Multifunctional enzyme that catalyzes the carboxylation of acetyl-CoA, forming malonyl-CoA, which is used in the plastid for fatty acid synthesis and in the cytosol in various biosynthetic pathways including fatty acid elongation. Required for very long chain fatty acids elongation. Necessary for embryo and plant development. Plays a central function in embryo morphogenesis, especially in apical meristem development. Involved in cell proliferation and tissue patterning. May act as a repressor of cytokinin response. acetyl-CoA + ATP + hydrogencarbonate = ADP + H(+) + malonyl-CoA + phosphate ATP + hydrogencarbonate + N(6)-biotinyl-L-lysyl-[protein] = ADP + H(+) + N(6)-carboxybiotinyl-L-lysyl-[protein] + phosphate Binds 2 magnesium or manganese ions per subunit. Lipid metabolism; malonyl-CoA biosynthesis; malonyl-CoA from acetyl-CoA: step 1/1. Homodimer. Expressed in roots, trichomes, epidermal leaf cells, siliques, petals, anthers, and seeds. Expressed in flower buds at stage 6 of development in tapetal cells and at stage 10 in the epidermal cells of growing petals and ovaries. In young siliques, expressed transiently in the inner integument of the ovules just prior to testal deposition. Embryo lethal with an arrest in development at the late globular stage in acc1-1 and acc1-2 null allele mutants. In the leaky pas3 and gk alleles, defect in embryo development, very short and thick hypocotyl and misshaped cotyledons that do not expand. Abnormal root development, abnormal fused leaves and compact rosettes with multiple shoots. Uncoordinated cell divisions in the apical region. Reduced levels of very long chain fatty acids in seeds. The acc1-1 and pas3-1 mutants can be partially complemented by exogenous supply of malonate.