Asparagine Cycling Alleles Strongly Impact Grain Protein Concentration

The Illinois Long Term Selection Experiment (ILTSE) for grain protein concentration began in 1896 and has generated phenotypic extremes for grain protein and nitrogen use efficiency traits. The high protein lines also distinctly hyperaccumulate asparagine. Asparagine is an amino acid that functions in storage and transport of nitrogen in maize. Asparagine concentrations are modulated by two asparagine cycling genes: asparagine synthetase and asparaginase. High and low protein lines from the ILTSE have lesions in the promoters of these genes that appear to change gene expression and have been divergently driven to fixation in the ILTSE populations. To test the role of these asparagine cycling alleles, near isogenic lines (NILs) were generated where high protein line alleles were introgressed into the low protein line and vice versa. The NILs differ for grain protein as measured by near infrared reflectance and an fl2zein-RFP transgene where red kernel color reports expression of the maize storage protein, zein. The introgression of high protein alleles into the low protein background increased grain protein, whereas low protein alleles decreased grain protein. The degree of change in protein mediated by asparagine cycling variants exceeds the variation observed for diverse maize inbred lines for the US Corn Belt.