DIFFERENTIAL EXPRESSION OF LIPOXYGENASE ISOENZYMES IN EMBRYOS OF GERMINATING BARLEY

Expression of lipoxygenase was studied in barley (Hordeum distichum L. ) embryos during germination. Total lipoxygenase activity was high in quiescent grains, dropped during the Ist d of germination, and subsequ ently increased to a level similar to that in quiescent grains. The co ntribution of two isoenzymes, lipoxygenases 1 (LOX-1) and 2 (LOX-2), w as studied at the activity, protein, and mRNA levels. Activity ratios of the two isoforms were determined via the ratio of 9- and 13-hydrope roxides, which are formed from linoleic acid. Isoenzyme protein levels were determined using specific monoclonal antibodies. mRNA levels wer e studied using the specific cDNA probes LoxA and LoxC, which correspo nd 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 thi s stage, LOX-1 contributed almost exclusively to total lipoxygenase ac tivity. LOX-2 activity rapidly increased until d 2 of germination. Fro m this time point onward, LOX-1 and LOX-2 showed similar patterns at b oth activity and protein levels. The tissue distribution of the two is oenzymes in the germinating embryo was closely similar, with the highe st expression levels in leaves and roots. The levels of LOX-1 and LOX- 2 may be regulated mainly pretranslationally, as suggested by the simi larity of the protein and mRNA patterns corresponding to the two isofo rms.