Parasitic weeds of the genera Striga and Orobanche are considered the most damaging agricultural agents in the developing world. An essential step in parasitic seed germination is sensing a group of structurally related compounds called strigolactones that are released by host plants. Although this makes strigolactone synthesis and action a major target of biotechnology, little fundamental information is known about this hormone. Chemical genetic screening using Arabidopsis thaliana as a platform identified a collection of related small molecules, cotylimides, which perturb strigolactone accumulation. Suppressor screens against select cotylimides identified light-signaling genes as positive regulators of strigolactone levels. Molecular analysis showed strigolactones regulate the nuclear localization of the COP1 ubiquitin ligase, which in part determines the levels of light regulators such as HY5. This information not only uncovers new functions for strigolactones but was also used to identify rice cultivars with reduced capacity to germinate parasitic seed.