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 RF2a. The heterodimer binds stronger to DNA than the homodimer. Expressed at high levels in roots, low level in leaf sheath, but not in leaf blade. Predominantly expressed in vascular tissues. Belongs to the bZIP family.