Plays a role in regulating directional growth at the meristem/organ boundary. Is required for the promotion of leaf and floral organ growth and for the prolongation of the plastochron. Promotes organ growth in a non-cell-autonomous manner and may generate a mobile growth signal distinct from the classical phytohormones that prevents premature arrest of proliferation, until the correct primordium size has been reached. Functions probably in association with CYP78A7 in regulating relative growth of the shoot apical meristem and plant organs. Is required locally in developing ovules to stimulates cell proliferation and promote seed growth. Expressed in the periphery of the shoot apical meristem and inflorescence meristem, on the adaxial sides of developing floral organs and in developing ovules in the region where the integuments emerge. Expressed in the inner integument throughout ovule development. Reduced size of leaves, sepals, petals and seeds. Increased leaf number. Plants over-expressing CYP78A5 have twisted and kinked stems, defects in floral organogenesis and reduced fertility. Belongs to the cytochrome P450 family.