Mutant populations are indispensable tools for investigating plant gene functions. Gain-of-function technology is one of the approaches used for the systematic production of mutant resources and activation tagging is a well-established method to generate gain-of-function mutants in plants. As an alternative approach for the systematic generation of a gain-of-function mutant population, we developed the Full-length cDNA OvereXpressor (FOX)-hunting system in which full-length cDNAs (fl-cDNAs) are overexpressed in plants to quickly identify candidate genes. The FOX-hunting system was used for high-throughput analysis of rice (Oryza sativa) genes heterologously expressed in Arabidopsis thaliana (rice FOX Arabidopsis lines). A large screening to identify and characterize rice genes with rice FOX Arabidopsis lines revealed that one of the isolated genes, BROAD-SPECTRUM RESISTANCE 1 (BSR1) conferred multiple or broad-spectrum disease resistance in both a dicotyledonous and monocotyledonous plant. We found that expression of rice fl-cDNAs without a homolog in Arabidopsis affected morphological traits. In addition, overexpression of homologous genes of rice and Arabidopsis led to a similar phenotype. Thus, we conclude that the FOX-hunting system is an excellent heterologous system and offers a new tool with which to explore gene function in rice.