Contribution of granule bound starch synthase in kernel modification of quality protein maize

Quality Protein Maize (QPM) is derived from natural mutations that increase the level of lysine and tryptophan in maize grain. The rate of QPM development has been slowed down due to frequently poor kernel texture associated with opaque-2. Identification of genes (opaque-2 modifiers) with ability to overcome negative effects of the mutation, while maintaining high nutritional value, paved for development of commercially attractive high lysine maize genotypes. These have been used to develop modified opaque-2 genotypes. Screening for opaque-2 modifiers by conventional means has been difficult and poses a big challenge to large scale adoption of the technology. The biochemical basis of modified kernel texture in quality protein maize (QPM) is poorly understood. Recent proteomic analysis of several QPM lines indicated increased levels of granule-bound starch synthase I (GBSSI) in the soluble non-zein protein fraction of opaque-2 modified genotypes compared to wild type (Hunter et al., 2002: Gibbon et al., 2003.The report by Gibbon and co-workers (2003) suggests that GBSSI conditions similar counteractive effects on the opaque-2 mutation in a similar manner to the 27 kDa λ-zein class gene. The zein gene counteracts the effects of opaque-2 which interferes with normal transcriptional process (Lopes et al., 1994), by restoring processes that create hard kernels such as filling of empty spaces by gamma-zein-rich protein bodies, creating a vitreous kernel phenotype. The goal of this study was to investigate the possible role of granule-bound starch synthase (gbss) in the kernel modification of quality protein maize for use as genetic markers in quality maize breeding.