Plant growth regulators are generally:

Single PGR has multiple functions, e.g. auxins are responsible for apical dominance, abscission in older parts of plants, controls differentiation in xylem and promotes cell division in cambium.
One type of PGR can be used to perform same functions on different plant species. For example, dilute solution of NAA and 2,4-D produced in laboratory induce flowering in litchi and pineapple. 
Plant growth regulators are generally produced in many parts of the plant. For example, auxins are formed in the cells found at root and shoot apices. Cytokinin formed in are produced in cells found in young fruits, developing shoot buds and root apices.
PGRs generally do not show their effects on the cells producing them. For example, auxins are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action like older leaves, fruits and flower; apical buds, cambial cells etc.