The present invention relates to methods for stimulating root growth and/or enhancing the formation of lateral or adventitious roots and/or altering root geotropism comprising expression of a cytokinin oxidase or comprising expression of another protein that reduces the level of active cytokinins in plants or plant parts. Also provided by the present invention are methods for increasing seed size and/or weight, embryo size and/or weight, and cotyledon size and/or weight. The methods comprise expression of a cytokinin oxidase or expression of another protein that reduces the level of active cytokinins in plants or plant parts. Methods and compositions for increasing seed yield are also provided. The invention also relates to isolated plant cytokinin oxidase proteins, nucleic acid sequences encoding cytokinin oxidase proteins as well as to vectors, host cells, transgenic cells and plants comprising such sequences. The use of these sequences for improving root-related characteristics including increasing yield and/or enhancing early vigor and/or modifying root/shoot ratio and/or improving resistance to lodging and/or increasing drought tolerance and/or promoting in vitro propagation of explants and/or modifying cell fate and/or plant development and/or plant morphology and/or plant biochemistry and/or plant physiology, is also provided. The invention also relates to methods for identifying and obtaining proteins and compounds interacting with cytokinin oxidase proteins as well as the use of such proteins and/or compounds as plant growth regulators or herbicides.