This invention relates to methods for altering cell proliferation in a plant by modulating the expression of an endogenous plant gene whose expression product affects cell proliferation. These methods can be used to modify plant characteristics, such as seed size. In particular, the invention provides methods for modifying cell proliferation in the integuments and/or seed coats of a plant by inhibiting expression of endogenous gene encoding Auxin response factor 2 (ARF2/MNT) or an orthologue thereof, such that the number of cells in the integuments and/or seed coat is increased, resulting in increased seed size. The plant or plant propagating material can be transformed, e.g., with a nucleic acid construct, to increase or decrease the expression of endogenous genes or the orthologues thereof that affect cell proliferation. The invention also relates to modified plants and reproducible plant material produced by the methods, and nucleic acid constructs for use in such methods.