Transcription factor that acts as regulator of iron homeostasis (PubMed:22755510). May act as negative regulator of iron transportation from root to shoot (PubMed:22755510). Does not seem to be involved in the suppression of the induction of iron deficiency responsive genes (PubMed:22755510). Strongly induced by iron deficiency in roots (PubMed:22755510). Slightly induced in shoots (PubMed:22755510). Slight reduction of shoot length and 2-fold reduction of seed set. Plants overexpressing BHLH133 are hypersensitive to iron deficiency. Belongs to the bHLH protein family.