Comparative molecular and biochemical characterization of segmentally duplicated 9-lipoxygenase genes ZmLOX4 and ZmLOX5 of maize

Lipoxygenases (LOXs) catalyze hydroperoxidation of polyunsaturated fatty acids (PUFAs) to form structurally and functionally diverse oxylipins. Precise physiological and biochemical functions of individual members of plant multigene LOX families are largely unknown. Herein we report on molecular and biochemical characterization of two closely related maize 9-lipoxygenase paralogs, ZmLOX4 and ZmLOX5. Recombinant ZmLOX5 protein displayed clear 9-LOX regio-specificity at both neutral and slightly alkaline pH. The genes were differentially expressed in various maize organs and tissues as well as in response to diverse stress treatments. The transcripts of ZmLOX4 accumulated predominantly in roots and shoot apical meristem, whereas ZmLOX5 was expressed in most tested aboveground organs. Both genes were not expressed in untreated leaves, but displayed differential induction by defense-related hormones. While ZmLOX4 was only induced by jasmonic acid (JA), the transcripts of ZmLOX5 were increased in response to JA and salicylic acid treatments. ZmLOX5 was transiently induced both locally and systemically by wounding, which was accompanied by increased levels of 9-oxylipins, and fall armyworm herbivory, suggesting a putative role for this gene in defense against insects. Surprisingly, despite of moderate JA- and wound-inducibility of ZmLOX4, the gene was not responsive to insect herbivory. These results suggest that the two genes may have distinct roles in maize adaptation to diverse biotic and abiotic stresses. Both paralogs were similarly induced by virulent and avirulent strains of the fungal leaf pathogen Cochliobolus carbonum. Putative physiological roles for the two genes are discussed in the context of their biochemical and molecular properties.