a ∣ In leaf primordia, miR165/6 is specifically produced in the abaxial epidermis and forms a diminishing gradient towards the adaxial side. This gradient leads to the all-or-none expression pattern of the miRNA target gene PHABULOSA (PHB), which in turn specifies adaxial identity. b ∣ In roots, miR165/6 is specifically produced in the endodermis (En) and forms a diminishing gradient of expression towards the inner parts of the root. This gradient results in graded expression of PHB, with increasing levels of PHB expression leading to the formation of the protoxylem (Px) and metaxylem (Mx) cell types. Pe, pericycle.