Catalyzes the formation of 1-aminocyclopropane-1-carboxylate, a direct precursor of ethylene in higher plants (By similarity). Required for the regulation of starch grain size in endosperm (PubMed:26792122). S-adenosyl-L-methionine = 1-aminocyclopropane-1-carboxylate + H(+) + S-methyl-5'-thioadenosine Alkene biosynthesis; ethylene biosynthesis via S-adenosyl-L-methionine; ethylene from S-adenosyl-L-methionine: step 1/2. Localizes to the amyloplast membrane surrounding starch grains in endosperm, pollen, and pericarp. Expressed in leaves. Enlarged starch grains in endosperm, spherical starch grains in pollen, decreased number of chloroplasts in leaves, enlarged chloroplasts with increased number of starch granules in leaves, reduced number of total panicles and slight reduction of seed weight. Belongs to the class-I pyridoxal-phosphate-dependent aminotransferase family.