Involved in the control of organ size (PubMed:24486766). Upon auxin signaling, is targeted by the transcription activator SMOS1 to regulate cell expansion during organ size control (PubMed:24486766). May promote cell expansion by modulating microtubule organization (PubMed:24486766). Expressed in leaf sheaths and nodes. Plants silencing PHI-1 exhibit partial dwarfism due to decreased cell size, increased cell number, and abnormal cell arrangement and microtubule orientation. Belongs to the EXORDIUM family.