The F-box protein Coronatine Insensitive (COI) is a key receptor for the jasmonic acid signaling pathway in plants. To investigate the functions of the six maize COI proteins (COI1a, COI1b, COI1c, COI1d, COI2a, and COI2b), we made single, double, and quadruple loss-of-function mutants. Double-mutantcoi2a coi2bpollen was inviable, and no homozygous mutant plants were obtained. Thecoi1quadruple mutant (coi1-4x) exhibited shortened internode lengths, decreased photosynthesis, leaf discoloration, microelement deficiencies, and accumulation of DELLA, a repressor of the gibberellic acid signaling pathway. Co-expression of maizeCOIandDWARF9(DELLA) genes inNicotiana benthamianashowed that the COI proteins lead to proteasome-dependent DELLA degradation. Downregulated genes in thecoi1-4xmutant were primarily ones that are normally induced by gibberellic acid, most of which were bundle sheath or mesophyll-specific genes, including those encoding C4-specific photosynthetic enzymes.Ectopic expression of maizeCOIgenes inN. benthamianashowed that COI2a is fully localized in the nucleus and interacts with maize JAZ proteins, the canonical COI repressor partners. However, maize COI1a and COI1c proteins showed only partial nuclear localization and lacked binding to most of the JAZ proteins. Together, these results show divergent functions of the six COI proteins in regulating maize growth and defense.