Significance Mutations affecting the transcription factor encoded by the gene o2 are important in maize agriculture because they result in improved grain nutritional quality. The mutations also cause detrimental effects by reducing kernel hardness and diminishing agronomic quality and food applications. The undesirable characteristics are not fully understood because the o2 product regulates multiple targets that could contribute to the phenotype. This study investigated one target that had not been previously mutated, pyruvate phosphate dikinase (PPDK), and showed that PPDK deficiency in isolation causes the negative phenotype associated with reduced kernel hardness. Thus, maize improvement may be better accomplished by targeting individual metabolic pathways determining protein and amino acid balance rather than pleiotropic regulators such as the o2 transcription factor.