Fertilization controls various aspects of cereal growth such as tiller number, leaf size, and panicle size. However, despite such benefits, global chemical fertilizer use must be reduced to achieve sustainable agriculture. Here, based on field transcriptome data from leaf samples collected during rice cultivation, we identify fertilizer responsive genes and focus onOs1900, a gene orthologous toArabidopsis thaliana MAX1, which is involved in strigolactone biosynthesis. Elaborate genetic and biochemical analyses using CRISPR/Cas9 mutants reveal thatOs1900together with anotherMAX1-like gene,Os5100, play a critical role in controlling the conversion of carlactone into carlactonoic acid during strigolactone biosynthesis and tillering in rice. Detailed analyses of a series ofOs1900promoter deletion mutations suggest that fertilization controls tiller number in rice through transcriptional regulation ofOs1900, and that a few promoter mutations alone can increase tiller numbers and grain yields even under minor-fertilizer conditions, whereas a single defectiveos1900mutation does not increase tillers under normal fertilizer condition. SuchOs1900promoter mutations have potential uses in breeding programs for sustainable rice production.