Profiling of oxylipins from young maize roots revealed complex patterns of products mainly originating from the combined actions of 9- and 13-lipoxygenases and allene oxide synthase (AOS). A distinctive feature was the high content of the cyclopentenone 10-oxo-11-phytoenoic acid (10-oxo-PEA). Incubations with [1-14C]linoleic acid led to the formation of the α-ketols 13-hydroxy-12-oxo-9-octadecenoic acid and 9-hydroxy-10-oxo-12-octadecenoic acid as well as the cyclopentenones 12-oxo-10-phytoenoic acid (12-oxo-PEA) and 10-oxo-PEA in a ratio of 10:2:1:3. Chiral phase radio-HPLC showed that the labeled 10-oxo-PEA was mainly (93%) due to the 9S,13S-enantiomer, whereas 12-oxo-PEA was racemic. Recombinant maize AOS CYP74A19 (ZmAOS2) converted linoleic acid 9(S)-hydroperoxide (9-HPOD) into an allene oxide, 9,10-epoxy-10,12-octadecadienoic acid (9,10-EOD), which did not undergo cyclization but was solely hydrolyzed into the α-ketol. A cyclase activity promoting the conversion of 9,10-EOD into (9S,13S)-10-oxo-PEA was detected in the 10(5)×g supernatant prepared by differential centrifugation of the maize root homogenate. The data obtained suggested the existence of a new type of allene oxide cyclase, which is active towards an allene oxide formed from a 9-lipoxygenase-derived hydroperoxide.