Journal
JOURNAL OF THE AMERICAN SOCIETY FOR HORTICULTURAL SCIENCE
Volume 146, Issue 5, Pages 297-+Publisher
AMER SOC HORTICULTURAL SCIENCE
DOI: 10.21273/JASHS05064-21
Keywords
AAT; alcohol acyl transferase; sensory; citramalate; fatty acid; Km; Vmax
Categories
Funding
- Michigan AgBioResearch
- U.S. Department of Agriculture National Institute of Food and Agriculture, Hatch project [MICL002688]
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The study found that the production of ester volatile compounds in apple fruit is regulated by ethylene, with ester production rapidly increasing as ethylene levels rise, reaching a peak during ethylene synthesis and then declining. Furthermore, acetate esters predominate throughout fruit maturation and ripening, indicating a potential saturation of acetyl-CoA for ester formation.
The volatile profile of 'Redchief Delicious' apple (Malus xdomestica Borkh.) fruit was evaluated at 18 time points from 3 weeks before to 8 weeks after onset of autocatalytic ethylene production to capture the dynamics associated with development from mature green to senescent fruit. Minor amounts of ester production began several days before the onset of ethylene production. Ester production rose rapidly as internal ethylene levels increased beyond 22 nmol.L-1 (0.5 mu L.L-1). Peak ester production roughly coincided with maximum ethylene synthesis, declining thereafter. Ester production was further evaluated according to the acid- (alkanoate) and alcohol- (alkyl) derived portions of the ester. The maximum rate of production for a given ester tended to occur later in development as the chain length of the alcohol-derived portion declined. The production rate for many esters paralleled the rate of emanation of their respective alcohol substrates, suggesting that availability of the alcohols limits ester production more than availability of the acid substrates. Combining production rates with sensory descriptors and human sensitivity to individual volatiles permitted approximations of aroma sensations likely engendered by the fruit throughout ripening. Overripe and alcoholic sensations are predicted to increase 2 weeks after the initiation of ripening in response to an increase in the production of ethyl esters. Acetate esters predominated, comprising 50% to 80% of esters throughout maturation and ripening, indicating that the substrate acetyl-CoA may be at saturating levels for alcohol acyl transferase (AAT) at the final step of ester formation. Acetate feeding did not enhance ester production, although label from C-acetate was extensively incorporated into esters. The data are consistent with the action of multiple AAT isozymes differing in activity and substrate preference. Incorporation of labeled C-13-acetate into precursors of esters, alcohols, and acids, reflected ester biosynthesis via 1- and 2-carbon chain elongation pathways in ripening 'Redchief Delicious' apple fruit.
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