The present invention relates to 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 in plants or plant parts, preferably in seeds, embryos and/or cotyledons. The methods result in increases in: yield, growth of seedlings, early vigor, and stress tolerance. 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.