Auxin plays a vital role in plant organ development, influencing organ initiation and patterning across all axes. The diversity in auxin patterning results from changes in the activities and expression of auxin signaling components, including the AUX/IAA repressors. Higher land plants have multigene AUX/IAA families, which leads to functional redundancy and a lack of phenotype in loss of function mutants. Instead, dominant mutations, which prevent AUX/IAA degradation in response to auxin, have highlighted the importance of these proteins in organ development. Here we report a new dominant AUX/IAA mutant in maize, Hoja loca1 (Oja). Oja has a mutation in the degron motif of ZmIAA28 and affects aerial organ initiation and medio-lateral patterning in the leaf. These phenotypes contrast with other maize AUX/IAA mutants that affect the root or inflorescence only. Oja illustrates the role of auxin signaling in the tight coordination of phytomer unit development and provides evidence of species-specific sub-functionalization of the AUX/IAAs.One Line SummaryThe maize AUX/IAA ZmIAA38 is involved in phytomer coordination, aerial organ initiation and mediolateral patterning, and illustrates species specific sub-functionalization of the AUX/IAAs.