To attract insects, flowers produce nectar, an energy-rich substance secreted by specialized organs called nectaries. For Arabidopsis thaliana, a rosid species with stamen-associated nectaries, the floral B-, C-, and E- functions were proposed to redundantly regulate nectary development. Here we investigated the molecular basis of carpel-associated nectary development in the asterid species Petunia hybrida. We show that its euAGAMOUS (euAG) and PLENA (PLE) C-lineage MADS-box proteins are essential for nectary development, while their overexpression is sufficient to induce ectopic nectaries on sepals. Furthermore, we demonstrate that Arabidopsis nectary development also fully depends on euAG/PLE C-lineage genes. In turn, we show that petunia nectary development depends on two homologs of CRABS CLAW (CRC), a gene previously shown to be required for Arabidopsis nectary development, and demonstrate that CRC expression in both species depends on the members of both euAG/PLE C-sublineages. Therefore, petunia and Arabidopsis employ a similar molecular mechanism underlying nectary development, despite otherwise major differences in the evolutionary trajectory of their C-lineage genes, their distant phylogeny and different nectary positioning. However, unlike in Arabidopsis, petunia nectary development is position-independent within the flower. Finally, we show that the TARGET OF EAT (TOE)-type BLIND ENHANCER (BEN) and APETALA2 (AP2)-type REPRESSOR OF B-FUNCTION (ROB) genes act as major regulators of nectary size.