In plants, variations in seed size and number are outcomes of different reproductive strategies. Both traits are often environmentally influenced, suggesting that a mechanism exists to coordinate these phenotypes in response to available maternal resources. Yet, how maternal resources are sensed and influence seed size and number is largely unknown. Here, we report a mechanism that senses maternal resources and coordinates grain size and number in the wild rice Oryza rufipogon, a wild progenitor of Asian cultivated rice. We showed that FT-like 9 (FTL9) regulates both grain size and number and that maternal photosynthetic assimilates induce FTL9 expression in leaves to act as a long-range signal that increases grain number and reduces size. Our findings highlight a strategy that benefits wild plants to survive in a fluctuating environment. In this strategy, when maternal resources are sufficient, wild plants increase their offspring number while preventing an increase in offspring size by the action of FTL9, which helps expand their habitats. In addition, we found that a loss-of-function allele (ftl9) is prevalent among wild and cultivated populations, offering a new scenario in the history of rice domestication.