The world's largest toy maker plans to close its only unionized plant in North America later this year. The closing of Mattel Inc.'s Power Wheels factory in Fort Wayne, Ind., will eliminate 600 jobs. Production of battery-operated riding toys will be moved to a plant in Murray, Ky. The Indiana plant assembles 1.5 million Power Wheels annually. The company could start laying off workers as early as August, but the first permanent layoffs are expected in the fall, El Segundo-based Mattel said.