Transcription factor probably involved in vascular development and shoot tissue organization. Binds to the DNA sequence 5'-CCGAGTGTGCCCCTGG-3' present in the promoter region Box II of the phloem-specific rice tungro bacilliform virus (RTBV) promoter. May regulate tissue-specific expression of the RTBV promoter and virus replication. Binds DNA as a homodimer or as a heterodimer with RF2b. The heterodimer binds stronger to DNA than the homodimer. Interacts with TBP2. Expressed at high levels in levels in leaf sheath, moderate levels in leaf blade, but not in roots. Predominantly expressed in vascular tissues. Belongs to the bZIP family.