Genetic analysis of amino acid accumulation in opaque-2 maize endosperm

The opaque-2 mutation in maize (Zea mays) is associated with an increased l evel of free amino acids (FAA) in the mature endosperm. In particular, ther e is a high concentration of lysine, the most limiting essential amino acid . To investigate the basis for the high-FAA phenotype of opaque-2 maize, we characterized amino acid accumulation during endosperm development of seve ral wild-type and opaque-2 inbreds. Oh545 omicron2 was found to have an exc eptionally high level of FAA, in particular those derived from aspartate (A sp) and intermediates of glycolysis. The FAA content in Oh545 omicron2 is 1 2 times greater than its wild-type counterpart, and three and 10 times grea ter than in Oh51A omicron2 and W64A omicron2, respectively. We crossed Oh54 5 omicron2 to Oh51A omicron2 and analyzed the F-2:3 progeny to identify gen etic loci linked with the high FAA level in these mutants. Quantitative tra it locus mapping identified four significant loci that account for about 46 % of the phenotypic variance. One locus on the long arm of chromosome 2 is coincident with genes encoding a monofunctional Asp kinase 2 and a bifuncti onal Asp kinase-homo-Ser dehydrogenase-2, whereas another locus on the shor t arm of chromosome 3 is linked with a cytosolic triose phosphate isomerase 4. The results suggest an alternation of amino acid and carbon metabolism leads to overproduction and accumulation of FAA in opaque-2 mutants.