Preferential induction of a 9-lipoxygenase by salt in salt-tolerant cells of Citrus sinensis L. Osbeck

Recent findings in our laboratory suggested that in citrus cells the salt i nduction of phospholipid hydroperoxide glutathione peroxidase, an enzyme ac tive in cellular antioxidant defense, is mediated by the accumulation of hy droperoxides. Production of hydroperoxides occurs as a result of non-enzyma tic autooxidation or via the action of lipoxygenases (LOXs). In an attempt to resolve the role of LOX activity in the accumulation of peroxides we ana lyzed the expression of this protein under stress conditions and in cells o f Citrus sinensis L. differing in sensitivity to salt. Lipoxygenase express ion was induced very rapidly only in the salt-tolerant cells and in a trans ient manner. The induction was specific to salt stress and did not. occur w ith other osmotic-stress-inducing agents, such as polyethylene glycol or ma nnitol, or under hot or cold conditions, or in the presence of abscisic aci d. The induction was eliminated by the antioxidants dithiothreitol and kaem pferol, thus once more establishing a correlation between salt and oxidativ e stresses. Analyses of both in vitro and in vivo products of LOX revealed a specific 9-LOX activity, and a very fast reduction of the hydroperoxides to the corresponding hydroxy derivatives. This suggests that one of the met abolites further downstream in the reductase pathway may play a key role in triggering defense responses against salt stress.