive Summary: Maize is used primarily for food and feed, but it is nutritionally limited by its content of lysine, tryptophan and methionine. Breeding with the o2 mutation has been used successfully to improve lysine levels. Maize developed by this process is called QPM (Quality Protein Maize). Because tryptophan and methionine can be nutritionally limiting as well as lysine, we examined the levels of these amino acids in QPM breeding germplasm. We found that tryptophan on average is lower in agronomically adapted QPM germplasm than in the starting material, and that methionine is on average lower in material carrying the o2 mutation than in normal genotypes. This result suggests that QPM breeding programs can be improved by incorporating strategies to maintain methionine and tryptophan levels. This will have a direct impact on plant breeders developing QPM. Ultimately, it will enable development of QPM with higher nutritional value than current QPM. This will result in more nutritious maize-based food and animal feed with lower cost, benefiting consumers.
Technical Abstract: Because maize is used often used in food or feed for monogastric animals, its amino acid balance is important. Quality Protein Maize (QPM) has an improved amino acid balance conferred by the o2 mutation, but lacks the agronomic deficiencies normally associated with this mutation such as opaque kernels that are soft and susceptible to mechanical and biological damage. Because the objective of QPM breeding programs is to develop maize with improved amino acid balance and superior agronomic traits, we determined the levels of the nutritionally limiting amino acids tryptophan and methionine. Tryptophan levels were negatively correlated with endosperm translucence, a measure of kernel hardness. This suggests that the process of selection for the hard-kernels reduces tryptophan levels. On average, germplasm containing the o2/o2 mutation had lower methionine levels than O2/O2 germplasm regardless of kernel hardness, suggesting that methionine levels are reduced by the o2/o2 mutation. A series of inbred lines was test-crossed to the O2/O2 inbred line Tx804. The predictive value of the characteristics of the inbred line for the characteristics of the hybrids was examined. The amino acid levels of the inbred lines were significantly correlated with those of the hybrids, although the predictive value was low (R2= 0.1 and 0.3 for methionine and tryptophan, respectively). The reduction in tryptophan during conversion to the hard-kernel phenotype and the reduction in methionine in o2 germplasm both reduce the nutritional value of QPM. It may be possible to correct these deficiencies by monitoring the tryptophan and methionine levels during breeding.