The LAFL transcription factors LEC2, ABI3, FUS3 and LEC1 are master regulators of seed development. LEC2, ABI3 and FUS3 are closely related proteins that contain a B3-type DNA binding domain. We have previously shown that LEC1 (a NF-YB type protein) can increase LEC2 and ABI3 but not FUS3 activity. Interestingly, FUS3, LEC2 and ABI3 contain a B2 domain, the function of which remains elusive. We showed that LEC1 and LEC2 partially co-localised in the nucleus of developing embryos. By comparing protein sequences from various species, we identified within the B2 domains a set of highly conserved residues (i.e. TKxxARxxRxxAxxR). This domain directly interacts with LEC1 in yeast. Mutations of the conserved amino acids of the motif in the B2 domain abolished this interaction both in yeast and in moss protoplasts and did not alter the nuclear localisation of LEC2 in planta. Conversely, the mutations of key amino acids for the function of LEC1 in planta (D86K) prevented the interaction with LEC2. These results provide molecular evidences for the binding of LEC1 to B2 domain containing transcription factors, to form heteromers, involved in the control of gene expression.