A process is provided for modifying growth or architecture of plants by altering the level or the functional level of a cell division controlling protein, preferably a cell-division controlling protein that binds phosphorylates retinoblasoma-link proteins, more preferably a cyclin, particularly a D-type cyclin within cells of a plant. Also provided are chimeric genes comprising a transcribed DNA region encoding an RNA or a protein, which when expressed either increases or decreases the level or functional level of a cell-division controlling protein, and plant cells and plants expressing such chimeric genes.