Plant-specific transcription factors are classified according to DNA-binding domains (DBDs) that were believed to be distinct from those of prokaryotes or other lineages of eukaryotes. Recently, structures of the DBDs including WRKY, NAC, B3, and SBP, which comprise major families of transcription factors, were determined by NMR spectroscopy or X-ray crystallography. In this review, we summarize the recent progress of structural biology in this field, especially on their DNA-binding mechanism and structural similarity to DBDs from other kingdoms. Unexpected structural relationships, together with recent identifications of homologous sequences in a variety of genomes, indicated that majority of the "plant-specific" DBDs originated from non-plant species, and that they largely expanded along with the evolution of higher plants.
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