Polyamines, small aliphatic amines that are ubiquitously found in almost all cellular organisms, are involved in a wide variety of physiological processes. Putrescine, which is produced from arginine, serves as a metabolic precursor for longer polyamines, including spermidine and spermine. In plants, biosynthesis of polyamines is induced in response to pathogen infection, as well as other abiotic stresses. Polyamine acetylation, which has important regulatory functions in mammalian cells, has also been observed in several plant species. Here I show that Arabidopsis thaliana (Arabidopsis) N-ACETYLTRANSFERASE ACTIVITY1 (NATA1) not only catalyzes acetylation of ornithine to Nδ-acetylornithine, but also putrescine to N-acetylputrescine. NATA2, encoded by a homolog of NATA1 that is directly adjacent in the Arabidopsis genome, efficiently acetylates both ornithine and spermine. The NATA1 and NATA2 acetyltransferases compete for polyamine metabolic flux and, hence, reduce H2O2 formation via polyamine oxidation. By doing so, they play a regulatory function in plants under stress. More specifically, the induction of NATA1 and NATA2, by jasmonate and heat stress, respectively, reduces H2O2-dependent defenses in Arabidopsis and makes plants more susceptible to biotrophic pathogens, such as Pseudomonas syringae. This is shown by the fact that nata1 knockout mutants are more resistant to a P. syringae strain DC3000. By producing coronatine, an effector molecule that mimics jasmonate signaling, P. syringae highjacks the polyamine acetylation mechanism described above. Arabidopsis nata2 knockout mutants showed similarly increased resistance to P. syringae under heat stress. Moreover, the chewing herbivore Spodoptera exigua also performed less well on nata2 knockout mutants under heat stress. Taken together, NATA1 and NATA2 highlight the regulatory function of polyamine acetylation, as well as the controlling role polyamines play in crosstalk between jasmonate signaling, heat stress, and salicylate signaling. Acetylated polyamines also provide an alternative pathway for polyamine formation. Duplication and neofunctionalization of ADC1 and NATA1, which co-occur in a small number of plant species in the Brassicaceae, constitute an alternative pathway in which ADC1, one of two known arginine decarboxylases in Arabidopsis, converts Nδ-acetylornithine to N-acetylputrescine. Subcellular localization data indicate that ADC1 and NATA1 share a common substrate pool, as they are both localized in the endoplasmic reticulum.