Expression of lipoxygenase was studied in barley (Hordeum distichum L.) embryos during germination. Total lipoxygenase activity was high in quiescent grains, dropped during the 1st d of germination, and subsequently increased to a level similar to that in quiescent grains. The contribution of two isoenzymes, lipoxygenases 1 (LOX-1) and 2 (LOX-2), was studied at the activity, protein, and mRNA levels. Activity ratios of the two isoforms were determined via the ratio of 9- and 13-hydroperoxides, which are formed from linoleic acid. Isoenzyme protein levels were determined using specific monoclonal antibodies. mRNA levels were studied using the specific cDNA probes LoxA and LoxC, which correspond to LOX-1 and LOX-2, respectively. The major difference in temporal expression of LOX-1 and LOX-2 was observed in quiescent grains. At this stage, LOX-1 contributed almost exclusively to total lipoxygenase activity. LOX-2 activity rapidly increased until d 2 of germination. From this time point onward, LOX-1 and LOX-2 showed similar patterns at both activity and protein levels. The tissue distribution of the two isoenzymes in the germinating embryo was closely similar, with the highest expression levels in leaves and roots. The levels of LOX-1 and LOX-2 may be regulated mainly pretranslationally, as suggested by the similarity of the protein and mRNA patterns corresponding to the two isoforms.
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