Carbohydrate partitioning from source to sink tissues is essential for plant growth and development. However, in maize (Zea mays L.), the molecular mechanisms by which callose synthase genes regulate this process remain largely unexplored. This study demonstrates that mutation of maize callose synthase12 (ZmCals12) results in increased carbohydrate accumulation in photosynthetic leaves but decreased carbohydrate content in sink tissues, leading to plant dwarfing and male sterility. Histochemical β-glucuronidase (GUS) activity assay and mRNA in situ hybridization (ISH) revealed that ZmCals12 expression mainly occurs in the vascular transport system. ZmCals12 loss-of-function decreased callose synthase activity and callose deposition in plasmodesmatas (PDs) and surrounding phloem cells (PCs) of the vascular bundle. The drop-and-see (DANS) assay indicated reduced PD permeability in photosynthetic cells and diminished transport competence of leaf veins in Zmcals12 mutants, resulting in decreased symplastic transport. Paraffin section analysis revealed that less-developed vascular cells (VCs) in Zmcals12 mutants likely disrupted sugar transport, contributing to the pleiotropic phenotype. Furthermore, impaired sugar transport inhibited internode development by suppressing auxin (IAA) biosynthesis and signaling in Zmcals12 mutant. These findings elucidate the mechanism by which ZmCals12-mediated callose deposition and symplastic transport regulate maize growth and development.