Plant 9-lipoxygenases(9-LOX)and α-dioxygenases(α-DOX)initiate the synthesis of oxylipins after bacterial infection.Here,the role of these enzymes in plants’ defense was investigated using individual Arabidopsis thaliana lox1 and dox1 mutants and a double lox1 dox1 mutant.Studies with Pseudomonas syringae pv.tomato(Pst)revealed the enhanced susceptibility of lox1 to the virulent strain Pst DC3000 and the partial impairment of lox1 and dox1 mutants to activate systemic acquired resistance.Notably,both defects were enhanced in the lox1 dox1 plants as compared with individual mutants.We found that pre-treatment with 9-LOX- and α-DOX-generated oxylipins protected plant tissues against bacterial infection.The strongest effect in this respect was exerted by 9-ketooctadecatrienoic acid(9-KOT),which is produced from linolenic acid by 9-LOX.Quantification of 9-KOT revealed its accumulation after bacterial infection.The levels were reduced in lox1 and lox1 dox1 plants but strongly increased in the dox1 mutant due to metabolic interaction of the two pathways.Transcriptional analyses indicated that 9-KOT pre-treatment modifies hormone homeostasis during bacterial infection.The nature of the changes detected suggested that 9-KOT interferes with the hormonal changes caused by bacterial effectors.This notion was substantiated by the finding that 9-KOT failed to reduce the growth of PstDC3000hrpA,a mutant compromised in effector secretion,and of the avirulent strain Pst DC3000 avrRpm1.Further support for the action of the 9-LOX- and α-DOX-oxylipin pathways as modulators of hormone homeostasis was the observation that lox1 dox1 seedlings are hypersensitive to the growth-inhibitory effect of ABA and showed enhanced activation of ABA-inducible marker genes as compared with wild-type plants.