LEAFY COTYLEDON 1 (LEC1) plays vital roles in the regulation of seed maturation in Arabidopsis. LEC1 encodes a homolog of yeast HAP3 or mammalian NF-YB/CBF-A subunit of trimeric CCAAT binding factor (CBF). Among the nine paralogs of NF-YB in Arabidopsis, LEC1-LIKE (L1L) is most closely related to LEC1, and can complement the lec1 mutation when expressed under the control of the LEC1 promoter. Although the nature of the B3-type seed maturation regulators as transcription factors have been investigated, knowledge of the molecular action of LEC1 is limited. When co-expressed with NF-YC2 in the presence of ABA, we found that LEC1 or L1L, but not other NF-YBs, activated the promoter of CRUCIFERIN C (CRC), which encodes a seed storage protein. However, additional expression of an NF-YA subunit interfered with the activation. The LEC1/L1L-[NF-YC2] activation depended on ABA-response elements present in the promoter, which led to the finding that LEC1/L1L-[NF-YC2] can strongly activate the CRC promoter in the absence of ABA when co-expressed with a seed-specific ABA-response element (ABRE)-binding factor, bZIP67. Functional coupling of LEC1/L1L-[AtNF-YC2] and bZIP67 was also observed in the regulation of sucrose synthase 2 (SUS2). Immunoprecipitation experiments revealed that L1L and bZIP67 formed a protein complex in vivo. These results demonstrate a novel plant-specific mechanism for NF-Y subunit function that enables LEC1 and L1L to regulate a defined developmental network.