The present invention relates to cytochrome P450 protein originating from Arabidopsis thaliana which can be used for increasing seed size or storage protein content in seed or for increasing water stress resistance of plant, a gene encoding said protein, a recombinant plant expression vector comprising said gene, a method of increasing seed size or storage protein content in seed and a method of increasing water stress resistance of plant by using said vector, plants produced by said method and transgenic seed of said plants. According to the present invention, by using cytochrome P450 gene of the present invention, seed size or storage protein content in seed can be increased or water stress resistance of plant can be increased.
