Adding at least one gene involved in plant host cell T-DNA integration enhances transformation by Agrobacterium. The histone H2A gene encoded by the Arabidopsis RAT5 gene increases transformation frequencies of plants, most likely by causing overexpression of a product needed for T-DNA integration. Agrobacterium tumefaciens genetically transforms plant cells by transferring a portion of the bacterial Ti-plasmid, designated the T-DNA, to the plant, and integrating the T-DNA into the plant genome. However, not all plants are transformable by Agrobacterium and transformation frequencies may be too low to be useful. Little is known about the T-DNA integration process, and no plant genes involved in integration have been identified prior to the present invention.