Catalyzes the formation of 1-aminocyclopropane-1-carboxylate, a direct precursor of ethylene in higher plants (Probable). Involved in defense response by producing ethylene after pathogen infection (PubMed:17012402, PubMed:23031077). Involved in several phospate deficiency-induced adaptive responses, such as lateral root elongation (PubMed:30810167). 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. Down-regulated by partial submergence (PubMed:9037160). Induced in leaves by infection with the fungal pathogen Magnaporthe oryzae (PubMed:17012402). Induced by phosphate deficency (PubMed:30810167). Lines with ACS2 transgene under the control of a strong pathogen-inducible promoter show significantly increased levels of endogenous ethylene and defense gene expression in response to pathogen infection, and exhibit increased resistance to Magnaporthe oryzae and Ralstonia solanacearum. Belongs to the class-I pyridoxal-phosphate-dependent aminotransferase family.