A Single Lipoxygenase in Apple and Its Free Linoleic Acid Substrate are Proposed to Constitute a Major Pathway in the Formation of Hexyl Esters during Fruit Ripening

During apple fruit ripening, only one of 22 putative lipoxygenase (LOX) genes was found to undergo a substantial increase in transcript accumulation.  The increase in expression (~100-fold) coincided with autocatalytic ethylene formation and the climacteric rise in respiration. This LOX (MdLOX1a) was originally annotated as a 9-LOX, which would primarily form 9-hydroperoxides and lead to the formation of 9-carbon aldehydes, which are not produced in apple. However, protein expression studies demonstrated that MdLOX1a was primarily a 13-LOX and formed 13-hydroperoxides, which are cleaved by hydroperoxide lyase to form C6 aldehydes.  Unlike many 13-LOX enzymes, MdLOX1a lacked a chloroplast transit peptide and did not co-locate with chloroplasts in a heterologous system using Agrobacterium-mediated transient expression in tobacco. The increase in MdLOX1a transcript accumulation paralleled an increase in free oleic (18:1) and linoleic (18:2) fatty acid pools during ripening. While 18:1 would not be acted upon by a 13-LOX, 18:2 would be expected to yield hexanal.  Emissions of hexanal, hexanol, and hexyl esters increased simultaneously with the abundance of MdLOX1a transcripts and the increase in concentration of free 18:2. Interestingly, free linoleic acid (18:3) did not accumulate.  The mechanism whereby free 18:1 and 18:2 could accumulate without a concomitant increase in free 18:3 is not clear.  To our knowledge, there is currently no synthetic or catabolic pathway characterized that might lead to this finding. Collectively, the data suggest that production of free 18:2 fatty acid during apple fruit ripening provides substrate for the up-regulated lipoxygenase MdLOX1a and results in the synthesis of hexanal and, subsequently, hexanol and hexyl esters.