Involved in jasmonate (JA) signaling. Required for jasmonate signaling in plant defense responses (PubMed:22529386). Can complement Arabidopsis coi1-1 mutant and restore jasmonate signaling (PubMed:23320078). Required for JA-regulated defense responses to infestation by the leaffolder Cnaphalocrocis medinalis (PubMed:22558386, PubMed:24003150). May act on an initial response of jasmonate-regulated gene expression toward drought tolerance as part of a BHLH148-TIFY11D/JAZ12-COI1A complex (PubMed:21332845). Component of SCF(COI1) E3 ubiquitin ligase complexes, which may mediate the ubiquitination and subsequent proteasomal degradation of target proteins, including TIFY/JAZ family (By similarity). Interacts with TIFY6A/JAZ3, TIFY6B/JAZ4 and TIFY11D/JAZ12 in a coronatine-dependent manner (PubMed:21332845, PubMed:23320078). Interacts with TIFY9/JAZ5, TIFY10A/JAZ6, TIFY10B/JAZ7 and TIFY11C/JAZ11 in a coronatine-dependent manner (PubMed:23320078). Interacts with TIFY11A/JAZ9 in a coronatine-dependent manner (PubMed:23320078, PubMed:25617318). Expressed in roots, shoots, leaf sheaths and leaf blades. By the leaffolder Cnaphalocrocis medinalis infestation and methyl jasmonate (MeJA) treatment. Plants silencing COI1A and COI1B show increased plant height and seed length, are hypersensitive to gibberellin, and display reduced sensitivity to inhibition of seedling growth by jasmonate (PubMed:22529386). Plants silencing COI1A show early and reduced tillering with few grain filling, and display increased sensitivity to the chewing insect Cnaphalocrocis medinalis (PubMed:22558386).