- The Arabidopsis aldehyde oxidases are a multigene family of four AAOs (1-4) that oxidize a variety of aldehydes, amongst them abscisic aldehyde, which is oxidized to the phytohormone abscisic acid.Toxic aldehydes are generated in plants both under normal conditions and in response to stress.The detoxification of such aldehydes by oxidation is attributed to aldehyde dehydrogenases but never to aldehyde oxidases. The feasibility of detoxification of aldehydes in siliques via oxidation by AAO4 was demonstrated, first, by its ability to efficiently oxidize an array of aromatic and aliphatic aldehydes, including the reactive carbonyl species (RCS) acrolein, hydroxyl-2-nonenal, and malondialdehyde (MDA). Next, exogenous application of several aldehydes to siliques in AAO4-Knock out (KO) Arabidopsis thaliana plants induced severe tissue damage, enhanced MDA levels and senescence symptoms but not in WT siliques. Further, abiotic stresses such as dark and UV-C irradiation caused an increase in endogenous RCS and higher expression levels of senescence marker genes, leading to premature senescence of KO siliques, whereas RCS and senescence marker levels in WT siliques were hardly affected. Finally, in naturally senesced KO siliques, higher endogenous RCS levels were associated with enhanced senescence molecular markers, chlorophyll degradation and earlier seed shattering as compared with WT. The aldehyde dependent differential generation of O2- and H2O2 by AAO4 and the induction of AAO4 expression by H2O2 shown here suggest a self-amplification mechanism for detoxifying additional reactive aldehydes produced during stress.Taken together, our results indicate that AAO4 plays a critical role in delaying senescence in siliques by catalyzing aldehyde detoxification.