Involved in cell elongation in rice through dual regulation by brassinolide and auxin. Homodimer and heterodimers. Mainly expressed in roots and leaf sheaths. Detected in shoot apical meristem, organ primordia, including branch root primordia, and vascular tissues. By brassinolide (BL) and auxin (IAA), synergistically. Belongs to the Casparian strip membrane proteins (CASP) family.