Required for cell expansion in leaves. May mediate brassinosteroid (BR)-induced leaf growth. May play a role in the control of BR responses in roots. May be involved in signaling processes that coordinate BR responses with environmental or developmental signals. Expressed in root tips, vascular tissue of roots, shoot apex, rosette leaves and embryos. By the synthetic auxin naphthaleneacetic acid (NAA) and 24-epibrassinolide. Down-regulated by kinetin. Reduced leaf size, root length and biomass production due to diminished expansion of epidermis and parenchyma cells. Over-expression of EXO promotes growth in both shoots and roots. Belongs to the EXORDIUM family.