Plant NAC-type transcription factor proteins contain a NARD domain for repression of transcriptional activation

Plant-specific transcription factor NAC proteins play essential roles in many biological processes such as development, senescence, morphogenesis, and stress signal transduction pathways. In the NAC family, some members function as transcription activators while others act as repressors. In the present study we found that though the full-length GmNAC20 from soybean did not have transcriptional activation activity, the carboxy-terminal activation domain of GmNAC20 had high transcriptional activation activity in the yeast assay system. Deletion experiments revealed an active repression domain with 35 amino acids, named NARD (NAC Repression Domain), in the d subdomain of NAC DNA-binding domain. NARD can reduce the transcriptional activation ability of diverse transcription factors when fused to either the amino-terminal or the carboxy-terminal of the transcription factors. NARD-like sequences are also present in other NAC family members and they are functional repression domain when fused to VP16 in plant protoplast assay system. Mutation analysis of conserved amino acid residues in NARD showed that the hydrophobic LVFY motif may partially contribute to the repression function. It is hypothesized that the interactions between the repression domain NARD and the carboxy-terminal activation domain may finally determine the ability of NAC family proteins to regulate downstream gene expressions.